Newly Planted Orchards: Weed Management

Managing unwanted vegetation is difficult in newly established orchards. Weeds grow fast due to the amount of water, fertilizer and sunlight that is available. Temperatures in the spring and summer often favor weed growth, which can quickly overgrow trees, reducing nutrient and water availability. Furthermore, control is difficult due to the sensitivity of the tree to herbicide damage. These factors make weed control one of the most challenging aspect when developing a new orchard.

A young almond tree with glyphosate drift. Please note the distorted new growth. Trees not killed by drift will take 4-6 weeks to recover and begin to grow.

Most operations rely on post-emergent (aka burn-down) materials for control of weeds in newly planted trees. These products kill the plants that they contact by a number of mechanisms. Burn-down materials, however, have risk in use as drift can damage or kill young trees. This is especially true with glyphosate (e.g. Round-up) which is a systemic herbicide that can persist for a long time within a plant. To reduce this risk, glyphosate should be avoided, and operations should rely on contact materials (e.g. paraquat, oxyfluorfen) for the first growing season.

Contact post-emergent products aren’t as effective assystemic products. These materials will require multiple applications throughthe year as they only kill the top part of the plant. Usually within 2-3 weeks,the plant begins to regrow, and follow-up sprays will be needed. Further managementchallenges exist for weeds that are “hairy” or have a waxy cuticle. Thesefeatures prevent the full contact of the herbicide with the plant surface,reducing kill.

Drift from the contact herbicide paraquat on almond. Damage is localized and trees generally recover within a few days.

To manage these variations within the population, coverageis critical. Utilize the upper limit of the label rate of the primary herbicideand make use of any water conditioners to improve efficacy. Increase the volumeof water to provide more thorough coverage. This often means that applicationsare in the 30-50 gal/acre range. Check and replace worn nozzles and perform therequired maintenance on the spray-rig to increase coverage and decrease drift. Finally,increase the amount of non-ionic surfactant to 0.5% as these products helpbreak water tension.

Pre-emergent products are often overlooked when establishingorchards. If planting has occurred early enough to take advantage of springrains, consider an application to reduce weed pressure. A very common program afterplanting is the use of oxyfluorfen and pendimethlan. This program is effectivein controlling broadleaves and grasses.  Thereare, however, several pre-emergent herbicides that can be used after plantingwith minimal risk. Most of these require 0.5-0.75 inches of rain to “set” andbecome effective. Burn-down partners and surfactants are also required to manageany germinated weeds.

When deciding on an herbicide, identify the weed spectrum present. In my experience, the weed spectrum usually contains grasses and broadleaves.  Tank mixes of multiple herbicides are usually needed to provide a thorough kill. Typically, the high rate of both herbicides is not needed. I often find myself applying the high rate of the product with the broadest spectrum or the chemistry which is targeting the most problematic weed, while adding the low rate of the product that is controlling the species missed by the first product. Resources identifying weed susceptibility and rates can be found at the UC IPM Website.

After the first season, weed management becomes easier. Dueto the reduce impact of potential drift, more burn-down products becomeavailable. Also, nearly all the pre-emergent products can used, depending onlocal soil and label conditions. Strongly consider the use of some type ofpre-emergent herbicide applied during the dormant period. Utilizing the right combinationof these products will increase weed control, allowing more water andfertilizer to go to the trees and decreasing the frequency of burn-downapplications.

More information can be found at the UC IPM Website or within the “New Orchard Handbook.”

Mentioning of any active ingredients is not an endorsement or recommendation. All chemicals must be applied following the chemical label. Please check with your pest control adviser to confirm rates and site-specific restrictions.