Chemical Weed Control
Unlike in most other crops, weed control materials are applied in orchards as directed sprays, in bands under trees, onto undisturbed soils. At the time of application, there are usually mixtures of perennial and annual weeds present in various stages of development. The weeds that are growing are usually killed or injured with a tank mix of glyphosate or other contact herbicide, and the residual materials in the mix are expected to prevent significant weed growth stemming from germinating seeds for as long as possible before the grower must re-apply herbicides. Zero, or very few weeds present in the band treated under the tree row for an entire growing season is not a reasonable expectation. However, proper planning and careful product choices and rates can greatly reduce the number of necessary trips through the orchard.
There are many reasons that weed control is variable. Prior to product application, the existing weed growth may be tall enough to cause uneven application, leading to poor performance of both contact and residual materials. Uneven applications of residual herbicides due to poor set up or operation of weed booms will lead to uneven weed control. Poor choices of tank mix residual herbicides may allow weed resistant to both of the products to return rapidly. In other cases, well-established perennial weeds that would normally be controlled when sprayed in summer through fall may have very limited above-surface foliage in late winter or early spring due to dormancy, and may not be controlled by the springtime application of the tank mix glyphosate or other contact herbicide. This leads to heavy growth of uncontrolled carry-over perennial weeds in the spring and early summer, and the apparent failure of the residual herbicide control, even though the residual herbicides may be preventing the growth of new seedling annual and perennial weeds.
Though weeds compete with orchard trees for water and mineral nutrients, and greatly aid the build-up of voles, the blocking of sprinklers and subsequent loss of irrigation efficiency should be considered the most important problem caused by orchard weeds. Recent changes in orchard irrigation technology has made this factor even more important, as micro sprinklers are generally lower and less resistant to weed interference than older style sprinkler heads.
Most residual weed management programs include at least two products, each of which covers the weakness of the other. Even then, weed materials should be rotated through the seasons to avoid the build-up of specific resistant species in the orchard. Nature has a way of filling open space.