Best Time to Use Crabgrass Preventer: The Soil Temperature Rule and Regional Timing

The best time to use a crabgrass preventer is just before crabgrass seeds start to sprout, which happens when soil temperatures reach about 55 degrees Fahrenheit in spring. In practice that means late winter in the Deep South, early to mid April across much of the central US, and late April to early May in northern states. Put it down a little before that trigger, water it in with about half an inch of rain or irrigation within a few days, and it will stop most of the season’s crabgrass before you ever see it. Put it down after the seeds sprout and most products will do nothing at all.

This guide explains the soil temperature trigger, the plant and weather clues that tell you the window is open, typical timing by region, what happens if you are too early or too late, how rain and mowing fit around the application, what to do if you also want to seed, and how to deal with crabgrass that has already come up. It draws on guidance from Iowa State University Extension, University of Illinois Extension, University of Georgia Extension and Purdue University’s turfgrass program.

The short answer

• The trigger: crabgrass begins to germinate when soil reaches about 55 degrees Fahrenheit. Illinois Extension puts it at 55 to 60 degrees for seven to ten consecutive days.
• The target: have the preventer down and watered in before that happens.
• The easy clue: Iowa State suggests applying when forsythia blossoms start dropping or redbud trees begin to bloom.
• Watering in: aim for about half an inch of rain or irrigation within three to five days of applying.
• Too late is worse than a little early: most preventers do not work on crabgrass that has already sprouted.
• Seeding this spring? Most preventers also stop grass seed. Check the label, or use a product made for new seedings.

Why timing matters more than the product

Crabgrass is an annual. The University of Georgia explains that it sprouts from seed, grows to maturity, sets seed and dies within a single growing season, so every crabgrass plant you see in summer started as a seed that spring. Iowa State adds that individual plants can produce thousands of seeds each, and Illinois Extension puts the figure at upwards of 150,000 seeds in a single season, with seeds remaining viable in the soil for three years.

A crabgrass preventer is a preemergence herbicide. Iowa State describes how these products work by disrupting the germination process, which is why they must be applied before the seeds germinate and are not effective on emerged weeds. That is the whole story of timing. A perfect product applied a week after germination begins has already missed part of its target, and one applied a month late may miss most of it.

Iowa State lists benefin, bensulide, pendimethalin and dithiopyr among the active ingredients that control crabgrass in established lawns. The label on whichever product you choose will give the rate and any restrictions, but none of them can fix bad timing.

The soil temperature trigger

The sources tie crabgrass germination to warming soil rather than to a calendar date. The details differ slightly by region and by how deep the reading is taken.

The “consecutive days” part of the Illinois guidance matters. One warm afternoon does not start germination. A run of warm days that brings the soil up and keeps it there does. That is why a cold snap in April can push the window back even after an early warm spell.

How to check soil temperature at home

You do not need special equipment beyond an inexpensive soil or probe thermometer. A few habits make the reading more useful.

• Match the depth. The Georgia guidance refers to a 4 inch depth. Push the probe to a consistent depth each time.
• Read at the same time of day. Soil warms through the afternoon, so a morning reading is a more conservative guide.
• Look for a trend, not one number. Several days in a row near the threshold is the signal the Illinois guidance describes.
• Check a typical spot. Soil along a south facing driveway or wall warms earlier than the middle of the lawn.

If you would rather not probe the ground, many state weather networks publish soil temperatures. The Georgia article, for example, points readers to the UGA Weather Network for current readings at a local station.

Nature’s calendar: forsythia and redbud

Plants respond to the same warming soil that crabgrass does, which makes them a handy timer. Iowa State suggests applying preemergence herbicides when forsythia blossoms start dropping or when redbud trees begin to bloom, noting that crabgrass germination typically, although not always, starts after these events.

The University of Georgia describes an older version of the same rule: get your preemergence herbicide down before the forsythia blooms. The two are not really in conflict. Georgia’s advice leans earlier, in keeping with its view that being a little early is better than being late, while Iowa’s gives the latest sensible moment. If you see forsythia in full bloom and have not applied yet, it is time.

Growing degree days

For those who like data, Purdue’s turfgrass program describes a growing degree day model. Research suggests that about 200 growing degree days need to accumulate, using a base of 50 degrees Fahrenheit, before crabgrass germinates. A growing degree day is a measure of accumulated warmth above that base, so the model captures a run of warm days rather than a single reading.

Several university and weather services publish growing degree day maps and calculators for turf. They are useful for spotting whether spring is running early or late in your area, and for confirming what the soil thermometer and the forsythia are already telling you.

Typical application windows by region

Because the trigger is soil temperature, the calendar window moves north as spring does. Here is what the extension sources we read give for their own states.

If your state is not listed, use the pattern. Southern lawns open the window in late winter, mid latitudes in April, and northern lawns in late April or May. Your local extension office will usually publish its own dates, and the soil temperature trigger works anywhere.

Adjusting for a warm or cold spring

Spring weather varies from year to year, and Iowa State recommends shifting within the normal window to match it. If March and April are consistently warmer than normal, apply early in the usual period. If the early spring is cold, apply later in the period.

This is where the soil thermometer and the flowering shrubs earn their keep. A calendar date is an average. A soil reading or a forsythia in bloom reflects this year’s conditions in your own yard.

Can you apply crabgrass preventer too early?

Yes, but the risk depends on where you live. Iowa State and Illinois both explain the tradeoff. Applied too early, a preventer gives good control at first, but crabgrass seeds that germinate late in the season may not be controlled, because crabgrass keeps germinating from spring into mid summer.

The University of Georgia takes a different view for its region. It advises that it is better to be a little too early than too late, and notes that recent research has found applications in January still gave acceptable crabgrass control, because the herbicide does not break down as rapidly in cold weather as it does once temperatures rise. Purdue makes a related point: preemergence herbicides do not break down in the soil until late spring, when soils warm and microbial activity increases.

Putting these together: an application a few weeks early is usually fine, because cold soil preserves the product. An application months early in a region with a long germination season is where late summer breakthrough becomes more likely. That is the case for the split application some lawn programs use, with half the labeled rate applied early and the rest several weeks later, but follow the product label for whether and how that is allowed.

What happens if you apply too late

Too late is the more common and more costly mistake. Illinois Extension describes the result: the first flush of crabgrass germinates while later germination is suppressed, so you lose part of the season’s control. The University of Georgia compares a late application to closing the gate after the horse has left the barn.

There is one partial exception. Purdue notes that dithiopyr controls crabgrass after germination until it reaches one tiller, the stage when the young plant starts to produce side shoots. So if you are slightly late, a product containing dithiopyr can still catch very young crabgrass. Once plants are larger, preventers are no longer the right tool.

A late application is still not wasted. Illinois points out that other grassy weeds germinating at soil temperatures of 60 degrees and higher will still be managed, even if part of the crabgrass flush got through.

Before or after rain?

Preventers need water to move them off the grass blades and into the top layer of soil where the seeds are. Purdue states that preemergence herbicides must be applied and watered in, through rainfall or irrigation, before crabgrass germination.

• Iowa State recommends timing the application to receive about half an inch of rainfall or irrigation within three to five days.
• The University of Georgia suggests applying before rain is expected, or watering in with half an inch of irrigation if the label requires it.
• Avoid applying just before a heavy storm that could wash granules off slopes and paving.

In short, apply shortly before a gentle rain, or apply and then water. A long dry spell after application is the situation to avoid.

Should you mow before or after?

Mowing just before applying is fine and makes the granules easier to see and spread. After applying, the University of Georgia recommends delaying mowing until after a rainfall or irrigation, so the product has settled into the soil before the mower passes over it.

Mowing height matters all season. Iowa State recommends keeping turf at a 3 to 4 inch height of cut, because taller grass shades out germinating crabgrass. Illinois Extension recommends mowing high and often with a sharp blade, and removing no more than one third of the grass blade at any mowing. If your mower is due for an upgrade, our guide to the best lawn mowers covers options for every lawn size.

Spreading it evenly

Most homeowners apply granular preventers with a spreader, and uneven coverage leaves gaps where crabgrass comes through in stripes. The University of Georgia lists calibrating your application equipment among its key steps.

• Use the spreader setting listed on the preventer’s label for your model, not the setting from your usual lawn food, since granule size and rate differ.
• Walk at a steady pace and overlap passes so the edges of each pass meet.
• Pay attention to edges along driveways, sidewalks and curbs, which warm first and are classic crabgrass spots.
• Sweep any granules off paving back onto the lawn.

A drop spreader or a broadcast spreader with an edge guard helps keep product on the lawn. For liquid preemergence products, a well calibrated sprayer does the same job. Our guide to the best lawn and garden sprayers covers the options.

Crabgrass preventer and grass seed

This is the conflict that catches many homeowners. Iowa State explains that most preemergence herbicides also prevent the germination of turfgrass seed. The University of Georgia gives the same warning: do not overseed soon after applying a crabgrass preventer, and check the label for the required interval between application and seeding.

• Seeding in spring: Iowa State says the only preemergence herbicide that can be used is siduron, which controls crabgrass without affecting turfgrass seed germination.
• Seeding in late summer: Iowa State notes grass seed can be sown successfully from mid August to mid September after a spring application of any preemergence herbicide, because by then the herbicide has broken down.
• Young grass: the Georgia article notes young fescue can be sensitive to some weed killers, so read labels carefully on newly seeded areas.

If you need to repair thin areas, the cleanest plan is often a spring preventer on the established lawn and seeding in late summer, which also happens to be an ideal time to seed cool season grasses.

Weed and feed products

Many crabgrass preventers are sold combined with fertilizer. Iowa State notes these combinations let you apply a preemergence herbicide and a turf fertilizer in one application. The convenience is real, but the timing is set by the crabgrass, not the fertilizer. Apply a combination product in the crabgrass window, and plan the rest of your feeding schedule around it.

Iowa State also points out that a thick, healthy lawn is the best long term defense, and it recommends fertilizing established turf in the fall. If you are planning a year of feeding, our guide to the best slow release fertilizers explains the different formulas.

Corn gluten meal as a natural option

Iowa State describes corn gluten meal, a corn milling byproduct, as a product that interferes with crabgrass seed germination when applied in spring. It also acts as an organic fertilizer, containing roughly 10 percent nitrogen. Two points from the Iowa State guidance are worth knowing before you choose it.

• It should be applied at the same time as chemical preemergence herbicides, so the timing rules in this guide apply equally.
• It becomes more effective after two or three years of repeated application, so expect gradual rather than immediate results.

Is July too late? Dealing with crabgrass that already sprouted

For prevention, yes. By early summer most of the germination you were trying to stop has happened, and a preventer will not kill plants that are already growing. What you can do instead depends on the size of the plants.

• Young plants: Iowa State says postemergence herbicides are most effective on small, actively growing crabgrass at the three to five leaf stage.
• Established plants: Iowa State notes postemergence herbicides are generally ineffective against large, well established crabgrass, and some selective products are only available through licensed applicators.
• A few plants: hand pulling or digging before they set seed reduces next year’s seed bank. Iowa State stresses that even one plant allowed to go to seed can produce thousands of seeds.
• A badly infested lawn: Iowa State suggests considering total lawn renovation in the fall.

Product choice matters here, because Iowa State notes some postemergence herbicides affect certain lawn grasses differently, so always match the label to your grass type. For warm season lawns, our guide to the best crabgrass killer for Bermuda grass covers products labeled for that situation.

Build a lawn that crowds crabgrass out

Preventers buy time, but the long term fix is turf that leaves no room. Iowa State calls a thick, healthy lawn the best way to prevent crabgrass, because crabgrass has a difficult time germinating and surviving in dense turf. Its recommendations are straightforward.

• Fertilize established cool season turf in the fall.
• Seed thin spots so there are fewer openings letting light reach the soil.
• Mow at 3 to 4 inches so the grass shades germinating seeds.

Illinois Extension notes that crabgrass takes advantage of thin or damaged areas, such as strips along the road damaged by winter salt, and that taller grass also keeps the soil cooler for longer, which delays germination. Because crabgrass seeds can stay viable for three years, expect to combine prevention and better turf for a few seasons before the seed bank runs down.

A simple spring plan

1. In late winter, look up your local extension’s suggested window and decide whether you will seed this spring.
2. Buy the right product: a standard preventer if you are not seeding, or a product made for new seedings such as one containing siduron if you are.
3. Start checking soil temperature and watching forsythia and redbud as the window approaches.
4. Mow if needed, then apply at the labeled rate with a calibrated spreader, a little before the trigger rather than after it.
5. Water in with about half an inch within three to five days, or apply just before a gentle rain.
6. Wait to mow until after that rain or irrigation.
7. Through the season, mow high and pull any escapes before they set seed.
8. In late summer, seed thin areas, and in fall, fertilize to thicken the lawn for next year.

Frequently asked questions

Watch the soil and the forsythia, apply a little before the trigger, water it in, and keep the lawn thick and tall. Crabgrass is predictable, and that is exactly what makes it preventable.