When to Apply Pre-Emergent: Soil Temperature, Regional Timing and Splits

Pre-emergent herbicides are absorbed by germinating seeds in the top inch or so of soil and stop root and shoot development, so the seedling dies before it ever breaks the surface. The barrier forms after the product is moved into the soil by rain or irrigation. Anything that disturbs that layer, such as core aerating, dethatching or tilling after application, can open gaps.

They are most useful against annual weeds that come back from seed every year: crabgrass, goosegrass, foxtail, prostrate spurge and, with fall applications, winter annuals like henbit, chickweed and Poa annua. The comparison with products that kill weeds already growing is on pre-emergent vs post-emergent.

Apply before soil at a 2 to 4 inch depth holds about 55 F for several consecutive days, which is when crabgrass begins to germinate. Germination accelerates as soil warms further, so the aim is to have the barrier in place and watered in just ahead of that point. A week or two early is better than a week late.

• Measure it: push a soil thermometer into a sunny part of the lawn in the morning, when soil is coolest, on several days in a row.
• Use soil temperature data: many state extension services and university weather networks publish soil temperature readings and maps.
• Watch plant cues: many extension services use forsythia bloom as a rough natural signal; crabgrass germination typically follows the fading of forsythia flowers.
• Remember microclimates: south-facing slopes and soil along driveways and sidewalks warm first, so crabgrass often appears there before the rest of the lawn.

These seasonal windows are general guidance for spring crabgrass prevention. Years run early or late, so confirm with soil temperature. Fall applications for winter annual weeds apply mainly to warm-season lawns and to cool-season lawns that will not be overseeded.

Splitting is a common professional practice when the label allows it. You apply part of the yearly rate at the normal spring timing and the rest roughly 6 to 8 weeks later, keeping the total within the label's maximum per year. The second application extends the barrier into summer, when late germinators such as goosegrass and prostrate spurge come up and when a single spring application may be fading, especially in the South with its long growing season.

Splits work best with sprayable concentrates, where you control the rate exactly: the Quali-Pro Prodiamine 65 WDG is a sprayable prodiamine sold for spring and fall pre-emergent programs. Granular bags make splits harder, because spreader settings for half rates are less precise. If you split, write down dates and amounts, because the label's yearly maximum still applies. For a sprayer, see best lawn sprayers.

Almost always. Most pre-emergents must be moved into the top layer of soil by about half an inch of rain or irrigation to form the barrier, and many labels give a time window for that, often within a few days. Product left sitting on dry leaves and thatch for weeks can break down in sunlight or be lost. Follow your label, because directions differ: the Scotts Halts listing calls for application to a dry lawn, and some products say to water in promptly.

• A light, soaking rain in the forecast is ideal. A heavy downpour right after application is not; it can wash granules off slopes and hard surfaces into storm drains.
• Sweep granules off driveways, sidewalks and streets back onto the lawn before watering.
• Do not core aerate, dethatch or power rake after applying; do those jobs before, or in a season you are not using a pre-emergent.
• Keep people and pets off the treated area for the time the label states.

Pre-emergents only stop seeds as they germinate, so they miss anything already growing and most weeds that spread from roots, rhizomes, stolons or tubers:

• Weeds that already emerged: visible crabgrass needs a post-emergent such as BASF Drive XLR8 (quinclorac) or Syngenta Tenacity (mesotrione). See crabgrass.
• Established perennial broadleaves: dandelion, white clover, creeping charlie, wild violet and broadleaf plantain come back from roots and need a broadleaf weed killer.
• Nutsedge: spreads from underground tubers; use a sedge product such as Sedgehammer Plus. See yellow nutsedge.
• Perennial grasses: quackgrass, dallisgrass and similar grasses are not stopped by crabgrass preventers.
• Weeds that germinate after the barrier fades or in spots that were missed, disturbed or washed out.

Pre-emergents do control many annual grassy weeds and several annual broadleaves when timed right, including goosegrass, foxtail, prostrate spurge and, in fall, henbit, chickweed and Poa annua. Check each product label's weed list.

If you can already see crabgrass seedlings across the lawn, a standard pre-emergent will not kill them, though it can still prevent later flushes over the rest of the season. Dithiopyr products are labeled with some early post-emergent activity on very young crabgrass, so they are the usual choice for a slightly late application; follow the label's growth stage limits. For established crabgrass, switch to a post-emergent labeled for your grass type, and plan to put the pre-emergent down earlier next year, based on soil temperature.

Not for a while. Most pre-emergents stop grass seed just as they stop crabgrass seed, and labels set waiting periods between application and seeding that can run several months depending on product and rate. Choose one per season: if you plan to overseed a cool-season lawn in late summer to early fall, do not use a fall pre-emergent, and if you must seed in spring, skip the spring crabgrass preventer.

Mesotrione is the main exception. The Syngenta Tenacity label allows application at seeding for listed cool-season grasses, where it prevents many weeds without stopping the new grass. Read the grass list and rate directions carefully. Full seeding steps are in how to overseed a lawn.

Most lawn pre-emergents use a handful of active ingredients, sold under many brand names. The table groups them as general classes. Residual length, rates and grass safety depend on the specific product and rate, so the label, not this table, is the authority. Only catalog products with listed active ingredients are named here.