By: Cesar Cappa, Weed Management Specialist – Horticulture, OMAFA
Published September 2025
Fall may signal the end of the growing season, but it’s not the time to wrap up weed management. In fact, it’s the ideal time to start planning for the following year and considering fall herbicide applications that could be effective on the whole range of weeds, from summer annual to perennial ones, even during the following grow season.
Why Apply Herbicides in the Fall?
While most soil-applied residual herbicides can be used in the spring, fall applications offer several key advantages:
- Control of fall and early spring-emerging weeds such as Canada fleabane, chickweed, shepherd’s purse, annual bluegrass, downy brome, and stinkweed.
- Reduced pressure on spring workload, when ideal herbicide application windows can be missed due to time constraints, increasing crop risk and reducing weed control efficacy.
- Improved reliability of incorporation, as fall’s more predictable precipitation helps ensure proper soil activation of herbicides.
- Lower risk of crop injury, particularly for products that must be applied during dormancy.
- Opportunity for effective post-emergence applications for perennial weeds such as for fescues, goldenrod, or Canada thistle, and summer annuals before they contribute further to the seedbank.
What Happens to Fall-Applied Residual Herbicides Over Winter?
Degradation of most herbicides in soil slows significantly at low temperatures and almost halts in frozen ground. This is because biological degradation and mineralization are highly temperature dependent. As a result, fall-applied herbicides may still provide effective residual control into the spring. However, the persistence and efficacy will depend on application timing, herbicide properties, and environmental conditions.
While degradation slows, it doesn’t stop completely, so fall applications generally won’t last as long and be as effective as ideally timed spring applications. The primary benefit is effective fall weed control, with the added advantage of residual activity into the next growing season, especially valuable if spring conditions prevent timely herbicide applications and the first flush of weeds is missed.
To maximize the residual potential of soil-applied herbicides, ensure the soil surface is clean and free of plant debris to allow even distribution and incorporation. When properly applied, late fall applications can be effective through the spring and even into early to mid-summer, especially for persistent products like Casoron and Alion.
When is the Right Time for Fall Application?
Timing will depend on the target weeds and the herbicide:
- For perennial weeds using post-emergent herbicides, apply early in the fall while the plants are still actively transporting carbohydrates to underground structures. Applications made after the first frost, leaf fall or dormancy will be less effective.
- For winter annuals targeted with residual herbicides, apply before the peak fall emergence.
- For residual spring weed control the later the application, but before the soil freezes or there’s snow cover, the longer the residual effect into spring.
- Some products require the crop to be fully dormant, and others, like Casoron, should only be applied after temperatures have dropped significantly.
- Weather conditions are critical, particularly rainfall forecasts, to ensure proper incorporation and avoid runoff.
Applications must always be made before the ground freezes.
Residual Herbicides for Fall Use in Blueberries and Raspberries
Below is a list of herbicides with sufficient persistence to provide residual control into the spring. Not all registered products are listed. Always consult the most up-to-date label before application.
Trade name | WSSA Group | Highbush Blueberries Registered | Raspberries Registered | Timing Notes |
|---|---|---|---|---|
Alion | 29 | ✔ | ✔ | Do not apply prior to any soil disturbance. Rain or irrigation after application will improve efficacy. Max one application per year. |
Casoron G-4 | 20 | ✔ | ✔ | Apply during dormancy. Late winter or fall when temperatures don’t exceed 15C. Light precipitation shortly after the application to incorporate is preferred. Max one application per year (low rate) or every other year (high rate) |
Chateau EZ | 14 | ✔ | ✔ | Max 2 application per year. The preferred application timing for Chateau EZ Herbicide is in the fall to maximize the potential for rainfall to activate and set the herbicide. Avoid contact with green bark, foliage and new shoots. If used after budbreak, a shielded or hooded application is required. |
Devrinol 2-XT | 15 | ✔ | ✔ | Rain is required for incorporation and activation. Max one application per year. Apply in the fall through early spring. |
Sinbar | 5 | ✔ | ✔ | Better results if there’s rainfall within two weeks of application. Moisture required for activation. Fall (during dormancy) or spring applications. Max one application per year. |
If weeds are already present in the field at the time of application, a post-emergence herbicide will be required in combination with a residual product.
For guidance on selecting the right herbicide combination for your field conditions and weed spectrum, consult the CropHub or the OMAFA Weed Management Specialist.