Best days to apply pre-emergent in St. Louis, MO this week
Here's the next 7 days for putting down pre-emergent in St. Louis, checked hour by hour against the published thresholds. Below it: what St. Louis's 1991–2020 climate averages say about the months that usually work.
This week in St. Louis
Asking the sky…
Fairsky is not a substitute for official warnings. For warnings, go to weather.gov. Verdicts are guidance, not guarantees. Your product's label always wins.
When putting down pre-emergent usually works in St. Louis
- On 1991–2020 averages, St. Louis reaches 200 growing degree days (base 50°F, counted from January 1) during April. Purdue's turf research ties crabgrass emergence to about 200 GDD.
- Fairsky aims to have pre-emergent down by about 150 GDD, which the averages reach during April. Real years run early or late, which is why the live soil estimate above matters.
- The normal daily mean temperature first reaches 50°F in April. Shallow soil follows the multi-day air average, and crabgrass starts sprouting once soil holds about 55°F.
| Month | Avg high | Avg low | Days ≥0.10" rain | GDD50 to date |
|---|---|---|---|---|
| January | 40° | 24° | 4.3 | 7 |
| February | 46° | 28° | 4.4 | 24 |
| March | 56° | 36° | 6.4 | 107 |
| April | 68° | 46° | 7.3 | 344 |
| May | 77° | 57° | 8.6 | 859 |
| June | 86° | 65° | 6.5 | 1615 |
| July | 89° | 69° | 6.0 | 2511 |
| August | 88° | 66° | 4.9 | 3346 |
| September | 81° | 57° | 4.3 | 3914 |
| October | 69° | 46° | 5.4 | 4185 |
| November | 55° | 36° | 5.3 | 4250 |
| December | 44° | 29° | 5.2 | 4263 |
Station: Cahokia St Louis Airport (NOAA NCEI USW00003960), about 7 km from central St. Louis. “Rain days” count days with 0.10" or more of precipitation.
The rules we check
- Soil temperature is estimated from the average air temperature over about five days (past and forecast); Fairsky labels it as an estimate.
- Spring: below about 50°F soil it's too early; roughly 50–58°F is prime time; up to 65°F is late but not lost; above that, crabgrass is already up.
- Rain helps: about a quarter to half an inch in the next three days waters it in. More than an inch in the next 24 hours means hold off, since a downpour can wash granules away. (Fairsky default: no source gave a number)
- Fall (August–October, soil 55–70°F): a window for winter weeds like henbit, chickweed and Poa annua.
- Wind under 20 mph while spreading. (Fairsky default: no source gave a number)
Your product's label always wins. Labels differ by product; in the app you can enter your label's temperature range and rain-free hours, and the verdict uses them instead.
Sources: Purdue Turf: pre-emergence timing · K-State: predicting crabgrass emergence · Iowa State: how to control crabgrass
More home and yard jobs in St. Louis
- Stain a deckDeck and fence stain: 50–90°F, dry wood, 24 rain-free hours after.
- Paint a house exteriorExterior latex paint: 40°F and up, 5°F above the dew point, 4 dry hours after.
- Pour concreteConcrete: 40–85°F, no rain for 24 hours, no freezing for 48.
- Spray weedsWeed killer: 3–10 mph wind, 45–85°F, 4 rain-free hours after.
- Plant grass seedCool-season grass seed: soil about 50–65°F, highs 60–80°F, no washout rain.
- Seal a drivewayAsphalt sealcoat: 55°F and rising, 50°F overnight, 36 rain-free hours.
- Cover plants for frostFrost watch: cover at 34°F, or 36°F on clear, calm nights; 28°F is a hard freeze.
The app covers 47 activities in all, from mowing to a round of golf: see every activity and its rules.
Apply pre-emergent in nearby cities
- Indianapolis, IN230 miles away
- Kansas City, MO238 miles away
- Memphis, TN240 miles away
- Louisville, KY242 miles away
- Nashville, TN253 miles away
- Chicago, IL262 miles away
All apply pre-emergent cities: Apply pre-emergent, by city · Home and yard in Missouri
Questions
How is this verdict made?
We take the National Weather Service hourly forecast for St. Louis and check each hour against the thresholds listed above. The result is guidance, not a guarantee; your product's label always wins.
When should I put down pre-emergent in St. Louis?
Before crabgrass germinates, which follows soil warming into the mid-50s °F. The verdict above estimates St. Louis's soil temperature from the forecast; follow your product's label.
Fairsky is not a substitute for official warnings. For warnings, go to weather.gov.
Live forecasts: National Weather Service / NOAA (public domain), fetched by your browser. Climate averages: NOAA NCEI U.S. Climate Normals, 1991–2020 (public domain). Activity thresholds: product data sheets, ACI and university extension guidance, linked on each page; your product’s label wins.