Quicklinks:
Are fall applied herbicides safe for tree health after season-long drought?
Yes, because when trees go fully dormant in late fall the risk of herbicide damage is low. Fall cleanup is a good strategy to start weed control for early next season.The drought conditions affected the proportion of weed species in orchards this season. Typically, during drought there is less emergence of small-seeded weeds that germinate close to the soil surface. On the other hand, you might have noticed that deep rooted perennials were less affected. Now that autumn rains have arrived, there will be a late-season flush of germination of winter annual weeds and resumed growth of perennials.
Some herbicide labels warn, “crops that are stressed may be more sensitive to herbicide application”, and labels list drought as an example of a stressor. Stressed crops may be unable to cope with the impacts of herbicides if the chemistries are taken up by actively growing trees or while storing reserves for winter dormancy. Our trees are in a drought-influenced state so we should heed these warnings.
Allow trees to go fully dormant by late November to early December and then practice fall cleanup. By this time, leaves have fallen and will scatter or may be mowed so they generally don’t interfere with herbicide coverage. The dormant state of the trees is a real advantage because there is little risk of uptake and tree injury. Just keep in mind that weeds should be treated with post-emergent (burndown) herbicides while they are green and growing, prior to a hard frost. Post-emergent and residual herbicides may be used in a fall program.
Come spring, trees may experience lingering stress from the drought when they restart growth and we want to give them the best chance at success. Efforts to reduce tree stress could be helpful. A late fall weed control program can reduce stress by:
- Minimizing early season weed pressure that competes with trees for resources.
- Reducing the need for post-emergent herbicides applied in spring when trees are actively growing and are therefore more susceptible to injury.
What is the influence of the drought on residual herbicide decisions?
In general, using herbicides in the fall is safe as explained above. But there is one additional consideration to keep in mind for the use of residual herbicides and that’s their persistence or carryover in extreme circumstances like drought.Many herbicides are degraded by environmental factors like sunlight, and microbial processes, but some herbicides degrade largely by microbial processes. Drought conditions slow microbial processes so some residual herbicides might carryover longer than expected and may in some cases lead to crop injury if:
- Reapplying the same product (increasing total residues)
- Replanting new orchard (exposing young trees to higher-than-expected residues)
- Herbicides with a moderate risk of soil persistence are Sencor and Prowl. I’m not aware of specific warnings in tree fruit.
- Herbicides with a high risk of soil persistence are Authority and Chateau. Label warnings suggest taking care.
Authority:
- Instead of waiting the usual 2 years for reapplication, you should wait 3 years since your previous application. The label states, “Sulfentrazone is persistent and will last in the soils (carryover) for one to two years. DO NOT APPLY AUTHORITY 480 HERBICIDE TO FIELDS PREVIOUSLY TREATED WITH ANY SULFENTRAZONE-CONTAINING PRODUCT IN CONSECUTIVE YEARS (24 MONTHS). In case of drought in any of those years, a subsequent application of AUTHORITY® 480 Herbicide should be further delayed by the equivalent number of years in which drought occurred.”
- The label advises a 3-year replant interval under normal circumstances and extended an additional year to a 4-year replant interval after drought.
- The rotational or replanting restrictions on the Chateau label do not state apple specifically so apples are categorized under “all other crops not listed.” The rotational interval is 1 year. However, the label also says, “After periods of extended drought longer rotational intervals may be needed.”
- You are also familiar with the long persistence of Alion herbicide. The label advises to, “Allow at least 12 months between the last application of ALION Herbicide and replanting an orchard or vineyard with labeled tree or vine crops.” There is no specific mention of drought on the Alion label in terms of replanting, but waiting longer might be advised.
There may be some carryover of these persistent residual herbicides but it is difficult to predict and can vary by circumstance. You could extend the length of time from your last application and replanting interval. Or if you are concerned, there is an option to perform your own bioassay.
A bioassay involves sampling soil and growing some test crops that are sensitive to herbicide residues. To learn how to do a bioassay, check out an article by OMAFRA that describes the process. Late October to mid-November is a good time to sample soil for the bioassay because soil temperatures are now low and have slowed microbial processes. Herbicide breakdown between now and planting time next spring is minimal and therefore soils now are representative of herbicide residues come spring.
I’m attending an intensive Weed Science Course on herbicide physiology and biochemistry this coming winter on Dec 8-12 in New Brunswick. Let me know if you have any specific questions that I should bring to the discussion.
https://www.pioneer.com/us/agronomy/Why-Dry-Conditions-Increase-Risk-of-Herbicide-Carryover.html
https://ucanr.edu/blog/uc-weed-science-weed-control-management-ecology-and-minutia/article/effects-drought-conditions
https://www.ndsu.edu/agriculture/ag-hub/ag-topics/crop-production/diseases-pests-and-weeds/weeds/herbicide-concerns-after-drought
https://www.canr.msu.edu/news/fall_weed_control_in_established_fruit_crops
Fall pruning is risky, but is it riskier than normal this year due to the drought?
Yes, fall pruning is riskier than normal this year and it should be avoided as much as possible.Drought stressed trees are less likely to achieve their normal level of winter cold hardiness. To respond to cold temperatures, plants use resources like carbohydrates as protection. This summer’s less productive photosynthesis suggests fewer root starch concentrations were accumulated as reserves going into winter. Water balance as affected by the drought might also challenge winter hardiness.
Pruning trees in the fall or early winter would compound the problem by reducing tree hardiness even further. Pruning in relation to winter injury is a challenging topic to research because the weather is unpredictable for setting up a controlled field trial and the circumstances of each cold event are variable. But observational reports after extreme cold events suggest that all pruning practices reduce tree hardiness. The pruning practices stimulate growth hormones and interrupt the dormancy process.
Apples are typically hardy to midwinter temperatures as cold as -31°C to -35°C. Depending on the winter, losing a couple of degrees of hardiness through pruning is usually not a concern - although it is risky. But consider that the combination of drought stress and pruning would reduce hardiness even further to the point of damaging temperatures being possible. If cold temperatures were to threaten, then the decision to prune in the fall could be the difference between a live and dead tree.
In recent years we’ve seen the riskiest times for cold temperatures are in late January and early February. Pruning should be delayed until after the highest risk period has passed to keep trees as cold hardy as possible.
If some pruning must be done earlier in the winter for labour reasons, keep the following in mind:
- Only prune trees in old plantings that are soon due for replacement and that you can afford to lose.
- Pruning in November and December is riskier than waiting until at least January.
- Prune lightly by making limited cuts and remove less wood than usual. (There is no guidance on the merits of few big cuts versus many small cuts. Big cuts would remove many resources whereas many small cuts may cause a widespread stimulation of growth hormones.)
- Delay pruning trees that showed premature defoliation due to drought.
- If you are pruning and then a cold snap is predicted, stop pruning immediately.
- Prune the young trees as late as possible, before bloom.
- Less girth and extension shoot growth occurred during the 2025 season so there may be less that is critical to prune off.
- During the ideal pruning time in spring, prioritize high value trees and then return to low value areas.
- Prior to bud break and bloom, ensure youngest blocks are pruned first so growth is directed into desirable leader and terminal extension.
- If you’re pruning near bloom time and a frost is predicted, stop pruning and wait for the cold weather to pass to conserve fruit bud health.
- On severely drought stressed trees, make only detailed pruning cuts for tree training and to expose fruiting wood. Wood contains usable carbohydrates and nitrogen so discarding the resources reduces growth potential. (Limiting pruning might be controversial and has pros and cons. Ultimately, if slight changes to pruning decisions is enough to impact tree health, then trees are not likely in a healthy state anyway).
- It might not be possible to prune all blocks. After bloom, making large pruning cuts can weaken trees. If the orchard is near the end of its life, then there is not as much concern about weakening trees.
- In unpruned situations, use growth suppression strategies like prohexadione calcium (Apogee and Kudos), root pruning, and summer pruning if needed.
- Neglected trees can be renovated over 2-3 years. Begin with large corrective cuts and limit detailed pruning. Reestablish the height and shape of the tree. Do not remove more than one third of the tree area in one year.
- I’m sure the pruning discussion will continue this winter and I'm open to hearing all thoughts.
A tree without maintenance pruning can be renovated but a dead tree can’t be revived. Therefore, let’s be proactive by avoiding early pruning and allow trees as much cold temperature hardiness as possible. Thankfully we have a good buffer between cold temperatures apple trees can withstand and cold temperatures we normally observe.
Pruning and winter hardiness references:
https://extension.psu.edu/tree-fruit-cold-hardiness-pruning-effects
https://creators.spotify.com/pod/profile/orchard-outlook/episodes/E2-S3--The-Coolest-Chat-about-Early-Dormant-Pruning-Guest-Dr--Richard-Marini-e1bmv55
https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/831/items/1.0106295
https://annforsci.biomedcentral.com/articles/10.1007/s13595-021-01052-5
https://www.ishs.org/ishs-article/903_166
