Stop Fall Weeds Before They Start
This week in San Angelo
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Rainfall year to date: 15.47 inches
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Difference from normal: 1.46 inches above normal
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Five-day average soil temperature: 101.7°F
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Forecast highs: Around 105°F
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Forecast lows: Around 78°F
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Rain chance: 20% Thursday, August 27
It certainly does not feel like fall, but this is when we need to begin planning for fall pre-emergent. If we wait until rescuegrass and other winter weeds are visible, we have missed the best opportunity to prevent them.
Rescuegrass, our biggest fall problem
Rescuegrass, Bromus catharticus, is one of the most troublesome winter grasses in West Texas lawns.
It is a cool-season annual or short-lived perennial. Seed generally germinates during the fall, the plant grows through winter, and the large seedheads become noticeable in spring. The seedhead has broad, flattened, pale-green spikelets that hang from the stem. Unlike several other brome grasses, rescuegrass seedheads generally lack long, noticeable awns.
Rescuegrass gets a head start while our bermudagrass, St. Augustinegrass and zoysiagrass are slowing down or becoming dormant. By spring, the rescuegrass is large, competitive and difficult to remove selectively without risking injury to desirable turf.
One mature plant can also return more seed to the soil, increasing next year’s problem.
Other winter annual weeds
Rescuegrass is not the only target. Fall pre-emergent programs may also help reduce certain populations of:
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Annual bluegrass, or Poa annua
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Little barley
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Chickweed
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Henbit
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Lawn burweed
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Shepherd’s purse
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Annual ryegrass
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Certain winter broadleaf weeds listed on the product label
The exact weeds controlled depend on the active ingredient and label. No pre-emergent controls everything.
What is a pre-emergent?
A pre-emergent herbicide does not normally prevent a seed from absorbing water or beginning germination. Most products interrupt root development, shoot development or cell division immediately after germination.
People often describe this as creating a chemical barrier. That description is useful, but it is not quite complete. The herbicide forms a treated zone near the soil surface. A susceptible seedling encounters that zone as it begins developing and fails to establish normally.
Pre-emergent usually will not kill weeds that are already established. That is the difference:
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Pre-emergent: Applied before the target weed emerges.
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Post-emergent: Applied after the weed is growing.
Once rescuegrass is standing above the lawn, the pre-emergent opportunity for that plant is gone.
Why soil temperature matters
Customers generally want a month they can remember. At BES-TEX, we use:
February, May and September
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February begins spring prevention.
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May extends protection against later summer germination.
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September targets winter annual weeds.
The month is the reminder. Soil temperature, moisture and weather determine the actual biological timing.
Winter annual weed pressure generally increases as soil temperatures decline through approximately 70°F toward 60°F, particularly when rainfall or irrigation provides moisture. This is a working management range, not a universal switch that controls every species.
Rescuegrass is especially flexible. Laboratory research found strong germination under alternating temperatures of approximately 50°F at night and 68°F during the day, but rescuegrass can germinate across a wider temperature range. Alternating temperatures and moisture can be as important as one particular average temperature.
Annual bluegrass commonly begins germinating as soil temperatures remain below approximately 70°F, with considerable activity in the 50°F to 70°F range. Henbit and chickweed also germinate during cooling fall conditions, but they do not have one dependable temperature that applies to every lawn.
That is why we do not wait for one perfect number. The application must be completed, activated and in place before the first major germination period.
Texas A&M recommends fall pre-emergent applications for lawns with a history of annual bluegrass and rescuegrass. Its bermudagrass guidance gives a general Texas window of mid-August through mid-September, although local weather must be considered. Texas A&M lawn-management guidance
With our current five-day soil temperature at 101.7°F, the major cool-season germination period has not arrived. However, now is the time to measure the lawn, select the proper active ingredient and prepare for the September application.
Common pre-emergent active ingredients
| Active ingredient | General use |
|---|---|
| Pendimethalin | Controls many annual grasses and certain broadleaf weeds in established turf and landscapes. |
| Prodiamine | Long-residual control of many annual grasses and certain broadleaf weeds. |
| Dithiopyr | Pre-emergent control of annual grasses and certain broadleaf weeds, with limited early post-emergent activity on very young crabgrass. |
| Isoxaben | Primarily used for pre-emergent broadleaf weed control. It does not replace a grassy-weed pre-emergent. |
| Indaziflam | Long-residual professional chemistry for labeled established turf and landscape sites. |
| Oryzalin | Commonly used around labeled ornamentals and landscape areas. |
| Trifluralin | Frequently used in gardens, ornamental beds and certain non-turf sites. It is not automatically a lawn product. |
| Oxadiazon | Specialty pre-emergent used in certain turf and ornamental situations. |
| Simazine | Pre- and early post-emergent activity on certain weeds in specifically labeled warm-season turf. Turf species and timing restrictions are important. |
The current Texas A&M pre-emergent publication explains product selection, application timing and the importance of matching the herbicide to the turf and target weed. Texas A&M’s 2021 pre-emergent publication
Why pre-emergent fails
When customers see weeds after an application, they often assume the product was bad. Sometimes it was the wrong product, but several other failures are more common.
1. It was applied too late
If the seedling has already emerged, most pre-emergents cannot control it.
2. It was not watered into the soil
Most products require rainfall or irrigation to move the active ingredient into the upper soil layer. Follow the product label for the required amount and activation period.
This week’s Thursday rain chance is only 20%, so customers should not assume rainfall will activate an application.
3. Coverage was uneven
Skipped strips, incorrect spreader settings and poor calibration leave untreated openings.
4. The wrong active ingredient was selected
A product that controls annual grasses may provide limited control of broadleaf weeds. A broadleaf pre-emergent may not control rescuegrass or annual bluegrass.
5. The soil was disturbed
Aeration, dethatching, trenching, digging and aggressive raking after application can disrupt the treated zone. Complete necessary soil-disturbing work before applying the pre-emergent unless the label directs otherwise.
6. Protection expired
Pre-emergents have limited residual life. Heat, moisture, soil conditions, application rate and active ingredient influence how long they remain effective. A properly labeled follow-up application may be needed.
7. The lawn has a heavy seedbank
A property that has allowed rescuegrass to produce seed for years may contain an enormous number of weed seeds. Even high control percentages can leave visible weeds when the starting population is extremely large.
8. The weed was resistant
Weeds do not become “immune” because they were exposed once. However, repeated use of the same herbicide mode of action can select naturally resistant plants that survive and reproduce.
For most homeowners, resistance is not the first explanation for failure. Timing, coverage, activation and product selection are much more common causes.
Commercial applicators should pay closer attention to mode-of-action rotation. Pendimethalin, prodiamine, dithiopyr, oryzalin and trifluralin are all Group 3 herbicides. Changing between those active ingredients does not create a true mode-of-action rotation.
Pre-emergent is only one step
A pre-emergent application is valuable, but it is not the foundation of weed control. The foundation is a dense, healthy stand of desirable grass.
Lawn Care 101
Mow it
Mow often enough that no more than one-third of the leaf blade is removed at one mowing. Maintain the correct height for the grass variety. Scalping weakens turf and opens the canopy for weeds.
Water it
Water deeply and infrequently, based on soil moisture, grass needs and weather. Frequent shallow watering encourages shallow rooting and may support repeated weed germination.
Feed it
Feed established turf while it is actively growing, using a balanced fertilizer selected from soil-test results and turf needs. More fertilizer is not automatically better. Excessive or poorly timed nitrogen can increase water use, weak growth and disease pressure.
A well-managed lawn does not eliminate every weed, but dense turf reduces the light, space and resources available for weeds to become established.
Pre-emergent helps stop the next generation. Good lawn care reduces the opportunity for that generation to take over.
The BES-TEX approach
Remember the simple schedule:
February, May and September
Then let us help determine the correct timing, active ingredient and application rate for your lawn.
Before purchasing, know:
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Your grass variety
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Total square footage
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Target weed
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Application equipment
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Whether you plan to seed or overseed
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When you can irrigate after application
Do not apply a pre-emergent before planting fescue, ryegrass or another desirable seed unless the label specifically permits it. The same chemistry that interferes with weed seedlings can interfere with desirable grass establishment.
Always read and follow the current product label.