Post-Harvest Prep for Walnuts: Weeds, Nutrients, and Bot


Harvest takes a toll on orchards, and as the hustle and bustle of walnut harvest comes to a close, trees need time to rest, recover, and refuel. Growing operations may be headed into a slower time of year, but post-harvest actions taken to prepare the tree for—or nourish the tree during winter—help ensure its health and productivity come spring.

Less active months are good times to check equipment and revisit the effectiveness of the previous season’s protocols, while also considering the winter management of weeds, nutrients, and diseases or infections.

Weed Control

Luckily for walnut growers who have mature trees, many weed populations can’t emerge simply because the canopy shades out the sunlight. But, weeds still make an appearance in the winter.

As the end of harvest approaches, growers do a postharvest irrigation, which causes flushes of weeds to sprout up. By the end of November, most growers have begun their residual programs, or are considering another burndown, to try to eliminate the weeds that have come up due to the application of irrigation water. In drier fields, growers wait for the rain.

If you’re using the postharvest time frame to scout for weeds, you might come up a little empty-handed. Ideally this season’s winter weeds were identified during the 18-19 winter season. Since the flora doesn’t generally change too much, last winter’s weed survey of your orchard will prepare you for what to expect this season. Sure, new seeds can blow in or make their way on the backs of critters, but anything new will be recorded in this year’s weed survey, which will be helpful for the following year. (The same is true for recording summer weeds.)

There are two broad categories for weeds: grasses and broadleaf, with each requiring their own type of herbicide. Preemergent herbicides are typically tank-mixed and put down based on a weed survey or specific knowledge about the field. There are about two to three mixes that are broad spectrum herbicides common amongst walnut growers, such as Chateau and Prowl H2O, which is a pretty common treatment for a berm spray or strip spray, or Alion and Matrix.

Ultimately, herbicide decisions are based on the species present. If you have never taken a survey and put the data in writing or are new to the walnut growing game, the collection and recording of your orchard data is important for knowing which treatments are ideal for your field.

There’s also the consideration of resistance. Growers have been experiencing herbicide resistance in weeds, which has increased the use of spot sprays. Where common practice was once a blanket application of glyphosate, media attention and weed resistance have required a different approach. If growers identify an escape or resistant species, spot sprays are more effective and financially sensible, instead of waiting for the weeds to become denser, a much larger problem, and spraying an entire field.

Good soil-herbicide contact is important for application effectiveness. So, be sure your orchard floors and berms are clean, with leaves and trash blown away first.

Nutrient Applications

Potassium is often the subject of nutrient discussions around this time of year. It’s typically applied in a narrow band because it easily sticks to the soil and remains accessible to the tree. Potassium is available in two forms—KCl and K2SO4. KCl, or potassium chloride, runs the risk of not being leached from the soil, which means there’s a high possibility that growers are putting salt on a tree that doesn’t need nor want it. This is also why this form is less expensive, and why many growers prefer the other form.

If you intend to regularly apply potassium with the goal of replacing what was lost, it’s important to know what the levels were in your July leaf samples. Unfortunately, growers can definitely experience a decline in yield if their walnuts are potassium deficient. So knowing whether a potassium application is simply maintenance or a catch-up decision with the use of a larger slug will determine your options. For information on potassium fertilizer rates, please see

Canker Monitoring

Fungicides are most effective in the warmer spring and summer months, when cankers grow more rapidly. In the fall, however, there will be a lot of dead wood. And while there are still leaves on the trees, it’s a good time to go through your trees to look for any kind of canker. Sometimes growers will come across Botryosphaeria or phomopsis (BOT) infections, and it’s best to cut out these infections in late summer. These fungal invaders kill large branches, small fruit wood, and nuts, are spread through splashed water or by air blown spores, both requiring water for infection.

A proactive step against BOT is limiting pruning wound susceptibility, and using the seasons to your advantage can help. Pruning is a common consideration at this time of year, and while pruning is on some growers’ agendas, autumn pruning is a better option than winter pruning because of the lower infection rates. Pruning wounds remain susceptible for a long time—about four months. It’s also important to avoid pruning when there’s rain in the forecast, as the long susceptibility period allows rainfall to threaten the wounds. But, pruning wounds made in fall are less susceptible since they’re made ahead of the colder winter temperatures. This also means that winter pruning that precedes the warmer months creates highly susceptible wounds as well.

(UC Davis plant pathologist Themis Michailides, Ph.D, based at the Kearney Agricultural and Research and Extension Center tested February pruning and found that infections favor warmer months for growth.)

By their very nature, these perennial cankers will continue to grow, so it’s really about prevention. If you are hedging, it’s possible that some of the infection will be cut away. Ideally, cankers are handled preventatively, but that’s not an activity for winter months.

Prepare well for the upcoming spring by taking care of your walnut trees postharvest. The perceived winter slowdown can be misleading—there are still many things to do as the trees rest and the cycle prepares for its next run.