Weeds are considered by many growers to be the number one problem in organic blueberry culture. It is especially important to control aggressive perennial weeds such as johnsongrass, bermudagrass, and quackgrass prior to crop establishment. Sites with these grasses should generally be avoided for blueberry establishment. Details of pre-plant and post-plant weed management for all fruit plants are provided in ATTRA's Overview of Organic Fruit Production. Some techniques, however, deserve additional elaboration.
In much of the country, blueberries are grown on mulched, raised beds. Rabbiteyes and old highbush plantings are commonly grown without mulch. Raised beds reduce the incidence of soil- and water-borne diseases. Thick organic mulches provide weed and disease suppression, soil temperature regulation, slow-release nutrients, organic matter, and moisture conservation. The latter is especially important because blueberry roots lack root hairs — the primary sites for water and mineral absorption on most plants. This characteristic makes water management of paramount concern and goes a long way toward explaining why irrigation and mulching are recommended practices.
The importance of maintaining a weed-free zone around blueberries was demonstrated in a Georgia study(NeSmith et al., 1995) using rabbiteye blueberries—which have a more vigorous root system than highbush. Researchers determined that an optimum vegetation-free zone during the first two to three years of growth extends roughly 1.5 to 2.5 feet from the plant. This translates to a 3- to 5-foot-wide, weed-free row bed.
Current recommendations suggest mulching a 3- to 4-foot-wide strip under the plants with 3 to 5 inches of sawdust, bark, wood chips, or wood shavings. Organic growers often prefer a deeper mulch of up to 6 inches over a strip at least 4 feet wide. Ideally, the mulch should be sufficiently coarse to minimize crusting, and the surface relatively flat to encourage water penetration and gas exchange.
While the mulch suppresses many weeds, the moist organic medium can also become a haven for annual weeds (annual ryegrass, stinging nettle, crabgrass) as well as perennial weeds (dandelion, horsetail, sheep sorrel) that find a niche in perennial plantings. Strategic attention to weed control, even in mulched fields, is a major cultural consideration. Tractor-drawn cultivation implements are impractical for in-row weed control on deep-mulched blueberries because blueberry roots often grow into the mulch, and significant plant damage can result from tillage. Shallow hoeing or hand-pulling weeds are two traditional options practiced by many organic growers.
Weeder geese can also eliminate most of the grass and many of the tender broadleaf weeds from a planting. They are prone to eating ripe fruit, however, and may damage some of the newly emerging canes, so their use should be timed accordingly. Obviously, goose stocking rates are much lower, and management easier, on clean cultivated plantings. Investigators at the Kerr Center for Sustainable Agriculture have used weeder geese for effective weed control in blueberries with sodded middles. The Center's strategy involves using movable electric fencing and intensive grazing. One possible drawback cited by Kerr Center researchers is the tendency of geese to compact the soil and mulch. ATTRA can supply further information on weeding with geese.
A promising alternative to organic mulching is the use of fabric weed barriers. While fabric mulches may not provide all the benefits of deep organic mulch, they are highly effective for weed control and allow water to pass through.
And, though the initial cost is high, it may prove reasonable when amortized over the fabric's expected lifetime of 10 to 12 years. All fabric mulches must be removed, however, before they deteriorate and decompose into the soil. Have a plan in place to deal with this eventuality. Available fabric mulches include Sunbelt by DeWitt Company (see References).
Non-porous black plastic mulches — commonly used in vegetable production—are not recommended for blueberries. Polyethylene plastic mulch encourages surface rooting—making the plants more susceptible to drought stress and winter injury—and the plastic does not allow water to pass through.
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