Preemergent season is here, or is it?

Southern locales are deep into applications, while northern regions are still waiting to apply preemergents. Stretches of warmer weather have certainly triggered that start-stop reaction as soil temperatures have risen and dropped again.

We generally target that 50 F mark to make sure we have time to get product down and watered in prior to seed germination. This way, the barrier is set up before the pressure of summer annual weeds begins.

Building on my fellow Ewing Tech Team member, Pat Gross’, blog article, 5 Reasons Why Preemergence Herbicides Sometimes Fail, I decided to take a deep dive into the issue.

1. Failing to Time Applications Correctly

Timing is the No. 1 reason preemergents fail. Preemergence herbicides must be in the soil/thatch interface when weed seeds begin to germinate.

One of the phenological markers that we associate with proper timing is the forsythia bloom. However, some years we’ve seen it bloom yellow early, creating a false trigger to apply preemergents. Our better indicator for germination is soil temperature, and much of the central U.S. has dropped below what we would expect to be optimal conditions.

If you’re looking for a more accurate predictor, you can use Growing Degree Day models. They use a minimum threshold for insect or plant development and can be very accurate. While this article was being written Columbus, Ohio, was at 133 growing degree days (GDD) and could expect 25% of large crabgrass to show first seedlings around 211 GDD and 25% emergence at 472 GDD. Smooth crabgrass should be at 25% emergence at 263 GDD and 50% emergence around 354 GDD. Many universities will have a GDD tracker you can use for preventative applications and observance predictions.

This all sounds great, but what if we respond to weather conditions and the application season starts earlier than years past? We risk failure on the back end of the season as early warmups can be tricky.

Most products will break down over a period of time, opening the opportunity for weeds to peek out into the lawn. This is where we have to look at raising application rates, where rate becomes a factor of control in terms of longevity, or giving greater consideration to split or successive applications.

Many of these situations may not have been considered when turf programs were first drawn up and will come with a price increase that you may not be able to pass on to your clients.

2. Calibrating Once Instead of For Each Product Application

Reason two actually lands ahead of reason one chronologically, but they are somewhat intertwined. Calibration may seem like a simple aspect of proper application but it is often easily bypassed or done improperly. Granular spreaders have a gauge of settings and bags have a recommended setting, but that is only a starting point. Many components of the spreader may be slightly bent or the slide itself may be mounted differently, causing some fluctuation in actual application rate.

It is important to calibrate your equipment for each material you plan to use for the upcoming season. Additionally, spray nozzles should be examined and changed in the recommended intervals to prevent inconsistent sprayer coverage for effective weed control. Winter is a perfect time to conduct calibration activities.

3. Incorporating the Preemergent to the Incorrect Depth

Potential failure No. 3 is incorporation. As mentioned above, preemergent herbicides must be physically incorporated to the depth where weed seeds will be germinating for the best results. By physical, I mean watered in. Materials that stay on the soil surface or get hung up in the thatch tend to lose a large percentage of efficacy. On the other hand, excessive soil moisture and puddling could cause the active ingredient to move offsite, become excessively diluted or not offer the opportunity to set up.

In many situations, rainfall is necessary for proper incorporation, or irrigation is recommended. If neither occurs or it’s not significant enough to properly move the active ingredient to the soil/thatch interface, then control will not occur. This is also where irrigation audits can be very important. Having the data to support sprinkler run times to properly move the material into place without the risk of runoff is key.

If left on the surface, these products also can begin to be broken down by sunlight, which is called photodegradation, a common failure of preemergent herbicides.

4. Not Cultivating a Thick, Dense Turf Canopy

Consider where areas previously treated with preventative products struggle with the greatest weed pressure. That is typically where the soil is exposed and lacking good turf coverage. Often this will happen in high-traffic areas, around hard surfaces like concrete, asphalt, structures and hardscapes, and poorly maintained areas.

Thin turf coverage allows for more sunlight to penetrate, which will warm the soil, leading to earlier crabgrass germination and allowing more rapid solar degradation of the herbicide.

5. Leaving Out Spray Additives and Wetting Agents

The last reason for preemergent failure is directly related to spray applications. Considering spray additives can be the “make or break” decision in terms of accurate spray applications. By these we mean dye indicators and foam markers to improve accuracy, which limits excess overlap and eliminates spray misses which is fertile ground for perceived breakthroughs.

Adding wetting agents also will improve penetration of the herbicide. Checking spray water sources to make sure buffering agents or mix aids will improve tank mix quality, and defoamers to make sure the tank can be filled properly with water.

The previously mentioned issues can be the least-considered problems, but they can be just as irritating as others. Callbacks are costly from a fuel and labor perspective, but can be catastrophic from a credibility and reliability perspective. It is important to pay attention to details to be successful in preventative weed control.

Lean on Us for More Turfcare Solutions

Contact myself or Pat Gross, Ewing’s Tech Team, for more information on preemergent herbicides and other turf questions. Email me at klewis@ewingos.com or call/text 480-669-8791. Email Pat at pgross@ewingos.com or call/text 714-321-6101. We’re happy to help.