Weed Management in Young Almond Orchards

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Posted on March 2 2022 by Franz Niederholzer

Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

Successful new plantings are key to the economic future of an almond growing operation. Development costs and income expectations are high. Any setbacks in new, non-bearing orchard growth or health can be costly, distracting and stressful.

Weed management is a major management focus and potential stress point in young orchard development. Driven by full sunlight and regular water and nutrient deliveries, rapid weed growth is often much more of a concern in non-bearing blocks than foliar/nut diseases and insects. The journey to a successful, bearing orchard can be rough if weeds get out of control. The following are some considerations to contribute to a smooth(er) trip.

Uncontrolled or poorly controlled weeds use water and nutrients intended for developing trees, hide tree-damaging voles and gophers from predators, and require extra labor to clean up. Heat (propane flamers, etc.) can control weeds, but requires repeat applications and is rough on plastic irrigation hose. Weed cloth controls weeds, but hides vole and gopher activity. Given the current lack of viable alternatives, cost effective weed control practices are largely limited to herbicide use – preemergent, postemergent or both. Both practices (pre or post emergence) can deliver effective weed control, but as challenges (herbicide-resistant weeds) and costs (labor and herbicide) grow, reconsidering costs and benefits of each approach is worth doing. Considerations differ for different aged plantings, so they are addressed by year.

Always consult with a licensed PCA regarding materials, rates and timings before applying a pesticide. Always read the label.

Preplant: Have a weed management plan before planting. Know, if you can, the history of weeds in a block and work to limit those when planting. Pre plant timing use of preemergent herbicides is allowed on a handful of labels ahead of planting. If a preemergent is used ahead of planting, use extra caution when planting, being careful not to place soil with residual herbicide (top couple inches of soil) back into the planting hole after the baby tree has been placed.

First leaf: Staying on top of weeds in the first year after planting is important to get early growth.  A strong weed control program in first leaf also helps prevent rodents, especially voles, from damaging the bark. Either pre- or postemergent herbicides can be used in the first leaf (if allowed by the label).

Applying preemergent herbicide(s) helps control weeds without the need for repeated postemergent herbicides through the season, although a small amount of rain or irrigation water is needed within several weeks of application to incorporate these materials. Preemergent herbicide application before or shortly after planting can help reduce risk of weeds out of control in a wet spring when the grower can’t get in to spray burndown (postemergent) herbicides. There are only a handful of registered preemergent herbicides for first year use (e.g. Prowl, Surflan, Trellis and Goal), but they are effective on many weeds (see the link to “UC IPM Susceptibility of Weeds in Almond to Herbicide Control” chart at the end of this article). Most of these preemergent herbicides work on the roots of seedling weeds and have limited uptake by aboveground parts of weeds or trees contributing to the margin of crop safety.

Postemergent herbicides such as Rely 280, Treevix, Roundup, Gramoxone, etc., are effective on weeds through the first leaf where trunk protectors (nursery cartons) are in place and in good shape. Without protective cartons/trunk guards, postemergent herbicides can damage vulnerable green bark on first leaf trees. For best results, include surfactant and/or activators on the label at the rates and mixing order listed. Repeat applications are needed when using a “ Postemergent only” plan for weed management in the first leaf. As always, match the herbicide to the weeds present and follow the label. For best results, include surfactant and/or activators on the label at the rates and mixing order listed.

Intact trunk protectors (nursery cartons) are a critical part of first leaf weed control.  Tree exposures to herbicides should be fairly limited as long as the spray stays below the top of the carton and off low branches. Smooth out row middles wherever possible to keep boom bounce down and the spray away from lower branches.

Second leaf: Postemergent herbicide use in second leaf trees is tricky as protective nursery cartons may have been removed but young trunk bark is still sensitive to certain herbicides. In recent UC research results, trunk protection practices, ranked from worst to best protection, were fresh 50/50 white latex paint/water < 9-week-old 50/50 paint < no paint, 9 weeks after nursery boxes pulled < nursery boxes still on the trees. The somewhat unexpected results about the latex paint was attributed primarily to hardening of the bark after carton removal rather than directly to the paint treatment.

Many preemergent labels allow application after trees have spent a year in the ground. Prior to second leaf, consider applying a preemergent (with postemergent for control of existing weeds) in the winter or early spring while nursery cartons are still protecting the trunks, then remove the cartons and allow the trunk to harden before following up with any postemergent spray(s) in the summer if needed.

Third leaf: The bark on trunks should be tougher at this time, so more options are registered for use at this time due to lower risk of crop injury, but efficacy still hinges on building a strategy based on the particular species you want to manage, when it is most vulnerable.

For the best postemergent herbicide for fleabane control, in research by Dr. Brad Hanson’s Lab at UC Davis, Rely 280 (and similar products) or Treevix were the most effective. Tank mixing with glyphosate (Roundup) often delivers the best control across weed species in an orchard. Field bindweed (morning glory) can be controlled with glyphosate but care should be taken to mnimize spray drift and almond trunk exposure because of potential crop injury. Timing is critical to effective weed control with postemergent herbicides. Early is better than later as smaller weeds are easier to control than larger ones. For example, once fleabane “bolts” (changes from the low growing rosette to a more upright form), herbicide control is very difficult.

  • Consider managing weeds in the row middles in young blocks so those areas don’t become a huge seed bank for the critical zones down the tree row. Cover crops can shade out summer weeds. Other strategies for killing young weeds in the row middle before they set seed include light tilling or herbicide spray(s).
  • How a herbicide is applied is important for good results. Spray application of herbicides must be done carefully to put the herbicide where it works on the weeds, not your trees. Smooth row middles reduce chances of boom bounce. This, along with low spray pressure and low-drift nozzles, reduce off-target spraying. Careful training of applicators and setup of spray rigs is also important to a safe and effective spray.
  • Systems that trigger sprayer nozzles to spot spray scattered weeds are used effectively in field cropping systems. To my knowledge, this technology is little used in orchards. In a year where herbicide availability is possibly limited, automated spot spraying could be a way to stretch materials while spraying problem weeds that are young and more vulnerable.

Rising costs of labor and postemergent herbicides and an increasing number of glyphosate-resistant weeds makes young orchard weed management an ever-increasing challenge. When looking for successful, cost-effective weed management solutions, consult with your PCA and consider all the timing, material and technological options available.

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Online resources for weed management

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