Weed control in organic farming systems combines many techniques to reduce weed pressure by preventing weed seed germination and propagule development. In addition to controlling weeds, a weed management plan in organic orchards should minimize erosion, enable farm equipment movement, and not interfere with IPM, water utilization, or soil fertility.
The first step in developing any weed management program is to identify the weeds infesting the orchard or planting site. Familiarity with each weed's growth and reproductive habits is crucial in order to choose the most effective management strategy. See the weed photos linked to the weeds in the list of COMMON AND SCIENTIFIC NAMES OF WEEDS.
Continuous monitoring and weed management throughout the year will help reduce weed pressure in organic olive orchards.
Transitioning an established orchard into organic production will allow the use of conventional herbicides to eliminate as many weeds as possible during the first years of a new orchard. This will help to reduce long-term weed control costs and should not impact first year production as organically certified. Full-canopied trees will require less intensive weed management than starting a new organic orchard. Mature, shady orchards often have limited weed growth, whereas weed pressure in newly planted orchards is more intense, because weeds under full sunlight can more effectively compete for water and nutrients with the trees.
Weed Management After Planting
Mulching with organic materials such as municipal yard waste, wood chips, straw, hay, sawdust, and newspaper, or solid film or woven plastics (although there are disposal problems associated with them) reduces weed pressure in organic orchards. For effective weed control, layer mulches 4 inches (10 cm) thick and replace to maintain thickness, since the thickness reduces by 60% after 1 year.
Cover crops can be grown that compete with weeds or grown and cut with a specialized mower that blows the cuttings into the tree rows to form a mulch. Again, this works well if the mulch layer is thick. Weeds that do emerge though the mulch can then be controlled using an organic contact herbicide or hand weeding.
Sprouting perennial weeds can penetrate organic mulches. Black plastic mulches generally stop most herbaceous perennials, except for nutsedge (Cyperus spp.) and blackberry (Rubus spp.). Bermudagrass will often grow out from under the edges of plastic mulch, forming runners that grow over and then root through the plastic.
Organically Acceptable Herbicides
Organic herbicides such as clove oil work on contact, therefore spray weeds until wet and use an organically acceptable surfactant to ensure good coverage. Spray volumes of 60 or more gallons per acre are needed for effective coverage. Organic herbicides have no residual activity so several applications may be necessary. Organic herbicides can damage leaves and young stems of olive trees but are safe for woody stems and trunks.
Organic herbicides do not control established perennial weeds, are weak on annual grasses, and may not be cost-effective for commercial production. Check with your organic certifier before use, since not all alternative herbicides are approved by all agencies.
Most weed control in organic systems is done using cultivation to uproot or bury weeds. Weed burial works best for small weeds. Slicing, cutting, or turning the soil to separate the root system from the soil controls larger weeds. Cultivation is effective for summer and winter annual weeds, but it is not as effective against summer perennial weeds like johnsongrass, nutsedge, bermudagrass, and field bindweed.
Close or deep cultivation can injure the tree's roots and crown. Avoid deep cultivation, which brings weed seeds to the surface where they can germinate.
Night tillage can reduce weed germination by avoiding weed seed exposure to light, which is needed for germination of some weed species. After night tillage, only the seeds left on the surface of the soil will germinate. Because the number of seeds on the soil surface can be quite large, several tillage passes may be needed to reduce weed germination. Night tillage works well for many winter annuals and most summer annual weeds like pigweed (Amaranthus spp.), lambsquarters (Chenopodium album), and barnyardgrass (Echinochloa crus-galli).
Several types of strip cultivators can be used to keep the tree row weed free, while allowing the cover crop or resident vegetation to remain between the rows.
Animal Weed Control
Some animals can be used for selective grazing of weeds in olive orchards.
Weeder geese prefer grasses and will eat other weeds and crops only after the grasses are gone. They appear to prefer the troublesome bermudagrass and johnsongrass and will even dig up and eat rhizomes if confined. Avoid placement of geese near grass crops such as corn, sorghum, and small grains. Geese also require water for drinking, shade during hot weather, and protection from dogs and other predators. Portable fencing can keep them in areas you want them to work on while keeping predators out. Young geese weed best. Geese can be messy and noisy.
Other grazing and browsing animals such as sheep and goats can be used to reduce weed competition by lowering the height of the weeds in olive orchards. Browsing animals such as goats must be managed to avoid damage to trees.
Food safety regulations require the removal of animals 120 days prior to harvest.
Propane-fueled models are most common for use in flame weeding. Weeds must have fewer than two true leaves for greatest efficiency. Grasses are harder to kill by flaming because the growing point is below ground. Foliage that retains a thumbprint when pressure is applied between thumb and finger has been adequately flamed.
The specific flaming angle, flaming pattern, and flame length vary with the manufacturer's recommendations. Typically, flaming can be done at 3 to 5 miles per hour (5 to 8 km per hour) through orchards, although this depends on the heat output of the unit being used. Avoid igniting dry vegetation, which could injure trees or start a fire. Flame weed in windless conditions to allow the flame's heat to reach the target. Observe the flame for adjustment in the early morning or evening.
With flame weeding, species and growth stage are most important for successful control of weeds. Repeated flaming can be used to suppress perennial weeds such as field bindweed. Cheeseweed (Malva parviflora) is resistant to flaming. For lambsquarters, three subsequent treatments are necessary for 95% control. Small dandelion (Taraxacum officinalis) plants are killed with one flaming, while bigger plants can survive four. Three passes achieve almost full control with annual weeds. Flame-tolerant or perennial weeds require several passes and complete reduction is unlikely (4 passes results in 75% control).
Hot-steam weeding is another alternative and eliminates the danger of flame application. Superheated water is delivered from a boom or spray nozzle attached to a diesel-fired boiler. After treatment the leaves will change color and the plant will wither. Steam is less effective than flaming and does not control all weeds.
Limitations on the use of flame and steam weeding are irregular weed control, low driving speed, repeated applications, and high energy consumption.