Post-harvest Weed Management – Vegetable Crops Hotline

Post-harvest Weed Management

The vegetable crops have all been harvested and it’s time to just relax, right?

Maybe. However, the decisions you make this fall will influence your weed control next spring.

In all likelihood, you did not control EVERY weed during the growing season. Inevitably, there were weeds that escaped control. If so, these weeds have already produced viable seeds (Figure 1).

Winter annual weeds have begun to germinate and grow. However, not every winter annual weed is problematic. Control measures in the fall should focus primarily on those weeds that will be more difficult to control next spring- marestail, for example.

Here are some control measures for managing this year’s weed seeds as well as emerging winter annual and perennial weeds.

Figure 1. Barnyard grass seeds shatter and fall to the ground in a harvested Indiana pumpkin field

Figure 1. Barnyard grass seeds shatter and fall to the ground in a harvested Indiana pumpkin field

Flail mowing: This is a minimalist approach- simply mowing the remaining crop residue and any existing weed plants.

Pros: Weed seeds are encouraged to fall to the soil surface. This makes it easier for them to be consumed by birds, mice, and insects.

Cons: Mowing does little to control winter annual weeds and perennial weeds like dandelion, Canada thistle, and docks (Figure 2).

Figure 2. Low-growing perennial weeds like dandelion (left) and curly dock (right) will not be controlled by mowing alone.

Cultivation: While it is tempting to run a disc through a harvested field, it may not always be the best option.

Pros: Shallow cultivation can uproot newly emerging winter annual weeds as well as perennial weeds.

Cons: Shallow cultivation moves weed seeds from the soil surface to below the soil, but not deeply enough to prevent them from germinating and growing. In other words, shallow cultivation when weed seeds are on the soil surface amounts to planting weeds seeds in your field. While it can be beneficial for some perennial weeds, shallow cultivation can also spread some weeds like Canada thistle (Figure 3) and yellow nutsedge throughout a field and make them more difficult to manage in the long-term.

Figure 3. A Canada thistle rosette grows in a harvested field.

Deep tillage: Sometimes referred to as inversion tillage, the idea behind this practice is to deeply burry plant residue (and weed seeds) several inches below the soil surface.

Pros: As a rule, small weed seeds will only germinate and grow when they are an inch or less below the soil surface. Burying these seeds can prevent them from germinating and encourage them to decay. Deep tillage is also a beneficial integrated pest management practice for some plant pathogens.

Cons: It is possible that deeply buried seeds from a prior deep tillage event can be moved to the soil surface. For this reason, avoid using this tactic every year.

Herbicides: Broadcast applications of non-selective or selective herbicides are suited for management of existing weeds post-harvest. Because most of the problematic weeds this time of year are broadleaves, consider using selective herbicides. Products containing 2,4-D, for example, can be a good option. Choose the warmest fall days to apply herbicides, and be aware that they will be slower to act in the fall than in the warmer months of spring and summer.

Pros: Herbicides can be used to control susceptible weed biotypes, even in no-till production systems.

Cons: Some winter annual weeds, such as marestail, have herbicide-resistant biotypes. It is important to be aware of the herbicide-resistant weeds in your fields.

Cover crops: Fall-planted cover crops are intended to compete with winter annual and perennial weeds. In Indiana, some cover crops will persist through winter while others will not. In general, weed control from cover crops is related to the amount of biomass the cover crop produces. Accumulation of biomass is largely determined by planting date and termination date. Early planting allows cover crops more time to accumulate sufficient biomass to suppress weeds.

Pros: Cover crops can reduce weed growth and reproduction. Many have additional soil health benefits. Cover crops, such as cereal rye, can be roller-crimped and used as an organic mulch for the following summer’s crop (Figure 4).

Cons: With the exception of cover crops that winter-kill, cover crops will need to terminated in the spring.

More information about cover crops can be found in the Managing cover crops profitability handbook

Figure 4. A handheld seed spreader containing cereal rye and crimson clover.

Pulling it all together:

Consider combining the above practices to match the weeds in your fields.

For example, inversion tillage followed by cover crops has been documented to control weeds better than either practice used by itself.

In a no-till system, flail mowing and herbicides can be used together.

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