Organic Weed Management in Orchards and Vineyards

http://www.attra.org/attra-pub/PDF/fruitover.pdf

Tree Fruits: Organic Production Overview

by National Sustainable Agriculture Information Service

excerpt pgs 22-24

Organic Weed Management

Some weed control methods, such as smother crops, are discussed in the Site Preparation section above. This type of cover cropping is an important tool for weed management that also contributes to good soil management, fertility, and pest management.

Mulches

Organic Mulch

Mulching is a powerful weed management strategy that can also contribute to good soil management, if appropriate natural materials are used. After a planting is established, weeds can be suppressed by applying thick layers of mulch. This can also create habitats for beneficial arthopods, including generalist predators such as big-eyed bugs, soft-bodied flower beetles, and spiders. Organic mulches are usually applied in a circle around tree trunks or vines, and down the whole row in blueberries.

Commonly, tree fruit growers keep mulches away from the tree trunks, particularly in winter, to prevent voles or mice from gnawing on the bark and damaging young trees. Keeping mulches 8 to 12 inches away from the trunk also reduces the likelihood of crown rot and other diseases in susceptible species—most notably apples on certain root- stocks.(23)

Mulch materials may include straw, spoiled hay, leaves, yard trimmings, woodchips, and sawdust. Many of these materials are inexpensive. Still, it’s wise to weigh the benefits and risks of each, including hauling costs and the risks of their containing impurities and prohibited materials.

Municipal greenwaste may be available, either raw or from municipal or commercial composting operations. Growers must monitor the incoming product and remove any trash to keep undesirable material out of their fields. Growers should ask compost producers about the sources of their materials and any pesticides that may persist in them. Of particular concern are clopyralid and picloram, herbicides that are extremely resistant to breakdown, even after composting. The sale and use of these materials is restricted in some areas. A Washington State University study showed treated grass clippings to be the primary source of clopyralid entering the organic waste stream. Experience from California, Oregon, and Washington shows that at levels of 1 to 10 parts per billion, clopyralid adversely affects sensitive vegetable crops.(21, 22)

Because organic mulches decompose over time, they require periodic re-applications in order to continue suppressing weeds. However, their decomposition provides other benefits. Mulching with organic matter enhances soil aggregation and water-holding capacity.(4) Researchers from 1937 to the present have consistently found that mulching is the best orchard-floor management system for retaining moisture.(15) In Michigan research, mulching was as effective as irrigation in encouraging tree growth.(24) Organic mulches can have positive effects on tree growth, with improvements in soil quality and shifts toward beneficial nematodes.(27) Mulch can also benefit the crop by moderating soil temperatures, thus reducing plant stress.

Organic mulches provide slow-release nutrients for the long-term health and fertility of the soil. Research indicates that potassium, phosphorus, and nitrogen (primarily from the slow breakdown of the mulch) are more available in mulched systems than in non-mulched systems.(4) Some growers express concern that sawdust may acidify their soil or bind nitrogen in the soil. However, these effects are minimal if the sawdust is not tilled into the soil.

Raising organic matter on the farm is one way to ensure sufficient, clean mulching material. Farm-raised hay grown outside the orchard can provide weed-free mulch. Cover crops may be grown between tree rows, mowed, and gathered around the trees. Some small-scale growers use the biomass from orchard alleyways, cutting cover crops with a sickle-bar mower and hand-raking the material under the trees. Larger-scale operations often use forage wagons, straw-bale spreaders, or specialized equipment to mecha- nize mulching jobs. King Machine Co. (25) offers a small, trailer- or truck-mounted square-bale chopper and blower suitable for most fruit crops. Millcreek Manufacturing Co. (26) has developed a row mulcher especially suitable to blueberry, bramble, and grape culture, but also useful in tree fruit orchards. The Millcreek machines are designed to handle bulk organic materials such as sawdust, wood chips, bark, peat, and compost.

Geotextiles

Geotextile mulches are paper or woven plastic fabrics that suppress weed growth. While they allow some air and water penetration, they may reduce water infiltration, whereas organic mulches increase infiltration.(27) Geotextile mulches do not provide the advantages of adding organic matter and nutrients to the soil, and if synthetic, they must eventually be removed. Geotextiles have a high initial cost, though this may be partially recouped in lower weed-control costs over the material’s expected field-life—5 to 10 years for polyester fabric; 2 to 3 years for paper weed barriers. Still, some growers find them useful for weed suppression in orchards, tree plantations, and cane fruit culture. The ATTRA publication Sustainable Pecan Production provides more detail on the use of geotextiles and outlines additional methods of weed control, including mechanical cultivation, mulches, wood chips, and living mulches.

Sheet Mulch

You can also create weed barriers by sheet mulching: laying down layers of card- board or newspaper and covering them with organic material. Sheet mulching increases the efficacy of organic mulch as a barrier against emerging weeds. Organic growers should avoid cardboard that is waxed or impregnated with fungicide, as well as color print and glossy paper, in order to be compliant with the National Organic Program standards (7 CFR §205.601(b)(2)(i) and 205.601 (c)).

Cultivation

Cultivation—using mechanical tillage and weed harrowing implements—is the most widely used weed-management practice in fruit production. In systems that maintain permanent vegetation between rows, cultivation may be limited to the tree row under the dripline in an orchard, or extended 1 to 3 feet from the edge of the hedgerow in bramble plantings. The reverse is true where mulches are used in the tree row, and cultivation is used to control weeds and incorporate cover crops in the alleyways. In any case, cultivation must be kept shallow to minimize damage to crop roots and to avoid bringing weed seeds to the surface.

Hand cultivation—enhanced with the use of a wheel hoe—can be effective in small-scale plantings. In large-scale plantings of trees or vines, where in-row tillage is desired, “mechanical hoes” such as the Weed Badger (28) or Green Hoe (29) are very useful. These tractor-mounted, PTO-driven cultivators can till right up to the tree or vine without damaging the plant. At- tachment options include powered rotary tillage tools and scraper blades that can move soil either away from or toward the base of the crop plants. Scraper-blade attachments, commonly known as “grape hoes,” have been used in vineyards for decades.

Herbicides Allowed for Use in Organic

Production

A few herbicides currently emerging on the market are allowable for organic production, with restrictions on the location of their use. There is ongoing research on using materials such as vinegar, corn gluten, and citric acid as herbicides, although they are not yet widely used by certified organic growers. Such materials may have applications in organic systems, such as for spot treatment of noxious weeds.

Weeder Geese, Chickens, and Ducks

For many years, farmers have used geese to control weeds in perennial and annual crops, including strawberries, blueberries, bramble fruits, and tree orchards. In Oklahoma, researchers at the Kerr Center for Sustainable Agriculture used weeder geese in commercial-scale blueberry and strawberry production, with portable electric fencing to keep the birds in a specific zone in the plant row.(30) Investigators at Michigan State University studied the impacts of populations of domestic geese and chickens in a non-chemical orchard system. They found that the geese fed heavily on weeds—especially grasses—and also on windfall fruit.(31) In general, geese are more effective against emerging or small grass weeds, and they have a particular preference for Bermuda grass and Johnson grass—weeds that can be especially troublesome in orchards.(32) ATTRA has additional information on weeder geese available on request.

Those who have raised chickens know how enthusiastically they devour fresh vegetation. If the area they inhabit is small, they will strip it to the dirt. Properly managed, however, their foraging characteristics can be used to the grower’s advantage.

Fred Reid is an innovative producer of raspberries and vegetables in Canada who has successfully employed his flocks of chickens in weed management. He uses a system of fencing to keep chickens in certain areas to accomplish a thorough job of weeding and insect control. He notes that if the vegetation has grown too high and the plants become too fibrous, the chickens will not eat them. However, if you mow tall vegetation in advance, the chickens will process it readily. He excludes the chickens from raspberry plots when the new, tender leaves are emerging and, of course, near harvest time.(33)

Flame Weeding

Flame cultivation uses directed heat to kill weeds. It works not by burning the weeds but by searing them and causing the plant cells to rupture. Farmers began using tractor-mounted flamers in orchard and row crops in the 1940s.(34) Tech- nology and technique have both been refined considerably in recent years. Several tools now commercially available, including flame, infrared, and steam weeders, make heat a viable option for some weed management applications. See the ATTRA publication Flame Weeding for more information.

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