Weed management in olive orchards should reduce the negative impact of weeds on trees, prevent buildup of hard to control weeds, and reduce plant debris at the base of the tree to facilitate harvest. Good management improves the growth and yield of both young and established trees. If not properly managed, weeds can create several problems. They can compete with trees, especially young ones, for water, nutrients, and even sunlight. Orchard productivity may be lowered and young orchards may take longer to come into production. Weeds can also enhance the activities of other pests such as insects, mites, nematodes, and diseases, and create a fire hazard when they dry up in the summer.

Growers have many weed management tools available to achieve control objectives; the best strategy for employing these tools, however, will vary from year to year and from orchard to orchard, according to local conditions.

We recommend the book from Ferguson and Sibbett as well as the book from Paul Vossen, both listed in our Sources below. We also list several UC Integrated Pest Management sources. Keep in mind that you must be certified to apply herbicides if you have a commercial operation. (You can apply non-restricted herbicides in your backyard without a permit). See the California Department of Pesticide Regulation for more information.

GENERAL GUIDELINES

Management differs for each orchard and depends on the weed species, soil type, irrigation method, amount of control desired, terrain, and the appearance of the orchard. For example, winter annuals are least troublesome because there is generally enough moisture during winter to support both the tree and the weed. They can be managed in the spring. Summer annuals, biennials, and perennials require stricter management. Perennials in particular should be eliminated completely.

Weed management starts before planting the orchard. See our Soil Preparation page.

It is also critical around young trees. Competition is most severe during the first 5 years of the tree's life or where root growth is limited. Weeds around the tree trunk not only compete directly with tree growth, but also provide a good habitat for field mice or voles, which can girdle and kill young trees. Gophers are most prevalent in non-tilled orchards and are common where broadleaf weeds, such as field bindweed and perennial clovers, predominate. They feed on the roots and weaken or kill young trees.

After about the fourth year, the effect of competition from weeds is somewhat lessened as trees become established and shading from the orchard canopy reduces weed growth. In older orchards, however, weeds result in colder orchard conditions, increased frost hazards, and the potential for olive knot. Weeds also increase humidity, making trees more susceptible to infection by the peacock spot fungus. In addition, weed growth can interfere with cultural practices and harvest. For example, weeds can disrupt the application pattern of water from sprinklers and low-volume spray emitters. Olive trees are shallow rooted and frequent cultivation near trees can injure tree trunks and promote suckering. Tree trunk injuries can result in crown gall or olive knot infections.

In established orchards, there are many options for weed control, including discing and mowing between rows with hand weeding around trees; discing and mowing between rows with a basal square or circle herbicide application around the base of the trees; strip treatment with herbicide down the tree row; and total reliance on herbicide.

Total reliance on herbicides has several disadvantages (besides the obvious impact on the environment). No single herbicide controls all annuals. Combinations of herbicides, sequential treatments, or pre-emergence plus post-emergence combinations are needed to maintain a weed free orchard. Soil erosion can be a problem on slopes if there is no ground cover. In some soils, compaction and development of a thin, silty surface layer, which impedes water infiltration, may become a problem. Pre-emergence herbicides do not control established perennials. These weeds spread rapidly in the absence of annuals. Some pre-emergence herbicides control only certain groups of plants, leaving the tolerant ones to propagate. An integrated program thus works best.

Soil characteristics are important to weed management. Soil texture and/or organic matter influence which weed species are present, the number and timing of cultivations required, and the activity and residual effects of herbicides. The irrigation method, terrain, amount of water applied, and pattern of rainfall also affect the frequency and timing of cultivation as well as the selection of chemicals and their residual activities.

MONITORING

Many different species of summer and winter annual and perennial weeds are found infesting California olive orchards. Weeds vary from area to area and year to year, even within orchards, so surveys should be conducted at least twice each year: once in late winter and again in late spring or summer to determine the spectrum of weeds present. Some experts recommend surveying three times a year: in November, February, and May.

These surveys are the basis for weed management decisions about herbicide choice or cultivation equipment and practices. Written records of survey results noting date and species observed in different parts of the orchard should be kept. Looking at historical records can be a very valuable tool.

MULCHES AND COVER CROPS

Mulches

Weeds in the tree row can be controlled using mulches. The mulch blocks light, preventing weed germination or growth. Different mulches vary in the depth necessary to block all light to the weeds. Organic mulches (cereal straw, green waste, composted wood chips, sawdust, newspaper) must be maintained in a layer 4” (10cm) or more thick. Shredded tree prunings make good mulch as well. It is also possible to use synthetic mulches of polyethylene, polypropylene, or polyester.

Mulches do not control perennial weed growth unless all light can be excluded. Some woven fabric mulches offer excellent weed control for several years, but the initial cost of purchase and installation is high.

Because organic mulches degrade, they must be replenished annually. As mulches degrade they become a perfect growth medium for weed species such as common groundsel, prickly lettuce, common sowthistle, and panicle-leaf willowherb. Always apply mulches when the soil surface is free of weeds.

In addition to weed control, there are several additional benefits to mulches. They create more uniform moisture conditions and conserve water, which in turn promotes tree growth. Soil temperature is better maintained and organic material is added to the soil on breakdown. However, mulches may also provide a good habitat for gophers, voles, field mice, and snakes or be a source of new weed seed that came with the mulch.

Cover Crops

Cover crops are planted in some orchards to replace the resident weed vegetation on the orchard floor. With cover crops, the species selected and management will differ from one orchard to the next. Be sure to select a cover crop such as fall-seeded cereal crops (wheat, oat, cereal rye, or barley), Blando bromegrass, Zorro fescue, rose clover, or subterranean clovers that will not compete with the trees.

The cover crops are seeded into a prepared seedbed between tree rows in late September through mid-November. Most plants will reseed themselves if mowed in January or early February and then allowed to re-grow into April and May. Mowing after the seeds mature ensures seeds for the next season.

Avoid invasive plants such as white clover and bermudagrass in a ground cover. Sometimes larger-seeded cover crops such as bell bean, purple or common vetch, or crimson clover are planted in orchards and tilled in as green manure. Perennial grasses (tall fescue, Berber orchardgrass, or perennial ryegrass) may also be grown but will require summer irrigation and may compete with tree growth.

Keep cover crops away from the trees. Changing cover crop species reduces the potential for buildup of disease pathogens, weeds, rodents, and insect pests. Cover crops can be planted between the tree rows, and in the spring a “mow-and-throw” mower can be used to cut the cover crops and throw them in the tree rows. This works well if the mulch layer is thick.

For more information on cover crops, consult UC ANR Publication 21471, Cover Crops for California Agriculture or UC ANR Publication 3338, Cover Cropping in Vineyards: A Grower's Handbook.

HERBICIDES

When properly used, herbicides registered for use in olive orchards can control most weed species. In many orchards, combinations and/or sequential applications of herbicides are required to provide effective, economical control. Before using any herbicide, identify the weed species to be controlled, then read and follow product label directions carefully.

Herbicides are traditionally discussed as two groups: those that are active against germinating weed seeds (pre-emergence herbicides) and those that are active on growing plants (post-emergence herbicides). Some herbicides have both pre- and post-emergence activity. Herbicides vary in their ability to control different weed species. Check the UC IPM Olive Susceptibility of Weeds to Herbicide Control tables, and consult product labels for specific weed control activity.

Pre-emergence Herbicides

Pre-emergence herbicides are applied to bare soil and are leached into the soil with rain or irrigation where they are active against germinating weed seeds. They must be moved by water (rainfall or irrigation) into the top 1 to 3” (2.5 to 7.6cm) of soil where weed seeds germinate. If herbicides remain on the soil surface without incorporation, some will degrade rapidly from sunlight. Weeds that emerge while the herbicide is on the surface, before it is activated by rain or irrigation, will not be controlled. Large weed seeds, such as wild oat, may germinate in the soil below the herbicide zone and still be able to emerge.

Some pre-emergence herbicides must be incorporated within a week by water to be effective, while some can wait longer. Some cannot be incorporated mechanically without reducing their effectiveness.

Pre-emergence herbicides can control weed emergence from several weeks up to a year, depending on annual rainfall, solubility of the material, soil properties, frequency and method of irrigation, weed species, and dosage applied. Prolonged moist conditions around low-volume emitters promote the breakdown and leaching of herbicides. Splitting a pre-emergence herbicide into two applications (with the same total dosage) can prolong control, particularly in areas with heavy rainfall, on sandy soils, in early fall treatments, or in orchards with a heavy growth of summer annuals. Split applications can be made by using one half to two thirds of the total required dosage in the fall and the remainder the following spring.

Pre-emergence herbicides are more phytotoxic to plants in sandy soils and in soils low in organic matter than in soils high in clay and organic matter. They leach from the surface more rapidly in sandy soils than in clay soils, which may allow some weeds to germinate above the herbicide. In orchards with sandy soils, therefore, split treatments are safer for the trees and provide longer control.

Because pre-emergence herbicides can persist in the soil for a few months to a year, their use should be discontinued 1 to 2 years before removing the orchard if the soil is to be replanted. Where a tree must be replaced, untreated soil should be backfilled around the root of the new tree to avoid damaging the tree itself.

Post-emergence Herbicides

Post-emergence herbicides are applied to control weeds already growing in the orchard. They may be contact herbicides or translocated (systemic) herbicides.

Contact herbicides kill only the parts of the plants that are actually sprayed; good coverage and wetting are therefore essential. A single spray kills susceptible annuals; retreatment is necessary if regenerating perennials are present or if annuals re-establish themselves from seed. Contact herbicides are most effective on young weeds where it is easier to get good coverage and less material is needed.

Translocated herbicides are transported from the sprayed part to the rest of the plant, including its roots, growing points, and storage structures. Thorough coverage is therefore not as important as with contact herbicides. Translocated herbicides are effective on both young and old weeds. No herbicide is effective on old, dusty mature weeds.

Because different herbicides work in different ways and on different weeds, they are sometimes combined. No single herbicide works on all weeds. In many instances, combinations or sequential applications of different herbicides provide better control than one product alone.

Treatment Options in Young Orchards

Weed management is critical around young trees where weeds compete for nutrients, water, and light. Weedy orchards may take 1 to 2 years longer than those that are weed-free to become economically productive. (From an economic standpoint, however, it is important to compare the costs of weed management with the benefits of earlier production.)

To control weeds after trees are planted and before bearing, apply a pre-emergence herbicide to either a square or circle around each tree at least 4–6’ (1.2 to 1.8m) across, or as a band down the tree row. Selective post-emergence herbicides are available for the control of most annual and perennial grasses. Young trees need to be protected from contact by some post-emergence sprays.

It is best to mow and/or cultivate (see below) in conjunction with the use of herbicides. Mow weeds when they get 6 to 8” (15 to 20cm) high, usually about four to eight times a year. Cultivation is required when weed seeds germinate after irrigation.

Treatment Options in Established Orchards

It takes 3 to 4 years for an orchard to become established under normal growing conditions. Established trees are more tolerant of many herbicides than newly planted trees, thus increasing the options available for weed control. Generally weeds are controlled between tree rows by discing or mowing, and a basal treatment of herbicide is applied around each tree or in a strip application down the tree row. For a detailed discussion of orchard floor management, see UC ANR Publication 8202, Reducing Runoff from Irrigated Lands: Orchard Floor Management Practices to Reduce Erosion and Protect Water Quality.

Pre-emergence herbicides can be applied either alone, in combinations of herbicides in fall after harvest, split into two applications as described above, or in winter with a post-emergence herbicide. It may be most beneficial to delay the pre-emergence application in winter until most weeds have germinated, and then to add a post-emergence herbicide. This allows longer weed control into the summer yet does not allow much competition from weeds to the tree. For greatest safety, herbicide sprays should be directed only at the soil or at the weed foliage, not at the tree leaves or 1- to 2-year-old wood. In orchards where tree rows are mulched or sprayed, there are often few weeds to treat, and a sprayer with a weed detection system can be used to reduce herbicide use.

For treatment of small areas, especially for perennial weeds, a backpack sprayer or low-volume controlled droplet applicator can be used. Extreme care needs to be exercised to avoid drift of herbicides to tree leaves or green stems.

Frequent wetting of the soil promotes more rapid herbicide degradation in the soil. Herbicide degradation is generally faster in moist, warm soils than in dry, cold soils. Degradation is also more rapid under drip emitters or micro-sprinklers than under furrow or sprinkler irrigation. The first irrigation following an herbicide application is the most critical in terms of how far the pre-emergence herbicide is moved into the soil; subsequent irrigation is less important to the movement of the herbicide. The optimum amount of water for herbicide activity is from 0.5 to 1” (13 to 25mm). Greater amounts of water (3 to 6”, 76 to 152mm) could move the herbicide far enough into the soil, especially in sandy areas, that it is absorbed by the tree's roots.

For specific pesticide application recommendations, contact your Farm Advisor, Pest Control Advisor, or County Agricultural Commissioner. You can also find information on the UC IPM Olive Herbicide Treatment Table. Don’t forget to check certification requirements. Always read and follow the entire product label before using any pesticide.

OTHER METHODS

Cultivation

Some growers prefer to manage weeds without herbicides for the first year or two after planting. This usually requires hoeing, cultivating, or using weed knives (less than 2 inches deep) around trees several times during spring and summer as well as cultivating or mowing between tree rows. This is best accomplished when weeds are still in the seedling stage; it becomes more difficult when weeds are allowed to get large. Hand tools are generally used close to the tree to minimize injury from mechanical cultivators, particularly when the trees are young. Mechanical control of weeds must be done repeatedly when weeds are immature. The equipment should be set to cut shallowly, to minimize damage to tree roots. As weeds mature, they are difficult to control, may clog equipment, and produce seed. When using any mechanical equipment around trees, be careful not to injure the feeder roots or trunk.

Cultivation can be used in established orchards to control annual and biennial weeds and seedlings of perennial weeds. It is often limited to between the rows because irrigation lines or berms preclude cross-tillage. Seedlings of field bindweed, bermudagrass, and johnsongrass should be controlled before they are 3 weeks old or they may form perennial structures such as rhizomes. Cultivating established perennials in an irrigated orchard often increases the weed problem. Cultivation also cuts and damages the roots of trees, reducing the ability of the tree to take up nutrients and allowing access to the tree by soil pathogens.

According to an Oregon study (Scopel et al. 1994), night tillage may reduce weed germination. Many weeds require a flash of red light (of microseconds) to germinate and it is thought that the weed seeds get this flash when they are suspended in soil during tillage. The study shows a 4- to 5-fold enhancement of weed germination when tilling is done during the day versus at night.

Flame Weeding (Flaming)

Flaming is a method that can be used to control very young weed seedlings. Propane fueled flamers are the most common. Heat causes the cell sap of plants to expand, rupturing the cell walls. This process happens in most plant tissue at 130°F (54.5°C). Flaming is not intended to burn the weeds, but rather to kill the tiny seedling with heat. Properly flamed weeds should have a matt finish on the leaves and pressing your thumb and forefinger together on a leaf should leave a fingerprint.

Weeds must have fewer than two true leaves for best burning efficiency. Grasses are harder to kill by flaming because the burning point is below the ground. Typically, flaming can be done at 3 to 5 miles/hour (5 to 8 km/hour) through orchards, although this depends on the heat output of the unit being used. Repeated flaming can be used to suppress perennial weeds such as field bindweed.

The specific flaming angle, flaming pattern, and flame length varies with the manufacturer’s recommendation. They vary from 30° to 40° at 8 to 12” (21 to 31cm) above the base of the plants, with flame length of about 12 to 15” (31 to 38cm). Best results are obtained in windless conditions, in the early morning or evening.

Do not use flaming around young trees because it may damage the thin, green bark. Adjust equipment speed for desired weed injury without damaging the tree trunks. Never use flaming where there is dry, dead vegetation, leaves, or duff around the base of the tree. This material may ignite, causing a fire that girdles the trees. Flaming may also damage or ignite mulches in the orchard. This method has varying degrees of success depending on the weed species.

Another similar alternative for non-chemical weed control is based on hot steam. High-temperature water provides a form of thermal weed control and eliminates the danger of flame application in dry and arid conditions where open fires are a hazard. Superheated water is delivered from a boom or spray nozzle attached to a diesel fired boiler. The high pressure and hot water damages the cellular structure and kills weeds within several hours or a few days. The first signs of effectiveness are a change of leaf color and plant withering. In general, steam has been shown to be less effective than flaming. Even with temperatures of 842°F (450°C), the steam is not able to control all weeds.

Animals

Weeder geese have been used for years for weed management for various crops and sometimes in orchards. All types of geese will graze weeds. Geese prefer grass species and will eat other crops only after all grasses are gone. They appear to have a particular preference for johnsongrass and bermudagrass, two weeds that can be particularly troublesome in some orchards. They will even dig out most of the stolons and rhizomes. Caution must be exercised not to place them close to grass crops, such as corn, sorghum, or small grains, which they would probably consider as a delicacy!

Sheep and miniature sheep will eat almost all weeds down to ground level, which reduces weed competition, but does not eliminate it. Goats are browsers, so they must be carefully managed to protect the trees.

The animals need water, protection from the heat during hot days, and protection from various predators. Movable fencing works well to keep them where they should be.

A FEW SPECIAL WEEDS

• Bermudagrass (Cynodon Dactylon): this is a vigorous, warm season perennial. It expands rapidly with an extensive system of rhizomes and stolons, which can form new plants after being cut by cultivation. Seeds also help bermudagrass expand to new locations.

• Johnsongrass (Sorghum Halapense): this is also a warm season perennial that spreads from seeds or from rhizomes. It can overtop small trees and is highly competitive for light, moisture, and nutrients.

• Yellow Nutsedge (Cyperus Esculentus): this is a perennial weed that reproduces from underground tubers. Viable seeds are rarely produced. A single plant can produce hundreds of new tubers in one year, with tubers able to survive for 2 to 5 years in the soil. Each tuber contains several buds, with each bud capable of producing a plant. Generally only a single bud grows from a tuber, but if a tuber is damaged by cultivation, a new bud is activated. Repeated cultivation at 3-week intervals destroys successive flushes of nutsedge and eventually kills the tuber.

• Field Bindweed (Convolvulus Arvensis): this is a vigorous perennial weed. It grows from seeds as well as from rhizomes and extensive, deep roots. It has been known to survive over 30 years. Because of the longevity of the seed in the soil it is imperative to destroy the plants before they can produce seeds. The plants may spread from stem or root sections that are cut during cultivations, but cultivation controls seedlings. Repeated cultivation at 2- to 3-week intervals depletes the carbohydrates in the root system and eventually kills the weed. Longer periods between cultivation would allow the energy reserves to replenish.

SOURCES

G. Steven Sibbett and Louise Ferguson: Olive Production Manual, University of California.

Paul M. Vossen: Organic Olive Production Manual, University of California.

Paul M. Vossen: Olive Weed Control

Paul M. Vossen and Chuck Ingals (Rewritten by Paul Vossen): Orchard Floor Management

Paul M. Vossen: Fruit Tree Weed Control

UC IPM Olive Weed Photo Gallery with Common and Scientific Names

UC IPM Olive Integrated Weed Management

UC IPM Olive Susceptibility of Weeds to Herbicide Control

UC IPM Olive Special Weed Problems

UC IPM Olive Herbicide Treatment Table

UC IPM Olive Weed Management in Organic Orchards

UC IPM Pest Management Guidelines