Weeds are awful and are the bane of most people’s existence. Growers spend a significant amount of time and resources trying to manage weeds, as they can have a negative impact on most crops, including:
- Competing for nutrients, water, light, and space
- Harbouring, sheltering or encouraging insect pests and diseases
- Affecting harvest operations
- Contaminating harvested products
Like it or not, weed management is a yearlong (not just season long) effort. To effectively manage weeds, you must keep chipping away at the actively growing annual and perennial weeds at different growth stages, as well as addressing the seemingly endless weed seed bank in the soil.
Late season or fall management practices are a critical tool in the weed management toolbox, particularly in perennial horticulture crops where the management windows are restricted and small. Many crops are too sensitive to allow any controls to be applied during their active periods or their harvest times, other than hand weeding or some careful mechanical operations. However, once harvest is complete, end of season cleanup practices may be implemented, to good effect. Applications can be made to active or dormant (or almost dormant) crops, depending on the product.
Late summer or fall chemical applications
These types of applications can play a significant role in reducing perennial weed populations, as some products are more effective at penetrating some weed species’ barriers and getting deep into the plant. This is because most weeds start shutting down as the season winds down, meaning that the direction of flow within the plant is reversed and aimed at gathering resources into the overwintering parts. Non-selective products, such as glyphosate, applied as spot sprays (they may be applied differently in field crops, but not horticulture crops) are carried back into the perennial, and can take a big bite out of the perennial plant network that you cannot see on the surface.
Some things to consider with this type of application are:
- Air temperatures
Air temperatures should be above freezing, preferably closer to upper single digits to double digits, at least for several hours after application, to ensure maximum uptake of the product. Warmer temperatures will ensure that weeds are actively growing and are vulnerable.
- Active growth and foliar condition
Weeds must be actively growing and have live, active leaves to take up any applied herbicides. If frosts have occurred, assess weeds carefully to determine if they are still susceptible to herbicides. Frozen plants may be dead and unable to take up herbicides.
- Post-application wait period
Wait for several days at a minimum before tilling weeds, if tillage was part of the plan. This allows a maximum amount of active ingredient to have been drawn back into the plant.
Late season applied granular or foliar products
These products, with this application timing, can be applied to knock back annual weed populations in perennial fruit crops, such as strawberries or raspberries. In strawberries, renovation opens the canopy, as well as narrowing rows, allowing between-row weeds to be cultivated out, and post-harvest, fall-applied herbicides to get at weeds. Alternatively, the late applications can take out late flushes of weeds, bringing the overall population down.
Some things to consider with this type of application:
- Apply the products according to the exact label instructions.
- Avoid applying these types of products to a weak crop, as they might not be able to withstand the effect of the herbicide.
- Some products will need precipitation or irrigation to move the product off the crop and onto the weeds.
- When an herbicide is applied in the fall, spring applications should be avoided, so as not to damage the crop. Always follow the label instructions to avoid issues.
Fall and soil-applied products
These products can help to establish an herbicide barrier in the soil that can give crops a head start in the spring. The concept is to apply the product to the soil surface and then either incorporate it carefully to establish the herbicide barrier layer or apply it in conditions that will move the product into place. Most of these products are going to have limited activity on currently growing weeds, but are more focused on emerging, germinating, and growing weeds the following spring. Some products can be applied as a directed spray to the area at the base of a woody plant, mainly when the crop is dormant.
Some things to consider with this type of application:
- Pay attention to application instructions.
- Pay attention to soil temperatures and conditions.
Some products are best applied when soils are quite cool and clear of snow. This ensures that they are not broken down or denatured prior to being locked into place in the soil, and that they can move into the soil and are not washed away.
- Consider the lasting impact of these products, as they are geared at a soil residual effect. They will affect perennial crops, as well as the weeds, and may have an additive effect if applied over and over. Check product labels for instructions on using them in various crops.
Conclusions
Fall weed management can be effective at helping to knock weed populations back for the next season. As with any type of management, care and consideration must be given for both current and future impacts.