Pre-Emergent vs Post-Emergent Herbicide: What Is the Difference?

Use a pre-emergent if:

• Crabgrass, foxtail or spurge filled your lawn last summer, so seed is in the soil.
• You are not planning to seed the lawn in the coming weeks.
• You can apply it on time, before soil reaches germination temperature.
• You have a warm-season lawn with winter annual weeds such as poa annua, henbit and chickweed, which call for a fall pre-emergent.

Use a post-emergent if:

• Weeds are already visible.
• Your problem is perennial broadleaf weeds such as dandelion, clover or plantain, which pre-emergents do not stop.
• You have nutsedge, which spreads from tubers and needs a sedge product.
• Crabgrass escaped your pre-emergent and needs to be knocked out while young.

When soil at a 2 in depth holds about 55 F for several days, which in much of the country lines up with early spring as forsythia finishes blooming. Crabgrass germination rises as soil warms past that point, so the product must already be down and watered in. A soil thermometer or a state extension soil temperature map beats the calendar, because the date shifts by weeks between a warm year and a cold one and between a sunny south slope and a shaded north lawn.

Many labels allow a split application, half the rate at the normal time and the rest several weeks later, which extends control through summer; prodiamine products such as Quali-Pro Prodiamine are commonly used this way and tank-mix with fertilizer. In warm-season lawns, a late summer to early fall application targets poa annua and other winter annuals. See crabgrass for identification and the best crabgrass preventers.

Match the product to the weed and to your grass type, because selective herbicides are labeled for specific turf species and some injure others, such as St. Augustine. The table covers the common lawn weeds; pages for dandelion and yellow nutsedge go deeper.

It depends on the bag. Scotts Weed and Feed5 is fertilizer plus a post-emergent broadleaf control, applied to a wet lawn so granules stick to growing weeds. Other combination bags pair fertilizer with a crabgrass pre-emergent. The honest problem with combination products is timing: the best moment to feed grass and the best moment to kill or prevent weeds do not always line up, so one of the two jobs is often mistimed, and you spread herbicide over the whole lawn when a few weeds needed spot treatment. Separate products applied at the right time generally work better.

The usual causes: it went down after crabgrass had already germinated; it was never watered in; gaps between spreader passes left untreated strips; aeration or dethatching after application broke the treated layer; the rate was cut to stretch a bag; or the weed was a perennial like dandelion or nutsedge that pre-emergents do not control. Fix the timing and coverage next season, and use a post-emergent on young escapes now.