How often should you put down crabgrass preventer?

Why bother

Miss the spring window and crabgrass owns the summer. A thorough-tier job. Skippable when life's busy, worth it when it isn't.

Why it's a range

Go down in early spring before the soil hits 55F, around forsythia bloom. The date shifts months by region, and a long spring may want a second round.

What the job involves

1. Time it to the soil, not the calendar. You want this down before the crabgrass wakes up, which is when the top 2 inches of soil sit around 55 degrees for a few days. The old gardener's cue is when the forsythia or redbud is in full bloom, usually mid-March to mid-April depending where you are.
2. Spread it even with a calibrated broadcast spreader at the rate on the bag. A patchy coat leaves gaps, and crabgrass walks straight through the gaps. Granular or a sprayed liquid, both work, just cover the whole lawn properly.
3. Water it in with about a quarter to half an inch, or get it down just before rain. The herbicide needs to soak in and form its barrier in the soil where the seeds are sitting. Dry on the surface, it does nothing.
4. Don't seed at the same time. A standard preventer kills new grass seed right along with the weed seed. Either seed first and skip the preventer, or wait about 60 days and two mowings before seeding, or use a seeding-safe one like siduron (Tupersan).

Do it yourself, or pay someone?

Yourself, for most lawns. A bag covers a typical yard for $30 to $72 and the only trick is timing it and spreading it even. Paying a pro runs about $90 or more for a standalone visit, or it's folded into a spring lawn program at $50 to $250, and that's mainly worth it for a big or sloped lot, or if you keep missing the window.

What skipping it costs

$400 to $1,800 to reseed a lawn once crabgrass has thinned it out, and a severe infestation pushes toward a full resod at $1.50 to $4.00 a square foot. Set that against a $30 to $72 bag, or a roughly $90 pro application, that would have stopped it. The preventer is a once-a-year, narrow-window job. Miss it and that summer's crabgrass germinates unchecked, a bad year seeds the soil for the next, and once it's established it crowds the turf out. Pre-emergent won't touch what's already sprouted, so the only fix left is killing it and renovating the bare patches it leaves behind.

Signs it's overdue

• Coarse, light-green grass sprawling in low clumps come mid-to-late summer, worst along the driveway and sidewalk edges where the soil warms first. That's crabgrass that got past you.
• The forsythia or redbud is in bloom, or the soil at 2 inches is hitting 55 degrees, and the bag's still in the shed. The germination window is opening.
• Last summer's lawn had crabgrass patches you never pulled before they set seed. That's a fresh batch already in the soil, just waiting on warm weather.

When to start thinking about it

Give yourself about 7 days' notice. That's when Grumpty starts grumbling, so it doesn't land on you the day it's due. Mark it done and Grumpty sets the next one, every 12 months, so you can go back to forgetting it exists.

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