- The 9 Most Common Reasons Pre-Emergent Fails<\/a><\/li>
- What to Do About Weeds Right Now<\/a><\/li>
- FAQs<\/a><\/li><\/ul> \t\t\t<\/div> \t\t<\/div><\/div>
The 9 Most Common Reasons Pre-Emergent Fails<\/h2>
1. Didn\u2019t Read the Label<\/h3>
The most common pre-emergent mistake? Skipping the fine print. According to Pawel Petelewicz, assistant professor of turfgrass weed science at the University of Florida, failures are almost inevitable when homeowners assume all pre-emergents work the same way.<\/p>
\u201cThe label tells you the rate, timing, activation requirements, reapplication intervals, and turf safety limits, all based on how that specific herbicide behaves in soil,\u201d he explains.\u00a0<\/p>
The fix for now:<\/strong> Check the product label. Did you apply at the right rate? Water it within the required window? You may find the answer to what went wrong and a solution in the sections below.
For next year:<\/strong> Before buying, compare labels. Products differ in how long they last, when they need water, and how often you can reapply. Choose one that fits your climate and schedule.<\/p>2. Applied Too Late<\/h3>
Pre-emergent must be applied BEFORE weed seeds germinate. For crabgrass, the most common target, that means applying when soil temperatures (not<\/strong> air temperatures) reach 50 F-55 F for 3-5 consecutive days. This is typically Feb. 1 in South Florida to as late as May in the northern states. <\/p>
\u201cApplying too late, after weeds have emerged, does pretty much nothing,\u201d says Petelewicz, unless your product has some post-emergent activity.<\/p>
The fix for now:<\/strong> Look for products with dithiopyr (Dimension is a popular brand), which also works as a post-emergent for crabgrass seedlings.<\/p>
For next year:<\/strong> Monitor local soil temperatures through university Extension websites or use a soil thermometer. Apply when soil temperatures consistently reach 50-55 degrees for about 5 days.<\/p>
See Related:<\/strong> <\/p>
- What Are Pre-Emergent Herbicides?<\/a><\/li>
- When to Apply Pre-Emergent Herbicide (Spring & Fall Guide)<\/a><\/li> <\/ul>
3. You Didn’t Water It In<\/h3>
Watering in the pre-emergent herbicide within 1-2 weeks of application is what activates the product. \u201cMost PRE herbicides have to be moved into the upper soil profile to be effective,\u201d says Petelewicz. \u201cIf that step is missed, we cannot really talk about an effective barrier.\u201d<\/p>
Without water, the product sits on the surface, breaking down from sun exposure. By the time it finally rains, less of the active ingredient reaches the weed seed zone. <\/p>
Most products require 0.5 inch of water (from irrigation or rainfall) within the first week after application \u2014 the sooner, the better.<\/p>
The fix for now:<\/strong> If you applied pre-emergent within the past two weeks and haven’t watered, irrigate immediately with 0.5 inch of water.<\/p>
For next year:<\/strong> Apply pre-emergent right before forecasted rain, or plan to irrigate within 24-48 hours of application. <\/p>
4. Uneven or Missed Coverage<\/h3>
Pre-emergent only works where you apply it. Miss a spot, and weeds will find it. Common causes include poor calibration, spreader streaking, skipped passes along edges, or simply walking too fast. <\/p>
\u201cEven small gaps can lead to very visible and persistent failures,\u201d Petelewicz warns.<\/p>
The fix for now:<\/strong> Apply post-emergent herbicide<\/a> to visible weeds in breakthrough areas. Make note of where gaps occurred.<\/p>
For next year:<\/strong> Use a professional weed control service<\/a> for even coverage, or split your application into 2 passes at half rate, with the second pass perpendicular to the first.<\/p>
5. One Application Isn\u2019t Enough<\/h3>
![\"A](\"https:\/\/www.lawnstarter.com\/blog\/wp-content\/uploads\/2025\/08\/weed-killer-herbicide.jpg?wsr\")
Pre-emergent herbicide spray. Photo Credit: Scott Habermann<\/a> \/ Adobe Stock<\/figcaption><\/figure> Many common pre-emergents protect for 8-12 weeks. But in warmer regions like the Deep South, Texas, and California, crabgrass can germinate for 4-6 months or longer, from late winter through midsummer. A single application simply can\u2019t cover the entire window, which is why a second one is recommended.<\/p>
The fix for now:<\/strong> Seeing new weeds in the summer? Treat with a post-emergent herbicide. If it’s still early enough in the season, do a second pre-emergent application (if the label allows) to prevent more weeds from sprouting.<\/p>
For next year:<\/strong> Plan for 2 applications: one in early spring and another 6-8 weeks later. <\/p>
See Related: <\/strong>When and How to Apply Crabgrass Preventer<\/a><\/p>
6. You Used the Wrong Product\u00a0<\/h3>
Not all pre-emergents control all weeds. Some products target grassy weeds like crabgrass but don’t prevent broadleaf weeds. Others work on cool-season weeds but miss warm-season ones.<\/p>
Common scenarios:<\/p>
- Used a crabgrass-only preventer, but got spurge, clover, or other broadleaf weeds.<\/li>
- Applied a spring pre-emergent, but got poa annua or chickweed (cool-season weeds<\/a> needing fall application).<\/li>
- Used a short-residual product (6-8 weeks) in a climate needing 12+ weeks of control.<\/li> <\/ul>
The fix for now:<\/strong> Identify your weeds correctly, then use targeted post-emergent herbicides. Grassy weeds need different treatments from broadleaf weeds.<\/p>
For next year:<\/strong> Match the product to your specific weed problems. Prodiamine (Barricade) and dithiopyr (Dimension) provide broad-spectrum control of most grassy and some broadleaf weeds. For poa annua and winter weeds, apply pre-emergent in fall (September\/October).<\/p>
See Related:<\/strong> How to Kill Winter Weeds<\/a><\/p>
7. Core Aeration Disrupted the Barrier<\/h3>
![\"A](\"https:\/\/www.lawnstarter.com\/blog\/wp-content\/uploads\/2025\/06\/aeration.jpg?wsr\")
Core aeration. Photo Credit: Shutterstock<\/a><\/figcaption><\/figure> Core aeration pulls up plugs of treated soil, creating thousands of small gaps in your herbicide barrier, where weed seeds can germinate. Even worse, aeration distributes untreated soil with weed seeds from deeper layers across the lawn surface.<\/p>
The fix for now:<\/strong> You may see breakthrough weeds in aeration holes. Spot-treat with post-emergent herbicide or hand-pull weeds.<\/p>
For next year:<\/strong> Time your services correctly. Aerate BEFORE applying pre-emergent. Avoid aerating and dethatching within 6-8 weeks after pre-emergent application.<\/p>
8. Heavy Rain Washed It Away<\/h3>
Pre-emergent needs water to activate, but too much works against you. According to Petelewicz, heavy rain can push the herbicide deeper into the soil (beneath where weed seeds actually germinate) or wash it sideways. Sandy soils, compacted ground, and slopes are especially vulnerable.<\/p>
\u201cOnce the herbicide is no longer concentrated near the soil surface, weeds can emerge above it,\u201d he explains.<\/p>
The fix for now:<\/strong> Don\u2019t panic \u2014 and don\u2019t immediately reapply. Check your product label for reapplication intervals and annual limits, advises Petelewicz. <\/p>
If weeds appear, apply a post-emergent herbicide.
For next year:<\/strong> Check the forecast. Avoid application if heavy rain is predicted within 48-72 hours \u2014 light rain helps, storms don\u2019t. In rainy climates, use the split-application strategy: half rate in early spring, the rest 3-4 weeks later.\u00a0<\/p>9. Application Rate Too Low<\/h3>
Soil rich in organic matter binds more herbicide, so less is available to work its magic on the weed seeds. If you have heavy clay soil, you\u2019ll need to use the highest recommended application rate for best results. <\/p>
The fix for now<\/strong>: Get your soil tested and ask the local Extension office for recommendations, or hire a weed control pro<\/a> to apply the right amount of pre-emergent.<\/p>
For next year:<\/strong> Make a note to use the highest recommended rate on the label.<\/p>
What to Do About Weeds Right Now<\/h2>
Since pre-emergent doesn’t kill existing weeds, you need a different approach for current infestations.<\/p>
Post-Emergent Herbicides for Active Weeds<\/h3>
Post-emergent herbicides kill weeds that have already germinated and are actively growing. Choose products based on weed type:<\/p>
- Used a short-residual product (6-8 weeks) in a climate needing 12+ weeks of control.<\/li> <\/ul>
- When to Apply Pre-Emergent Herbicide (Spring & Fall Guide)<\/a><\/li> <\/ul>
- What to Do About Weeds Right Now<\/a><\/li>
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