Weed-control tools available for organic orchards
By STEVE BROWN
Though organic orchardists have a more limited toolbox than conventional growers, they still have effective tactics available, researchers say.
The orchard floor performs several functions -- including water intake and storage, physical support, gas exchange for roots and nutrient cycling -- and controlling weeds is critical, said sustainable agriculture specialist David Granatstein, of Washington State University's Center for Sustaining Agriculture and Natural Resources.
Weeds compete with young trees for nutrients and water, provide habitat for rodents and are hosts for pests and disease inoculum.
Orchardists can employ various options, often in combination: mechanical, thermal, mulch/weed fabric, cover crops, biocontrol using insects and pathogens and soil conditions.
Tillage is the most common practice in many regions, but it costs in terms of lost organic matter, less tree vigor, lower fruit yield, smaller fruit and trees falling over from root damage.
One option is flaming, which is comparatively inexpensive at $70 to $80 an acre for four passes in a season. The only problem is it kills only above the surface of the ground, Granatstein said during a webinar.
Weed fabric is effective, though it is initially expensive. In an Oregon State University test orchard in Hood River, Ore., initial costs were $2,125 per acre more than bare ground. Gross returns the first year of production were $3,240 per acre more than bare ground, but the trees produced at an earlier age and maintained higher yields.
The fabric must be removed at the end of its life before it begins to fall apart, but some fabrics last 10 or more years.
One downside is that the fabric creates "wonderful habitat" for voles and protects them from predators. Granatstein said one grower told him, "Weeds don't kill trees, but voles do."
By using two pieces of fabric on either side of the trees, a grower can fold back the fabric in winter to expose the habitat and disrupt the voles. Come spring, the grower can put down compost or other soil amendments and cover the area.
Wood chip mulch is the least attractive to voles, and it contributes to increased fruit size and tree growth.
"It's pretty good weed suppression for one to three years, but it's not recommended as a stand-alone practice," he said. Monitoring of one apple orchard put the cost of three years of mulch at $1,429 per acre; after three years, the net value of that investment was $3,000 more.
Voles are a big challenge in Washington, Granatstein said, and researchers have tried different living mulches to evaluate their effectiveness. Planting Galium odoratum -- sweet woodruff -- resulted in a significant reduction in vole pressure.
Using a living mulch is tricky because most vegetation increases voles. Some growers have found sweet alyssum beneficial as part of a mix. White clover can improve the soil, but voles have an appetite for it.
Using plastic mulch can have a negative effect in a hot environment, elevating temperatures up to 10 degrees Celsius more than bare ground. Some growers deal with this by opening up the plastic in the summer, he said.