Diagnose and fix
Crabgrass Prevention in Northern Virginia
By Patrick Klippen, Owner & CEOB.S. Landscape Horticulture · VDACS certified applicator #141054
Published Updated
Short answer
Crabgrass is a summer annual that returns every year from seed, so prevention matters far more than cleanup. Put pre-emergent down before soil at two inches holds near 55°F, typically mid-March to early April in Northern Virginia, and split it into two applications to cover the late May flush. Killing mature crabgrass in August does almost nothing for next year.
Why prevention matters most with crabgrass.
Prevention matters most because crabgrass is a summer annual, so every plant dies at the first hard freeze and only the seed returns. Killing mature plants in August changes almost nothing since they've already dropped seed, which means the crabgrass you can actually control is next year's.
When does crabgrass germinate in Northern Virginia?
Crabgrass germinates once soil at two inches holds around 55°F for several consecutive days, typically mid-March to early April here. A warm February pulls that forward a week or more, and a cold, wet March pushes it back. Pre-emergent applied after germination is wasted, because it only works on seed.
| Approximate window | Soil temp at 2 inches | What to do |
|---|---|---|
| Late February | Usually below 50°F | Aerate now if you must, and pick up a soil thermometer |
| Early to mid-March | Upper 40s, climbing | Put the first pre-emergent down while forsythia buds are swelling |
| Mid-March to early April | About 55°F for several days | Germination begins, so anything applied now is late |
| Late April into May | 60 to 70°F | Make the second application ahead of the late flush |
| June onward | Above 70°F | The window is closed, so mow tall and plan for fall |
How to track the timing on your own lawn.
Push a soil thermometer two inches into your own turf, since a cheap probe beats any regional calendar.
- From late February, check mid-morning every couple of days in a sunny spot.
- Apply the first round when it reads in the upper 40s and is still climbing.
- Forsythia in bloom is a rough backstop, and once the flowers drop you're behind.
- Write down the date so you can time the second round.
Why one application is usually not enough.
One application usually isn't enough because pre-emergent breaks down in the soil and crabgrass doesn't all germinate at once. We split the season's rate into two applications, usually six to eight weeks apart, so the barrier lasts through the May flush. Our lawn weed control program is scheduled that way, and it comes with every fertilization visit. A typical 8,000 square foot lawn is $65 a visit.
The mistakes that undo a pre-emergent.
Two mistakes undo a pre-emergent, and those are aerating after you apply it and seeding in spring. Aerator holes let seed germinate untreated, and pre-emergent can't tell crabgrass seed from fescue seed. Do aeration and overseeding in fall, which is when seeding belongs here.
Mowing tall is the free half of prevention.
Mowing at three and a half to four inches is the cheapest crabgrass control there is, because crabgrass seed needs light and tall turf shades the soil. Thick fescue also leaves no bare soil, which is why crabgrass shows up first along driveway edges, sidewalk strips, and thin ground under trees. Around Haymarket, those same margins collect Japanese stiltgrass, and one spring application at the right time covers both.
For this year, yes. Pre-emergent only stops seed, so once plants are up the window has passed. Post-emergent can clean up what's already there, but the real fix is timing next March.
No. Pre-emergent blocks grass seed as thoroughly as crabgrass seed, so doing both cancels one out. Use it in spring and seed in fall, when new fescue survives here.
A little, if you catch a few plants early. By midsummer they've set seed, so pulling clears the eyesore without reducing next year's pressure, and you should fill the bare spot afterward.
Some is normal. Even prevention that's timed right leaves a little along hard edges and in thin shade because the local seed bank is deep, and a scattered nuisance means the program is working. The Northern Virginia lawn care calendar shows how the rest of the year supports it.
Related guides
Keep reading.
Lawn care fundamentals
The Northern Virginia Lawn Care Calendar
What to do to a Northern Virginia lawn in every month of the year, and why fall matters more than spring.
Diagnose and fix
How to Get Rid of Japanese Stiltgrass in Virginia Lawns
Identifying Virginia's most overlooked invasive lawn grass, and why spring timing beats pulling it by hand.
Lawn care fundamentals
Tall Fescue Lawn Care in Virginia
The grass most Northern Virginia lawns are made of, and the one yearly habit that keeps it from thinning.
