Best Practices for Growers

These Best Practices are based on California research conducted by UC Davis, UC Cooperative Extension (UCCE) and UC Agricultural and Natural Resources (ANR).

They were prepared by the UC Davis Olive Center and G. Steven Sibbett, UCCE Farm Advisor Emeritus, Dr. Louise Ferguson, ANR Extension Specialist and Dr. Elizabeth Fichtner, UCCE Farm Advisor. We recommend that growers also review comprehensive research information available through ANR, including the Olive Production Manual, Organic Olive Production Manual and UC IPM Online.

It is critical to assess climate before planting olives in a particular site:

  • Winter. Ideally, winter temperatures should fluctuate between 35 °F (2 °C) and 65 °F (18 °C). Temperatures below freezing cause progressively more tree damage, from small shoot and branch lesions that provide entry points for olive knot bacteria at 23 °F to 32 °F (-5 °C to 0 °C), greater tissue damage at 14 °F to 23 °F (-5 °C to -10 °C) and death of large limbs and entire trees at temperatures below 14 °F (-10 °C).
  • Spring. The bloom development period should be free of prolonged cold and wet or hot and dry weather. These conditions hinder flower development, pollination, fertilization and fruit set. Long or sudden cold spells particularly increase the negative impact.
  • Summer. Long, warm and dry summers promote good fruit development. Avoid areas with summer rainfall and high humidity, which promote fungal and bacterial diseases.
  • Fall. Temperatures below freezing often damage processing quality of fruit destined for either table or oil. Pay special attention to low-lying areas, which are especially vulnerable to colder temperatures. Fall rains improve size and value of fruits destined for table processing, but make fruit destined for oil processing more susceptible to damage, fermentation and mold and may contribute to emulsions that hinder oil extraction Rains at harvest can also hinder mechanical harvest equipment from accessing the orchard.

Research the site history. Find out crop history from the previous landowner and, where relevant, the local agricultural commissioner’s office. Avoid soil previously planted with crops (such as cotton, cucurbits, eggplant, peppers, potato, or tomato) susceptible to the Verticillium wilt fungus, a soil-borne disease that kills olive trees. There are limited Verticillium wilt management strategies available to growers.

Analyze the soil profile. Managed correctly, olives perform well in many soils, even those considered marginal in quality. Soil maps do not provide sufficient detail for specific orchard sites. Use a backhoe or augur and dig pits in representative places on the planting site. Examine the soil’s physical condition, including layers that are texturally different, to identify limitations on root and water penetration. Olives do not grow well in poorly drained soils. The best and most productive soils are those un-stratified, moderately fine-textured of at least 4 ft (1.2 m) in depth.

Determine the soil chemistry. Take a representative soil sample from the orchard site and submit it to a laboratory for analysis. The best soils are those moderately acid to moderately alkaline (pH between 6.5 to 8.5). Soil below pH 5.5 can have aluminum and manganese toxicity, while soil above pH 8.5 have poor structure and may have sodium toxicity. Avoid soils high in salinity (≥4 dS/m). To avoid water penetration problems due to poor soil structure, avoid soils with an exchangeable sodium percentage of > 4 or be prepared to amend such soils to leach excessive sodium.Avoid soils with excessive boron (≥2 ppm), and chloride (10-15 meq/l) as these conditions may reduce productivity unless corrected or managed with soil amendments requiring additional expense.

Assess water availability. Although olive trees are drought tolerant, they will grow faster and produce more consistently in California with supplemental irrigation. Supplemental irrigation water can be available as irrigation district water (surface water) or by farm wells. Sites served only by irrigation districts are at risk of water shortage during drought years.Inadequate water during floral development can lead to poor fruit set especially if adverse weather occurs during or shortly after bloom. Inadequate water through the growing season can limit fruit size for table olive growers. Choose sites that can supply olive trees with approximately three acre-feet per year for table olives and two acre-feet per year for oil olives, although more water will be necessary if irrigation efficiency is compromised by conditions such as runoff or poor weed control.

Evaluate water quality. Knowing the site’s water chemistry will help growers manage chemical hazards and avoid excessive fertilizer use that increases orchard maintenance cost, reduces productivity and potentially pollutes water sources. Excessive sodium in water supplies concentrate in soil, causing infiltration problems. High nitrogen levels produce excessive vegetative growth hindering fruit production, encourages pest development (e.g. black scale) and adds additional pruning expense. Take a water sample and request a qualified laboratory to conduct an analysis of the elements in this table.

Acidity/alkalinity (pH) 6.5 – 8.5
Electrical conductivity (ECw) < 3.0 dS/m
Exchangeable sodium percentage (ESP) < 4
Bicarbonate (HCO3-) < 3.5 meq/l
Sodium absorption ratio (SAR) <6
Chloride (Cl-) <3 meq/l
Boron (B+) ≤ 1 - 2 mg/l
Nitrate nitrogen (NO3-N) <5 ppm

Assess infrastructure. Ensure essential supplies and services are within a reasonable distance from the orchard. An isolated orchard requires excessively high costs to extend transportation and water infrastructure, obtain essential supplies and services, access labor and deliver the crop.

Traditional California olive orchards were large trees at planted at low densities; typically less than 85 trees/ac (205 trees/ha), hand-harvested and hand-pruned. Increasing harvest costs and uncertain labor availability have resulted in many California olive oil growers and a few table olive growers planting higher-density, hedgerow configured orchards. In hedgerow orchards, both the in-row and between-row spacing is tighter, ranging from “high-density” (HD) orchards of about 200 trees/ac (482 trees/ha) to “super-high-density” (SHD) orchards of about 650 trees/ac (1567/ha). Both HD and SHD orchards facilitate mechanical harvesting and pruning while also intercepting the optimal percentage of incident light to maximize yield.

Most California olive oil acreage favors the SHD format, which requires clones of specific varieties developed for these orchards. Typically SHD oil olives are harvested with over-the-row, canopy-contact harvesters. A few California olive oil growers and table olive growers have adopted HD orchard densities, which can be harvested with trunk shakers for the first 15 years, and then possibly harvested with canopy-contact equipment when trunk-shaking efficiency declines. University of California (UC) researchers have developed specifications for an efficient canopy-contact harvester head for oil and table olive hedgerow orchards. UC has provided these specifications to commercial harvest equipment manufacturers.

UC cost studies for establishing and producing table and oil olives in a hedgerow orchard are available at UC cost studies are based on specific assumptions -- but not all practices, and particularly quoted prices, may be applicable to every grower’s situation. Growers should use these cost studies as beginning templates and modify them to a given situation and local costs. The grower then can use the ranging analyses in the studies to determine if table or oil olive production is feasible in a specific location.

The following best practices for establishing hedgerow orchards were based on UC research. When UC data-based information was unavailable, the best practices were based on research outside of California as well as current predominant California industry practices. Except where indicated, the recommendations apply whether the orchard is intended for oil, table or dual varieties.

Nurseries may need a year’s notice to ensure selected varieties are available for spring planting. California’s “ripe” table olive processors primarily favor Manzanillo with Sevillano (Gordal) as a pollinizer, while Sicilian-style and Spanish-style processers prefer Sevillano (Gordal). Small table olive processors have used other varieties for these and other curing styles. Arbequina is California’s most widely planted oil variety, although California oil producers have marketed all varieties in the table below as single-varietal olive oils, even traditional table olive varieties such as Ascolano, Mission, Sevillano and Manzanillo.

When selecting varieties, consider:

  • processor preferences
  • cold hardiness
  • disease resistance
  • vigor and yield
  • pollination requirements (most olive varieties perform better with an appropriate pollinator variety planted no more than 200 ft (61 m) away

When choosing varieties for olive oil processing, also consider:

  • oil content
  • extraction efficiency
  • flavor characteristics
  • stability (which correlates with high polyphenol levels and high oleic acid content)

When choosing varieties for table olive processing, in general, also consider:

  • high flesh-to-pit ratio
  • large fruit size (larger sizes typically receive a higher price than smaller sizes)
  • excellent flesh quality (such as resistance to bruising if the olives will be cured green, and desirable sensory characteristics for the anticipated curing method)

No single olive variety has every desirable characteristic. Comparison of some common olive varieties are in the table below, primarily based on information from the World Catalogue of Olive Varieties, modified by other sources to include varieties not addressed in the catalogue.

View entire table here

* IRTA considers the Arbequina SHD clone to be sensitive to peacock spot while the World Catalogue of Olive Varieties lists Arbequina as resistant to the disease. Arbosana SHD clone information is from California nurseries and California field observations. California field observations indicated Koroneiki SHD clones as high & alternate bearing rather than high & constant bearing as indicated by the World Catalogue of Olive Varieties.

Soils that do not have a minimum, un-stratified depth of 4 ft (1.2 m) will need to be deep-tilled to 3 - 5 ft (1 - 1.5 m) to ensure uniform water penetration. Use a slip plow to mix stratified soils and a ripper to break up cemented hard pans. The best time for deep tillage is when soils are dry.

Till un-stratified sub-soils to 2.5 ft (0.7 m) by chiseling and shallow ripping to break up compacted plow pans, thereby improving drainage and facilitating root development. Till when soils are dry to avoid compacting soil.

Super-high-density orchards in California have accommodated harvest equipment with grades as high as 15 percent, but grades above 6 percent can lead to severe erosion if not designed properly.

Control weeds prior to planting as weeds will compete with trees for water, nutrients and sunlight, thereby delaying production. Control annual weeds by disking or applying pre- or post-emergent herbicides. Control most perennial weeds (e.g., bermudagrass, dallisgrass, and johnsongrass) by repeated disking and drying during summer. Control field bindweed by irrigating to produce a vigorous plant and then treating with glyphosate or 2,4-D, followed in 10 days by disking and drying the soil.

Add soil amendments (such as gypsum, sulfur, sulfuric acid and lime) if necessary to correct sodic (alkali), acid (pH7) in reaction. A thorough soil analysis through the profile is essential to determining need for, type, and amount of chemical amendments prior to planting.

Consider establishing berms, which are flat, raised strips of soil of about 1.5 ft (50 cm) high and 3.5 ft (1 m) wide located within the tree row on which orchard trees are planted. Berms can allow better moisture retention and facilitate drainage, but they can also limit irrigation hose placement and decrease trellis integrity. Berms may also decrease drainage thereby increasing the potential for root disease when the orchard slope is perpendicular to the berms. Berms are have been more useful in rainy Northern California orchards to promote drainage away from the tree but generally are not employed in drier Southern California orchards.

High-Density (HD) hedgerow orchards in California have ranged from 200 trees/acre to 269 trees per acre, while Super-High-Density (SHD) hedgerow orchards, which are used exclusively for oil production, have ranged from 453 trees/acre to 908 trees/acre, with most SHD orchards planted at about 650 trees/ac (1567/ha).

Tree spacing in hedgerow orchards should be dictated by harvest equipment. Most SHD olive oil growers use over-the-row harvesters while UC research has demonstrated trunk shakers and canopy-contact harvesters can be efficient for HD table olive harvesting. The SHD system works best with clones of specific varieties such as Arbequina, Arbosana and Koroneiki, while the HD system can accept many olive varieties.

In general, SHD orchards are more expensive to establish than HD orchards, but SHD orchards will achieve maximum income sooner than HD orchards. Both orchards have similar maximum income and expenses over time based on data from the Australian olive oil industry.

North-south oriented hedgerows will maximize sunlight exposure, decrease humidity and reduce wind resistance.

Plant pollinizer trees or rows no more than every 200 feet (61 m) to ensure that wind-blown pollen is most effectively distributed.

A trellis and tree stakes support trees, prevents misshapen trees due to wind and ensures a straight tree row to facilitate the over-the-row harvesters in SHD olive oil orchards. A trellis is essential in an SHD orchard but not essential or usually needed in HD orchards. Stakes, however, are necessary to develop a straight single trunk.

Trellis and stakes. A standard SHD trellis uses 8 ft (2.4 m) treated round end posts, with 8 ft (2.4 m) metal T-posts placed at 55 ft (16.8 m) intervals and 12-gauge galvanized wire installed at 4.5 ft (1.4 m) and 6 ft (1.8 m), secured to the T-posts and bamboo tree stakes with wire clips. SHD growers in California are moving toward smaller trellises with 6 ft (2 m) end posts and 6 ft (2 m) T-posts driven 2 ft (0.6 m) into the ground, and a single wire placed at 3 ft (1 m). If one wants to use a trellis in a standard HD orchard, use 12 ft (3.7 m) treated-wood or iron posts that are round or 4 x 4 inches (10 x 10 cm) square, placed at the ends of rows and at 100 ft (30 m) intervals, with 12-gauge galvanized wire installed at 3, 6 and 9 ft (1, 2, and 3 m.) SHD and HD trellised orchards provide a 7 ft (2.1 m) bamboo stake at each tree and tie the central leader to the stake at 8 to 14 inch (20 to 35 cm) intervals as the leader grows.

Stakes only. When using stakes without a trellis in an HD orchard located in windy areas, use treated wood stakes that are 2 x 2 inches (5 cm) square and 8 ft (1.5 to 2.5 m) in height. Tie the central leader to the stake at 2.5 ft (75 cm) intervals. Lighter bamboo stakes can be used in areas with little wind.

A micro-irrigation system such as drip, fan jet, or micro-sprinkler, provides higher efficiency (85-95%) than large sprinklers (75-85%) or furrow irrigation (65-75%) by minimizing water losses due to runoff and evaporation. The higher efficiency of micro-irrigation systems also reduces orchard humidity compared to larger sprinklers, reducing risks related to fungal diseases.

Micro-irrigation systems are often designed to permit fertigation, the application of plant nutrients or water amendments in the irrigation water. Fertigation has the advantages of maintaining moisture in the wetted area, encouraging the development of new roots, extending the period of maximum root activity, improving plant nutrient absorption and providing a constant supply of nutrients based on the tree’s needs. The two objectives of fertigation are to maximize profit and reduce adverse environmental effects. Profit is maximized when fertilizer remains in the wetted zone where root density is greatest. Adverse environmental impacts are minimized by not allowing fertilizer to leach below the wetted root zone and pollute groundwater.

Filtration is a significant cost in installing micro-irrigation systems and is essential to mitigate clogging due to bacterial growth. Chemigation valves are also required to prevent back flow of chemicals as well as chemical precipitation in the laterals and emitters.

Maximizing benefits from drip and micro-irrigation systems requires the system to be designed properly, installed correctly and managed effectively. Neglecting these factors can lead to poor results.

Install the irrigation system prior to planting. Irrigate the orchard prior to planting if soils are dry. Planting in dry soil will cause moisture to migrate from the root ball to the soil, resulting in root death in very young trees.

There are two objectives to tree training during the first three years: (1) developing a sturdy, well-spaced framework of uncongested scaffold branches to support heavy crops and mechanical harvesting, and (2) bringing the tree into bearing as quickly as possible. Pruning must be sufficient to achieve the first objective, but minimized to promote the second.

Avoid training and pruning during dormancy. Such pruning requires heavier cutting and renders the tree more susceptible to freeze injury and disease.

For HD orchards, remove shoots below a 2.5 ft (75 cm) height, and pinch the leader at 36 to 40 inches (1 m) to stimulate lateral growth if lateral shoots above 2.5 ft (75 cm) have not already begun in the nursery. Remove suckers and waterspouts (which are the angular, succulent herbaceous growth, often pale green in color, that shoot straight upward) as early as possible.

For SHD orchards, remove shoots within the protective carton and 8 in (20 cm) above the carton to maintain a single leader. Remove competing limbs as the leader approaches the top wire. When the leader is 12-18 in (30-46 cm) above the top wire, prune a handful of shoots, and tip 10-20 shoots along the sides of the tree to promote dense foliage and more fruitwood.

Further training of trees depends on the method of harvest:

  • Trunk shaker. Prune to direct the growth into stiff upright scaffolds that transmit the force from the trunk shaking. Remove branches below a 45o angle from the vertical as they shake less efficiently. Remove branches extending laterally into the row middle, which reduce harvester efficiency.
  • Canopy-contact and over-the-row harvesters. Canopy contact harvesters remove olives with two different mechanisms. Some compress and agitate the canopy with a series of horizontal bars called bow rods that agitate the canopy with a vertical motion. Some have a head with vertical rows of radiating tines that extend into the canopy. The horizontal whipping motion at the tips of these tines against the vertical branches removes the olives. Both removal methods remove the fruit on the outer canopy more efficiently than interior fruit. Train the tree into an upright narrow canopy, with primary scaffolds parallel to the tree row and shorter branches extending into the row. Such training, achieved by hand pruning, mechanical pruning or both, will facilitate production of a continuous flat fruiting wall. Remove branches that extend into the row middle. These branches will interfere with harvester efficiency, can damage harvest equipment, and will probably be broken by the harvester.

Proper irrigation can significantly improve grower returns by providing greater yield, higher fruit quality, and improved water and nutrient efficiency.

Inadequate soil moisture can lead to poor production from reduced flower formation, flower development, fruit set, and shoot growth, as well as from exacerbating alternate bearing and inadequate nutrient absorption. Poor irrigation can also lead to poor fruit size and shrivel.

Excessive soil moisture can result in poor shoot growth, yellow foliage and tree loss due to asphyxiation or root disease (phytophthora).

In California, mature table olives need about 36 acre-inches per acre of water per year to promote optimal fruit size and productivity. Oil olives require about 24 acre-inches of water per year.

Olive tree water needs are based on evapotranspiration (ETo) adjusted by an olive crop coefficient (Kc). ETo is the sum of evaporation of water from soil and transpiration of water from a reference crop then adjusted by the Kc to reflect the olive tree’s foliage transpiration.

Oil olives produce more oil with regulated deficit irrigation, while reduced irrigation in table olive orchards to 50 percent of normal from June through mid-August, did not have a significant negative effect on fruit weight, yield or gross revenue. See more information here.

Maximize water efficiency and orchard productivity by using information from site variables such as crop ET, rainfall, and irrigation system efficiency.

Olive trees need 16 essential elements to fulfill a cycle of growth. An olive orchard’s supplemental nutritional needs vary depending on soil fertility, orchard age, soil composition, nutrients in irrigation water, and cultivation practices.

Leaf nutrient levels vary normally throughout the growing season – some accumulate, some are relatively static, and some diminish. In addition, annual variations in nutrient levels can occur for other reasons, such as the crop load, water availability in the soil, crop management techniques, and interactions between nutrients.

Nutrient deficiencies observed in California olive orchards have been limited to nitrogen, and, rarely, potassium and boron. Nitrogen is the only nutrient supplement that might be needed on an annual basis. Investigate the cause of a mineral deficiency, because the solution is not always to simply provide fertilizer.

Determine orchard fertilization needs through annual leaf tissue analyses and visual deficiency symptoms. For annual leaf tissue analyses, collect at least 100 leaves from several trees in each homogenous area of an orchard from the middle of non-bearing, current-season shoots. Low nitrogen levels are visually indicated by poor shoot growth (less than 8 inches) and leaves with a light green-yellow shade, but must be verified by an objective, quantitative leaf tissue analysis.

Seek to achieve nutrient levels consistent with this table:

Table. Critical nutrient levels from July olive leaf tissue analysis from subterminal leaflets of current year’s non-fruit bearing growth.

Element Deficient Sufficient Toxic
Nitrogen (%) 1.40 1.50 - 2.00
Phosphorus (%) 0.10 – 0.30
Potassium (%) 0.40 > 0.80
Calcium (%) > 1.0
Magnesium (%) > 0.10
Manganese (ppm) > 20
Zinc (ppm) unknown
Copper (ppm) > 4
Boron (ppm) 14 19 - 150 > 185
Sodium (%) > 0.20
Chlorine (%) > 0.50

Avoid applying fertilizers if leaf tissue nutrient levels are adequate. Excessive fertilization increases production costs, nutritional imbalances and environmental pollution.

Fertigation, the application of fertilizers dissolved in irrigation water, is the most efficient fertilizing method. Fertigation delivers nutrients to areas of greatest root activity and density, maximizing a tree’s nutrient absorption. Fertigation requires cleanliness and irrigation system maintenance, because some fertilizers can clog the system. Fertigation with some fertilizers can also increase soil salinity.

Organic fertilizers such as compost or cover crops have advantages in releasing nutrients slowly year-round, developing soil structure and aiding water infiltration, but also may require additional water (e.g. for a cover crop). Any fertilizer can leach nutrients during cold and rainy periods (when trees are not taking up nutrients) and allow nitrogen runoff into water resources.

The primary olive pests in California are Olive Fly and Black Scale. For more information on olive pests, see UC Integrated Pest Management.

Olive Fly

The larva of olive fly, Bactrocera oleae (Gmelin), feed on olive fruit, leading to losses for table olive growers and potentially imparting a defective flavor to olive oil. Olive fly can be controlled but not eradicated in California.

Monitor olive fly with either yellow sticky traps containing a sex pheromone and/or ammonium carbonate, ammonium bicarbonate, or diammonium phosphate bait; or with MacPhail traps containing yeast hydrolosate and the same ammonia-producing chemicals as used with yellow sticky traps.

Control olive fly populations by collecting and destroying fruit on the ground and in trees after harvest, by use of bait sprays, and by “attract and kill” devices. Use Danitol late in the season if heavy infestations emerge after the summer. See more information about olive fly control here.

Black Scale

Black scale, Saissetia oleae (Olivier), feeds on olive leaves and twigs and excretes sugary honeydew that supports sooty mold growth impairing photosynthesis. Black scale infestation can reduce fruit bud formation, cause leaf drop and twig dieback and reduce the crop in the following year.

Monitor black scale in April by checking the inner canopy of about 40 trees per block for honeydew droplets on the leaves, and in May by checking at least 10 branches on 10 trees per 10-acre section for adult scale. Examine the terminal 20 inches of the branches, count the number of scale, and calculate the average per branch for the 10 trees.

Control black scale by pruning the trees to open up the canopy in the trees’ center. Heavy infestations (more than four scale per branch) also treat with insecticide in-season. See more information here.

Primary olive diseases in California are Olive Knot, Olive Leaf Spot (“Peacock Spot”), and Verticillium Wilt. For more information on olive diseases, see UC IPM.

Olive Knot

Olive knot, Pseudomonas syringae pv. savastanoi (Smith 1908), is a bacterial disease that produces galls (knots) on twigs and small branches at wounds, even minor wounds such as leaf scars. Cracks in bark caused by freeze injury can also lead to severe damage from olive knot. Cultivars that are sensitive to freezing (such as Manzanillo) are more susceptible to olive knot. The disease can kill young trees, reduce productivity in more mature trees, and produce off-flavors in the fruit.

Infection almost always occurs with moisture, particularly rain but also humid conditions, and can be carried by pruning shears and harvest equipment.

Copper has traditionally been used to control olive knot but has had inconsistent and diminishing effectiveness in recent years. UC research is underway to identify new methods of control, and new chemicals are in the registration process. See more information here.

Olive Leaf Spot (“Peacock Spot”)

This fungal disease is caused by Spilocea oleaginea (Cast.) Hughes. Lesions most commonly appear on upper leaf surfaces, beginning as tiny sooty blotches and progressing to green or black spots, with some lesions developing a yellow halo, thus the name Peacock Spot. Infected leaves drop prematurely, weakening small wood and reducing productivity.

Most infections occur during the coldest part of the California winter, and it may take several years before the disease causes economic loss.

Control Peacock Spot by a spraying a copper-containing fungicide, once in late fall before winter rains begin. See more information here.

Verticillium Wilt

Verticillium wilt, Verticillium dahlia Kleb., is a soil-borne fungus infecting a wide range of crops grown in California, including cotton, melon, pepper, pistachio, stone fruit and tomato, and some indigenous weed species. The fungus persists in soil as microsclerotia, which resemble small grains of sand. Leaves suddenly collapse and die on one or more branches soon after the first warm summer weather, reducing productivity significantly.

There is no control method available for Verticillium Wilt. Conduct a soil test for microsclerotia prior to establishing a new orchard. Previously planted crops that harbor the Verticillium Wilt fungus are likely to leave high levels of microsclerotia. See more information here.

Weed management is essential for increasing orchard productivity (by eliminating competition for water and nutrients and increased frost protection, for example), facilitating harvest, removing fire hazards, and eliminating habitat for damaging insects and rodents.

The proper weed management strategy depends on many factors including weed species, soil type, irrigation method, amount of control desired and organic certification. Strategies include hand-weeding, mowing, disking, mulching, weed fabric, and spraying with pre-emergent and post-emergent herbicides.

Groves in the San Joaquin Valley are almost exclusively weed-free. Most California olive growers manage weeds by mowing the row middles and spraying herbicide in the tree row. Weed management is significantly more expensive in organic orchards. Begin weed management in new orchards to speed tree growth and productivity. See more information on weed management.

Reducing the fruit load (thinning) in heavy crop years may be necessary to achieve adequate table olive size and mitigate alternate bearing.

Chemical thinning is the most useful fruit thinning tool available to table olive growers. Post-bloom application of naphthalene acetic acid (NAA) regulates crop size to improve fruit size and results in better shoot growth for return bloom the following year.

Pruning is not ideal as a thinning method because pruning removes both leaves and fruit (leaf-to-fruit ratio is an important factor in fruit size). In addition, pruning is labor intensive. Pruning is, however, the most traditional method and is useful when chemical fruit thinning is not practiced or available.

Hand thinning is an effective method but is too labor intensive to be practical for most growers. When hand-thinning, wear gloves to protect fingers. Strip fruit from trees with both hands but be careful not to damage or remove leaves. Thin the twigs from which at least five or six olives can be removed from one pull. Complete within four weeks of full bloom.

Prune to optimize orchard productivity, mitigate alternate bearing, facilitate efficient fruit removal, manage crop size, rejuvenate productivity in older trees, reduce pests and disease, and reduce damage from mechanical harvesters.

Prune in spring and summer after winter rains have passed, to minimize diseases such as olive knot, attacks by pests, and susceptibility to freeze damage.

How a tree is trained and pruned is determined by the olive’s natural growth and crop-bearing pattern. Olives have “apical dominance,” where the central stem grows more strongly than lateral stems. The apical stem, which is vegetative rather than fruiting, produces several lateral buds that are almost exclusively fruiting buds. The two kinds of pruning cuts, heading and thinning, produce a different result with this apical vegetative and lateral fruit-bearing pattern:

  • Heading cuts decrease apical shoot growth by removing the apical vegetative bud and may stimulate latent lateral vegetative growth further back on the shoot. If the tree is vigorous and the heading cuts are severe this strong response could be exclusively vegetative and therefore delay fruit production for two years beyond pruning.
  • Thinning cuts remove a shoot where it emerges from the branch, producing a much weaker vegetative response and allow more light interception than heading cuts.

Moderate hedging cuts on alternate sides of the tree in alternate years will reduce the response of vegetative and fruiting shoots to produce consistent annual crops, thereby mitigating the alternate bearing tendency of many olive cultivars. Trees should be mechanically topped annually to maintain tree size and maximize fruit production on the lower lateral canopy (topping less frequently has been demonstrated to produce strong vegetative growth responses and erratic fruit production.)

Hand-harvesting.Hand-harvested orchards can be assisted with hand-held implements, hand-held pneumatic rakes, hand-held limb shakers, and by beating the trees with poles.

  • Prune to achieve a lobular shape when viewed from above and to allow light channels into the trunk so that growth and fruiting is not restricted to the outer shell.
  • Prune to maintain a safe tree height below 18 feet so that harvesters do not have to climb too high on ladders.
  • Prune nonproductive parts of the tree, or not at all, during light-crop years, to minimize crop loss. Pruning can be more severe in heavy crop years but does not result in larger fruit desired by table-olive growers since fruit size is largely determined on each branch by leaf-to-fruit ratio.
  • Prune mature trees from traditional orchards over several years so as to not dramatically reduce crop and stimulate excessive non-fruiting growth.
  • Harvest by sliding a cupped, protected hand down an olive shoot in a milking action, removing olives into a container or onto a tarp. Avoid removing leaves as to minimize the potential for disease, and avoid stepping on olives in tarps.

Trunk-shaking equipment.

Trunk-shaking equipment includes side-by-side harvesters and umbrella trunk shakers. These harvesters operate most efficiently when the orchard is young enough that the shaking provides efficient fruit removal. Eventually the efficiency of trunk shaking will be reduced as the trunk grows thicker, making it necessary for the grower to switch to other harvest methods, such as limb-shaking or canopy-contact equipment.

  • Direct the growth into stiff upright scaffolds, which efficiently transmit the shaking force. Top the trees at no more than 12 feet with scaffolds having no more than a 45° angle from the trunk. Prune with a combination of mechanical hedging and topping (or a single, gabled cut) with additional hand pruning to remove lateral branches extending from the scaffolds at less than a 45° angle.
  • Decrease canopy density with hand thinning cuts, as a heavier canopy will dampen shaking and decrease removal efficiency.
  • Prune generally to a V-shape with visibly filtered light.
  • Remove branches extending laterally into the row middle, which can reduce harvester efficiency.
  • Prune annually. If using a double-sided mechanical pruner, the annual pruning schedule can be every other row middle every other year. If using single-sided mechanical pruner, prune alternate tree row sides on alternate years. Top all trees annually.

Limb-shaking equipment. Limb-shaking equipment

  • Prune to establish a maximum of five well-spaced, upright scaffold branches (more that five scaffold branches increases the harvest time for each tree).
  • Decrease canopy density with hand-thinning cuts, as a heavier canopy will dampen shaking and decrease removal efficiency.
  • Remove branches extending laterally into the row middle, which can reduce harvester efficiency.

Canopy-contact equipment. Canopy-contact harvesters come with one of two mechanisms. Some compress and agitate the canopy with a series of horizontal bars called bow rods, while others have a head with vertical rows of radiating tines that extend into the canopy and remove the olives with a horizontal whipping motion. Both removal methods remove the fruit on the outer canopy more efficiently than interior fruit, but, unlike trunk-shaking equipment, it is not necessary to thin the canopy as the removal force is applied directly to the canopy.

  • Super-high-density (SHD) orchards in California typically have hedgerows spaced at 5-6 feet within the row and 12-13 feet between rows. High-density (HD) orchards with hedgerows spaced at 8-10 feet within the row and 16-18 feet between rows have been demonstrated to be optimal for harvesting and yield for California table olives.
  • Train the tree into an upright narrow canopy, with primary scaffolds parallel to the tree row and with shorter branches extending into the row middle, to facilitate production of a continuous, flat, fruiting wall.
  • Train SHD orchards to an espalier within the row. This can be accomplish by tying to a trellis or by simply weaving the branches through the trellis wires when adequately stiff. Train HD orchards to no more than 12 feet high after a season’s growth, 6 feet wide from row middle to row middle, with the bottom of the canopy 4 feet from the ground. Canopy width within the row will be a function of tree spacing within the row.
  • Remove branches, which can reduce harvester efficiency and damage harvest equipment, and will probably be broken by the harvester.
  • Prune annually. If using a double-sided mechanical pruner, the annual pruning schedule can be every other row middle every other year. If using single-sided mechanical pruner, prune alternate tree row sides on alternate years. Top all trees annually.

Understanding the olive tree can help growers better understand how to site and manage a productive, high-quality olive orchard.

Olive is the only species of the Oleaceae family (which includes ash, privet and lilac among others) producing edible fruit.

Olive trees typically range in height from 8 to 20 feet (2.4 to 6 m). The trees can be productive for centuries, although estimates for modern hedgerow orchards is 25 – 40 years. Characteristics such as growth habit, internode length, and canopy density vary by cultivar.

While olive wood is decay-resistant, heart-wood can decompose, resulting in a hollow core.

Olive trees produce fruit best on one-year-old shoots 8” – 12” (20 – 30 cm) long. Moderate pruning stimulates vegetative growth fruitful the following season. Heavy pruning can result in excessive shoot vigor that will be unfruitful.

Most commercially grown olive trees are produced from own-rooted cuttings. Many roots are formed at the cuttings’ base.

The root system extends downward to a maximum of 3 or 4 feet (1 – 1.2 m), even in deep soil. Soil type and irrigation conditions determine lateral root growth, which can extend up to 49 feet (15 m) from old trees.

The youngest roots, white in color, are constantly undergoing renewal and are most active in absorbing water and minerals. Root hairs, which are plentiful and short, increase absorption. Roots turn brown in maturity and have less absorption activity.

Leaves adapt to water loss, making the tree relatively resistant to drought.

Olive leaves are arranged opposite each other, growing at right angles. Leaves take about two weeks to reach final size and can live for a period of up to three years.

The leaf’s pores (known as stomata) are found on the lower leaf surface and control the exchange of oxygen, carbon dioxide and water. The leaf’s upper surface has a shiny, thick cuticle.

The leaf’s size and structure enables the exchange of heat in warmer months. The epidermal outgrowth on undersides of leaves, called trichomes, protect stomata from UV rays. Trichomes alternatively restrict water loss

Flowering and pollination are very critical stages when producing olive crops.

Olive buds are induced to develop flowers initially under stimulus of warmer temperature the previous summer.

Olive flowers are initiated in November, and rapidly differentiate flower parts over two months’ time. Olive flowers are small and can be white or yellow.

An inflorescence refers to a main branch, or arrangement of branches, that have stems producing a grouping of flowers. If flowers remain dormant for longer than a year, this usually results in unviable inflorescence; instead, these older buds form mostly vegetative shoots.

There can be an inflorescenses emerging from each leaf axil on each one-year old shoot, with each inflorescence containing close to 30 flowers. In common California table olive varieties, there are usually 12-20 flowers per infloresense.

Perfect flowers have both stamens (male) and pistils (female). Within a pistil, two carpels contain two ovules, and only one of these ovules is fertilized to develop an olive. Thus, young olive fruit evolve from one developed pistil and explains why an olive fruit only has one seed.

Flowers and organs develop when bloom takes place, from March until late spring (May-June). In southern regions of California, flowers bloom at their fullest in May; in Northern California, bloom can occur weeks later due to weather disparities.

Within four to six weeks of full bloom final fruit set occurs. Large commercial crops occur when one to two percent of flowers set fruit.

Pollen is disseminated by wind between flowers or by simply gravity, i.e. falling from a flower’s anthers to a flower’s stigmas. Successful pollination occurs when pollen grains land on stigmas and germinate. Fruit set occurs when this fertilized flower develops into an olive that stays on the tree until harvest.

Olive fruit set occurs by self- or cross-pollination. Self-pollination occurs when pollen from a cultivar results in fertilization and fruit-set of the same cultivar (also referred to as “self-compatible”). Cross- pollination occurs when pollen from one olive cultivar is capable of fertilization and setting fruit on another cultivar.

Acceptable fruit set can occur through self-pollination for most cultivars under ideal climatic conditions and with proper orchard management. Cross-pollination can enhance fruit set considerably when environmental conditions and orchard management are not optimal.

Possible factors that can reduce fruit set are: 1) low levels of viable pollen, resulting in reduced crop; 2) incompatible cultivars used for cross-pollination, which results in unsuccessful fertilization; 3) improper nitrogen management; 4) hot dry winds that prevent successful fertilization; and 5) lack of proper insect and/or disease management.

Alternate bearing refers to when a tree produces a large crop in one year and a smaller crop in the year following, which is common with fruit trees, including the olive.

A heavy crop results in small fruit size (reducing crop income to table olive growers) and reduces shoot growth (and thus fruit production in the following season.)

Various cultural practices can exacerbate alternate bearing, such as lack of nutrients or water at critical times, leaving a large crop on trees past normal harvest timing and environmental effects (such as harsh weather or pests that diminish tree vigor.)

The longer olives stay on the tree past normal harvest time the more likely that next season’s harvest will be reduced, particularly when there is a big crop and the grower delays harvest to attain better fruit size or oil content.

“Abscission” occurs when a plant discards parts, such as leaves, flowers, seeds and fruit, which are no longer necessary for function or reproduction. Abscission helps plants conserve energy.

Like other types of evergreen trees, olive leaves abscise in spring. A common indicator of olive leaf abscission is leaves that are yellow in color. A tree’s olive leaves are often a combination of current-season leaves and leaves of two previous seasons.

In most olive cultivars, fruit abscission due to non-fertilization occurs closely following full bloom.

Pests, lack of essential nutrients, and water stress induce leaf abscission. Leaf scars occurring as leaves abscise are susceptible to infection by olive knot bacteria and diploidea fungus. Good management is important, as leaves that abscise at inflorescense sites decrease both flower quality and fruit production.

In this section

Check out our best practices, review UC Davis olive publications from 1885 to the present, and access essential UC resources for olive growing and processing.