· by Michael Blair, Nashville's Weed and Feed
Pre-emergent herbicide stops seeds germinating. Applied after they have germinated it does nothing, which makes it the one treatment where timing decides the outcome entirely.
At what soil temperature does crabgrass germinate?
Crabgrass germinates as soil temperature at seed depth sustains around 55°F. In Middle Tennessee that can arrive in late February in a mild year and mid-March in a cold one.
Watching soil temperature rather than a date is what gets it right, and several local sources publish it.
Why split pre-emergent into two applications?
A single application often loses effectiveness before the germination window closes. Splitting the rate into two applications six to eight weeks apart extends the barrier through the season.
That second application matters more here than in cooler regions because our germination window is long.
- Watch soil temperature, target around 55°F
- Split into two applications six to eight weeks apart
- Water it in unless the label says otherwise
- Do not disturb the barrier by raking or aerating after
- Never apply where you plan to seed
Do not break the barrier
Pre-emergent works as a chemical layer at the soil surface. Raking, aerating or heavy cultivation after application breaks it and lets seeds through.
That is why aeration belongs in autumn rather than after a spring pre-emergent.
The seeding conflict
Pre-emergent does not distinguish crabgrass seed from fescue seed. Applying it and then overseeding wastes the seed entirely.
That is the main reason autumn overseeding and spring pre-emergent sit at opposite ends of the year in a Middle Tennessee program.
FAQ
Pre-Emergent Timing Decides Your Whole Year — questions
When should I apply pre-emergent in Nashville?
When soil temperature at seed depth is approaching 55°F sustained, which in Middle Tennessee can be late February in a mild year and mid-March in a cold one. Watching soil temperature rather than a calendar date is what makes the difference, and several local sources publish it.
Can I apply pre-emergent and overseed at the same time?
No. Pre-emergent prevents seeds germinating and does not distinguish crabgrass from fescue, so you would be paying for seed that cannot come up. That conflict is why overseeding belongs in autumn and pre-emergent in late winter in a Middle Tennessee program.
Is one pre-emergent application enough?
Often not here, because our germination window is long. Splitting the season rate into two applications six to eight weeks apart maintains the barrier through the whole period. A single early application frequently loses effectiveness before crabgrass has finished germinating.
How does pre-emergent work?
It forms a chemical layer at the soil surface that stops seeds from germinating, so it does nothing to weeds that are already up. Raking, aerating or heavy cultivation after it goes down breaks that layer and lets seeds through. It cannot tell crabgrass seed from fescue seed, so it stays off ground you plan to seed.