Organic weed and nitrogen management for tree fruits

Organically grown tree fruits generally sell for a substantially higher price than conventional produce.

Whether the price premium actually results in greater profitability depends on yields, fruit size, and fruit quality, as well as the cost-effectiveness of the growing practices. Organic production often requires more labor, bulkier fertilizers and amendments, and increased monitoring than conventional.

Far fewer insecticides and fungicides are available in organic pear production, so during high-pressure years (such as for diseases in wet springs), insect pests and diseases can reduce marketable yields. In most years, however, growers find that in-row weed control and providing sufficient nitrogen are the greatest challenges, and the two are linked.

Surveys conducted in Washington have shown that the top three production issues in organic tree fruit production were crop load management, weed control, and soil fertility.

Organic fertilizers

Organic fertilizers tend to have fairly low nitrogen (N) content. The most cost-effective fertilizer is often poultry manure, which generally includes wood shavings and rice hulls. It averages about 3 percent N, and also has phosphorus, potassium, calcium, and magnesium.

Feather meal is sometimes used in Pacific Northwest orchards. It has up to 13 percent N, but is still much more expensive than poultry manure and it has no other nutrients. Compost is an excellent soil amendment, but the N in it is largely unavailable during the first year. To maintain organic certification, uncomposted manures may not be applied within 90 days of harvest.

A portion of the N contained in manure and compost will volatilize into the atmosphere (up to 30 percent) if not disked into the soil. With most orchards in this area being no-till, applied manure will lose N to the air, although less is lost if it is irrigated fairly quickly. Manure and compost also release plant-available N at different rates, which is largely based on the C:N ratio – the lower the ratio, the faster the release. With poultry manure, the majority of the N will be available to plants in the first year; poultry also has the highest volatilization potential. The “decay series” of manures was studied by UC researchers in the 1970s, and the proportion of N availability over a three-year period (years one, two, and three) was shown to be:

– Chicken (.90, .10, .05)

– Dairy (.75, .15, .10)

– Feedlot (.35, .15, .10)

These values may vary widely for any given manure based on many factors. Composts generally fall well below feedlot manure in N availability in year one.

Cover crops often provide the most cost-effective way to add N. A vetch cover crop, often used with peas and/or bell beans, can supply up to 150 pounds N/planted acre. But this mix is usually used in disked orchards. The reseeding annual subterranean clover, with or without other annual clovers or medics, can provide similar amounts of N, but N loss through volatilization can result from leaving the clippings on the soil surface. Perennial clovers have been shown to add large amounts of N also, but they are invasive and they compete with trees for water. All clovers also attract gophers.

Organic in-row weed management

The greatest concentration of tree roots is under the canopy in the tree row, so weeds in the tree row compete with trees for nutrients and water. This competition is especially problematic for young trees, but yields and fruit size of mature trees can also be reduced by in-row weeds, especially warm-season grasses.

Organic weed management practices include mowers and cultivators that move around trunks and sprinklers, organic herbicides, flame or steam weeders, geese or sheep, and organic and synthetic mulches. In-row mowers are generally cost effective, but weeds still grow and compete with trees for nutrients and water. In-row cultivation can effectively control weeds, although tree roots near the surface can be damaged. Some implements are hydraulically driven with a vertical axis cultivating head, whereas others are ground-driven, rolling cultivators that can be used at speeds up to 8 mph.

Growers have found that some implements tend to break down fairly frequently. In a 2002 Yakima Valley orchard study, cultivation-based weed control was more than four times the cost of a typical herbicide program of two applications per season.

Available organic herbicides, mostly based on clove or cinnamon oil, or acetic and citric acids, are effective mainly on very young weeds so they must be applied often, and they have limited efficacy on perennial weeds. Flame weeders are fairly effective on young weeds, but they often require multiple passes for some species, and they use substantial amounts of fuel. Sheep or geese can be very effective; they require fencing and they must be removed 90 days before fruit harvest.

Mulches, either organic or landscape fabric, provide a practical – but expensive – method of preventing or greatly reducing weed growth in tree rows and improving the nutrient and moisture status of trees. Wood chips add organic matter and nutrients, but they are less effective against perennial weeds. A 2004-2006 Washington study showed that the total cost for applying a 5-foot wide, 6-inch thick layer of wood chips was $924/acre. In this study, wood chip mulch provided the best weed control in all three years, although it needed reapplication in year three. This treatment also produced the greatest tree growth and fruit size.

In a related trial, a Gala apple block was used to compare a 4-inch wood chip mulch in the tree row with a herbicide strip. In the first year, mulched plots consistently had 15 percent to 20 percent higher soil moisture at the end of each irrigation cycle than the bare ground plots. In the second year, the two treatments were watered independently according to need, and mulching reduced cumulative irrigation application by 20 percent to 30 percent.

Synthetic fabric allows water penetration but it excludes light to act as an effective barrier to weed growth. A 3 foot to 4 foot width of fabric is placed on either side of the row and they overlap and are pinned where they join, although weeds sometimes grow between the overlapped fabric. The outside edges are buried or pinned. Weed seeds may germinate on top of woven fabric and roots may grow through and establish in the soil, so some growers pull back the fabric each year and apply fertilizer or compost before re-pinning the fabric. Mulches, especially fabric, can also lead to the buildup of voles (meadow mice).

In a five-year study begun in a newly-planted cherry orchard in Hood River, Ore., researchers reported over 30 percent greater tree growth and fruit yield where 6-foot wide, in-row synthetic fabric was used compared to herbicide strip alone. Cumulative cash costs for the first four years before fruit production were $2,123/acre higher with ground cover relative to no cover. However, these costs were offset quickly by the increased returns from enhanced fruit yield and size.


– Weed management in organic pear orchards,

– Organic weed management in walnut orchards,

– Organic Orchard Floor Management – Papers and presentations from the WSU Tree Fruit Research & Extension Center, Click on Organic and Integrated, then on Orchard Floor Management.

TAGS: Management
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