After a scorching summer and warm fall, winter looks to be finally arriving. As I write this, chilling units are starting to accumulate, and rain is in the short-term forecast. The National Weather Service’s Climate Prediction Lab is forecasting “equal chances” for normal rainfall. Current reservoir levels are at or above historic averages up and down the state. Here’s hoping our rainfall is at least average and reservoirs are heading toward full bloom next February.

Most of what follows are practices to consider this December in almond. However, many of the topics (weeds, irrigation, sanitation, scale, gophers, sprayer calibration) can be useful in walnut and pistachio. Links to further information on many topics are at the end of the article.

Irrigation

Will winter irrigation be needed this year? Water stress delays almond flower development and bloom timing. Wilbur Reil, retired UCCE orchard advisor in Solano/Yolo Counties, recommended 2 inches of water, either rain or irrigation, per month from November through January. In most soils, that water rate should allow the root zone soil to reach field capacity (excess moisture drained away) by bloom. Soil moisture sensors (Watermarks, etc.) are great tools to evaluate root zone “water bank” status.

Plan to get a “checkup” for irrigation systems ahead of next season. That could be this winter using well water or early next irrigation season with surface water. There are mobile irrigation labs (links at the end of the article) that provide these services.     

Sanitation

There is no higher-priority activity than orchard (almond, walnut or pistachio) sanitation between harvest and bloom. Orchard sanitation was a big part of the almond nut quality improvement from 2023 to 2024. Don’t let up.

For almond, between now and early March is the chance to get navel orangeworm (NOW) out of an orchard until hull split. How? Get mummy nuts on the ground (mummy shake, polling) by bud swell and mowed up and destroyed by March 1. This will also help control Carpophilus beetles. Count mummies before and after knocking out of the trees to check the quality of sanitation. Check 20 trees per acre with the goal of two mummies or less per tree by February 1. Fewer than that target is better, but at least get to 2 or less per tree. Make sure mummies on the ground are destroyed by March 1. I know at least one grower who mowed mummies three times this spring to get all mummies destroyed. Specialty sanitizing flail mowers are commercially available and may be worth considering if there are concerns about how well mummies are mowed using standard flail mower(s). Infested grounder mummies host NOW that will emerge and infest any mummies remaining in the trees next spring. Don’t “leave the door open” to let NOW into your orchard next spring. Sanitize walnut orchards as well as almond and pistachio.   

Scale

Do a spur sample/check for scale (and mite eggs) to see if a dormant spray for scale is needed. This is a good practice every year but is especially important after a year when pyrethroids use for NOW and/or bug control was widespread. Those insecticides harm mite predators but can also hit scale biocontrol insects and allow scale populations to grow. Mild winter temperatures also reduce scale mortality. If spur monitoring shows scale populations exceeding UC thresholds, talk with your PCA about materials and rates. Uncontrolled scale populations kill spurs and branches, reducing yield.

Stink Bugs

Brown spot kernel damage is reported to be higher this year than last. This damage is caused by summer (June/July) stink bug and/or leaffooted bug feeding. If brown spot damage in your orchard is unacceptable, talk with your PCA about a control program in 2025. UC guidelines suggest one dormant (green stinkbug only) or in-season spray every three years should be enough to prevent economic damage. Effective insecticides for bug control (pyrethroids like Brigade or Warrior) are toxic to predator insects that provide biological (free) mite and scale control. In-season bug control should be approached carefully.

If multiple sprays were needed to control mites this past summer, poor calibration and coverage might be part of the problem. Put a note on the calendar to check calibration and coverage once the canopy fills in next spring.

Weeds

Scout orchards for winter weeds in late fall, recording species and location (see link to the UC IPM winter weed survey form at the end of the article). Match winter weeds to effective herbicides when planning herbicide spraying. Preemergent herbicides need a little (quarter to half an inch) rain to incorporate and hold in the soil. Heavy rain (inches) can move certain herbicides lower in the soil than the depth of many seeds, reducing control. Watch the weather forecasts when planning preemergent applications and blow leaves away from the tree row ahead of herbicide application to ensure best weed control results.

Gophers

Now is a good time to control gophers. Trapping and baiting are proven practices. Check with your Ag Commissioner regarding bait and fumigation regulations. Consider installing owl boxes (one per 5 to 10 acres) to help keep gophers and other rodent numbers down. Owls will not eliminate rodents but do help keep numbers down.

Calibration

Winter is the time to give sprayers a careful checkup and calibration.  If used on many acres per season, consider replacing all nozzle tips and swirl plates. Even ceramic tips wear after several years.

Tree Care

Full dormant season (now) is the best time for pruning and harvest trunk damage cleanup. Insects that spread bark disease like Ceratocystis are not active, and pruning wounds are least vulnerable to infection at this time. It’s the relatively cool temperatures that make now such a good time for this work, not rain that often falls in December. Watch the weather forecast and, as much as possible, avoid opening wounds ahead of rain. Topsin-M (thiophanate methyl) provides the most consistent control of pruning wound cankers. Treat cuts within 24 hours of pruning and ahead of forecast rain.

To protect shaker damage from fungal infection, clean up the loose bark back to tight bark with a sharp knife or draw knife on younger trees and a hammer and chisel on older trees with thicker bark. Painting the cleaned-up bark with mixtures of oil and/or latex paint and certain pesticides reduced Certatocystis infection in almond in UC research done in the 1980s and 90s. Check with your PCA regarding label status of fungicides used in that research. The best way to limit Ceratocystis infection is to avoid/limit bark damage at harvest through careful shaker operator training and monitoring.

Nutrition

Deciduous trees don’t absorb nutrients from the soil between leaf drop and leaf out. Because of this, soil application of key nutrients, such as nitrogen and boron, which can be leached out of the root zone with winter rains, is ineffective. However, because potassium can be loosely held on the surface of clay and organic matter particles and available for plant use in the spring, fall/winter-applied K fertilizer is an option. Fall banding of dry K fertilizers (muriate of potash and sulfate of potash) is a common practice. Many growers have shifted to in-season applications (banding of dry sulfate of potash or injection with irrigation water). Since K demand is linked to crop load and the bloom weather and crop set next season is unknown in December, a full fall application to match expected crop demand may be more than what is needed. Also, banded fertilizer in a dry winter can result in poor incorporation (little rainfall to dissolve the fertilizer) and fertilizer loss when berms are blown ahead of preemergent herbicide application or to windrow sanitized nuts.

Banding dry K is one way to practice low-tech precision ag and save money by just applying to varieties in the orchard with low K levels in summer leaf samples.  Aldrich, in my experience, often shows lower summer leaf levels than NP in the same orchard. Banding dry potassium near drip lines or targeted broadcasting under microsprinklers are best management practices.

Bees

Now is a good time to set up pollination contracts with beekeepers for 2025. Contracts should include language stating hive strength of the bees to be delivered and how that strength is checked. Weak hives will not deliver a crop. Strong hives outperform moderately strong hives. Some growers have reduced pollination cost per acre by renting fewer hives of higher quality (frames of bees) compared to more hives with fewer frames of bees per hive. At the end of the day, don’t rent boxes, rent bees.

Don’t pass on renting bees for Independence trees. In recent UC research, Independence yields were reduced in orchards with no rented hives compared to those with one or more hives per acre set out in the orchard. The recommendation is 0.5 to 1 hives per acre in the orchards. Bees from outside the study orchard did visit flowers in the study orchard, but not at the same rate as when bees were rented and stocked in the orchard. The study didn’t examine bee activity in other self-fertile varieties, such as Shasta and Yorizane. If I were a grower, I would follow the Independence stocking rate recommendation in those varieties.

Best wishes for a Happy Holidays with family and friends.

Resources

Weather/water:

National Weather Service’s Climate Prediction Lab: https://www.cpc.ncep.noaa.gov/

California major supply reservoir status: https://cdec.water.ca.gov/resapp/RescondMain

Mobile irrigation labs:

North Sacramento Valley: tehamacountyrcd.org/mobile-irrigation-lab

South Sacramento Valley: https://sutterrcd.specialdistrict.org/mobile-irrigation-lab

Pests:

UC IPM Navel orangeworm management

ipm.ucanr.edu/agriculture/almond/navel-orangeworm/

UC IPM dormant spur monitoring in almond

ipm.ucanr.edu/PMG/C003/m003dcdmtspursmpl.html

San Jose Scale IPM (with economic thresholds)

https://ipm.ucanr.edu/agriculture/almond/san-jose-scale/

Walnut scale IPM

ipm.ucanr.edu/agriculture/walnut/walnut-scale/

San Jose scale IPM in walnuts

ipm.ucanr.edu/agriculture/walnut/san-jose-scale/

Stink bugs IPM

https://ipm.ucanr.edu/agriculture/almond/stink-bugs/

Leaffooted bugs IPM

https://ipm.ucanr.edu/agriculture/almond/leaffooted-bug/

Beehive strength/pollination contracts

giannini.ucop.edu/publications/are-update/issues/2020/24/1/going-nuts-for-more-bees-factors-influencing-calif/