Eric Yu, David Nicolai, Ryan Miller, and Liz Stahl, UMN Extension crops educators, and Debalin Sarangi, UMN Extension Weed Specialist
With temperatures rising and adequate soil moisture across much of the region, weeds are growing rapidly, creating ideal conditions for early-season competition in row crops. Even if a pre-emergence herbicide was applied, it is important to scout fields now for weed emergence. Early detection and timely intervention are key.
Postemergence herbicide applications are essential for maintaining crop yield and protecting farm profitability. Weeds that emerge alongside crops compete aggressively for sunlight, nutrients, and water, especially during critical early growth stages. Targeting weeds while they are still small improves control effectiveness, reduces the risk of seed production, and limits the need for costly rescue treatments later in the season.
Larger weeds are more likely to survive an herbicide application, especially under less-than-ideal conditions such as poor coverage, incorrect spray rates, unfavorable temperatures, or suboptimal timing during the day. Bigger plants often have more growing points than smaller seedlings, allowing them to better tolerate or recover from herbicide injury. In addition, herbicide uptake and coverage tend to decline as plant size increases, reducing overall efficacy.
Weeds like waterhemp and giant ragweed can grow very rapidly once they obtain the ideal height to spray. At this height growth can be up to one inch per day under favorable weather conditions. This means a plant can grow well past the 4-inch threshold in just a few days. Early-season scouting is therefore essential to catch these weeds before they grow too large. Timely detection and treatment ensure that postemergence herbicides are applied when control is most effective. Figure 1 illustrates how rapidly waterhemp and giant ragweed can grow beyond the height at which herbicide control is most effective.
While early postemergence applications may sometimes require a follow-up spray to manage later flushes of weeds, controlling them at the seedling stage provides the best return on investment. Delaying treatment not only puts yield potential at risk but also increases the chances of herbicide resistance developing in the weed population.
Because systemic herbicides rely on movement within the plant, visible symptoms like yellowing or wilting may take several days to appear, but the results are usually more thorough and long-lasting. For optimal uptake, ensure the weeds are actively growing at the time of application. Avoid mowing or disturbing the plants before or immediately after treatment, as this can limit absorption.
Contact herbicides generally provide quicker visible results and plants may begin wilting or browning within hours of treatment. However, regrowth is possible if root systems remain viable.
This strategy is especially important in fields planted early, such as in late April or early May. Early planting often leads to early preemergence herbicide applications, and the effectiveness of those products can taper off by midseason. As the residual activity of the preemergence herbicide declines, fields become more vulnerable to new flushes of weeds. A postemergence tank mix that preferably includes a different residual herbicide, with a different mode of action, can help sustain control through the critical periods of crop development and delay resistance.
Common waterhemp is a problematic weed species in many MN fields that can germinate well into August. This extended emergence period means that a single early-season application may not be enough. Fields with a history of late-season waterhemp pressure can benefit significantly from the addition of a residual herbicide in the postemergence program to suppress later flushes and reduce seedbank replenishment.
In corn, for example, many products have cutoffs based on the number of leaf collars (V-stage), plant height, or both. Applying herbicides beyond the labeled crop stage may reduce crop safety and increase the risk of visual injury such as leaf burn, stunting, or delayed growth. While some level of cosmetic injury may not impact yield, severe injury, especially during early vegetative stages, can result in long-term developmental delays or reduced stand vigor.
To ensure crop safety:
Waterhemp resistance article highlighting resistance screenings across Minnesota.
Postemergence herbicide applications are essential for maintaining crop yield and protecting farm profitability. Weeds that emerge alongside crops compete aggressively for sunlight, nutrients, and water, especially during critical early growth stages. Targeting weeds while they are still small improves control effectiveness, reduces the risk of seed production, and limits the need for costly rescue treatments later in the season.
Timing is everything: Target weeds when they're small
Postemergence herbicides are most effective when weeds are small, ideally no more than 3 to 4 inches tall. This size threshold is critical for achieving consistent control and minimizing the risk of yield loss. Once weeds grow beyond this stage, control becomes more difficult and less reliable.Larger weeds are more likely to survive an herbicide application, especially under less-than-ideal conditions such as poor coverage, incorrect spray rates, unfavorable temperatures, or suboptimal timing during the day. Bigger plants often have more growing points than smaller seedlings, allowing them to better tolerate or recover from herbicide injury. In addition, herbicide uptake and coverage tend to decline as plant size increases, reducing overall efficacy.
Weeds like waterhemp and giant ragweed can grow very rapidly once they obtain the ideal height to spray. At this height growth can be up to one inch per day under favorable weather conditions. This means a plant can grow well past the 4-inch threshold in just a few days. Early-season scouting is therefore essential to catch these weeds before they grow too large. Timely detection and treatment ensure that postemergence herbicides are applied when control is most effective. Figure 1 illustrates how rapidly waterhemp and giant ragweed can grow beyond the height at which herbicide control is most effective.
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| Figure 1. Growth of giant ragweed and waterhemp over time following emergence. Plants were monitored and measured regularly at Rochester, MN, beginning at the date of emergence. |
While early postemergence applications may sometimes require a follow-up spray to manage later flushes of weeds, controlling them at the seedling stage provides the best return on investment. Delaying treatment not only puts yield potential at risk but also increases the chances of herbicide resistance developing in the weed population.
Know your herbicide: Systemic vs. contact
Understanding the differences between systemic and contact herbicides is essential for making effective post-emergent weed control decisions. Each type has specific modes of action, application strategies, and ideal use cases depending on the target weed species and environmental conditions.Systemic herbicides
Systemic herbicides are absorbed through the leaves or stems of the weed and translocated throughout the entire plant via its vascular system. This makes them highly effective against perennial weeds and species with extensive root systems, such as dandelions, bindweed, or Canada thistle. Systemic products target both the above-ground and below-ground portions of the plant, helping to prevent regrowth from root reserves. Glyphosate (e.g. Roundup), Enlist, 2,4-D, and dicamba products are some commonly used systemic herbicides.Because systemic herbicides rely on movement within the plant, visible symptoms like yellowing or wilting may take several days to appear, but the results are usually more thorough and long-lasting. For optimal uptake, ensure the weeds are actively growing at the time of application. Avoid mowing or disturbing the plants before or immediately after treatment, as this can limit absorption.
Application tips
- Use medium to coarse spray droplets to minimize drift and improve leaf contact.
- Apply during warm, calm weather when plants are not drought-stressed.
- Allow enough time post-application for the herbicide to be absorbed before rainfall or irrigation.
Contact herbicides
Contact herbicides destroy only the plant tissue they come in direct contact with, causing rapid cell damage and desiccation. They are most effective on young, actively growing annual weeds that don’t rely on underground reserves for regrowth. Because they don’t move within the plant, they may not completely kill established perennials unless coverage is extremely thorough or repeat applications are used. Liberty, Flexstar and Cobra are some commonly used contact herbicides.Contact herbicides generally provide quicker visible results and plants may begin wilting or browning within hours of treatment. However, regrowth is possible if root systems remain viable.
Application tips
- Achieve complete and even spray coverage, especially for larger or multi-branched weeds.
- Use medium spray droplets to maximize surface area coverage, drift reducing spray tips can minimize the number of driftable fine droplets.
- Apply at a minimum spray volume of 15 gallons per acre to ensure adequate leaf wetting.
- Spray volumes of 20 gallons per acre will work better
- Reapplication may be necessary for regrowth or new flushes of weeds.
Watch the weather: Environmental conditions matter
Light and temperature
Contact herbicides, like Liberty and Cobra, are more effective under hot, sunny, and humid conditions. Under cool or cloudy skies, their effectiveness drops. Avoid spraying when temperatures are below 50°F, especially during spring burndown.Drought stress
Under dry conditions, both crops and weeds slow their growth, reducing herbicide uptake. Adjuvants (like surfactants or water conditioners) can help improve control in hot, dry weather.Rainfast period
Each herbicide has a rainfast period which is the time needed for the product to absorb before rain. In general, aim for 4 to 6 hours of dry weather after spraying.Wind
Spray when wind speeds are between 5 and 10 mph to keep herbicide droplets on target. Spraying in high winds increases the risk of drift onto non-target plants. Avoid spraying when wind speeds are 0-3 mph as light winds can be unpredictable and variable in their direction and can also be an indicator of a temperature inversion.Temperature inversions
Inversions trap herbicide droplets in a stable air layer close to the ground, increasing drift risk. These conditions usually occur around sunrise and sunset.Choosing the right herbicide: Mix for maximum effect
Use herbicides with different modes of action to broaden weed control and slow resistance development. For example, one product may target broadleaf weeds while another focuses on grasses. Rotate and mix herbicide groups to:- Broaden weed control spectrum
- Reduce herbicide resistance pressure
- Avoid over-reliance on a single product (like glyphosate)
Add residuals for longer protection
Mixing a residual herbicide with your postemergence spray can help manage weeds that emerge later in the season. While these late-emerging weeds may not significantly impact yield, they can interfere with harvest operations and contribute seed to the soil seedbank, increasing weed pressure in future seasons. Including a residual herbicide helps prevent this seed production and extends the overall window of control.This strategy is especially important in fields planted early, such as in late April or early May. Early planting often leads to early preemergence herbicide applications, and the effectiveness of those products can taper off by midseason. As the residual activity of the preemergence herbicide declines, fields become more vulnerable to new flushes of weeds. A postemergence tank mix that preferably includes a different residual herbicide, with a different mode of action, can help sustain control through the critical periods of crop development and delay resistance.
Common waterhemp is a problematic weed species in many MN fields that can germinate well into August. This extended emergence period means that a single early-season application may not be enough. Fields with a history of late-season waterhemp pressure can benefit significantly from the addition of a residual herbicide in the postemergence program to suppress later flushes and reduce seedbank replenishment.
Always consider crop stage and herbicide labeling
When planning a postemergence herbicide application, it is just as important to consider the crop growth stage as it is to evaluate weed size. Each herbicide label specifies crop stage restrictions for application, and exceeding those limits can increase the risk of crop injury or yield loss.In corn, for example, many products have cutoffs based on the number of leaf collars (V-stage), plant height, or both. Applying herbicides beyond the labeled crop stage may reduce crop safety and increase the risk of visual injury such as leaf burn, stunting, or delayed growth. While some level of cosmetic injury may not impact yield, severe injury, especially during early vegetative stages, can result in long-term developmental delays or reduced stand vigor.
To ensure crop safety:
- Follow label directions closely for both weed and crop stage.
- Avoid tank-mixing incompatible products that may increase the risk of injury.
- Apply under favorable weather conditions that support crop tolerance and herbicide performance.
Take-home tips
- Spray early, when weeds are small, ideally before they exceed 3 to 4 inches tall.
- Match the herbicide to the weed type and field conditions.
- Watch the weather as temperature, wind, and rain can make or break your spray.
- Use herbicide mixes and rotate chemistries to manage resistance.
- Include a residual to stop late-season weeds from taking over.
- Be sure to check the label and follow any recommendations and restrictions regarding application timing, methods, tank-mix partners and conditions at application.
Related Crop News Articles
Postemergence herbicide application article with similar information including herbicide application cutoffs and guidelines.Waterhemp resistance article highlighting resistance screenings across Minnesota.
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