Post-Emergent Herbicide: When and How to Apply It Correctly

Post-emergent herbicide is the tool for weeds that are already growing in your lawn — but it only works if you match the product to the weed and catch it while it's still young. Get either of those wrong and you either waste the application or damage the grass you're trying to protect.

Here's how to get it right.

What Post-Emergent Actually Does

Post-emergent herbicides kill weeds that have already germinated and are actively growing, unlike pre-emergent, which only prevents weed seeds from sprouting in the first place. If crabgrass, dandelions, or clover are already visible in your lawn, pre-emergent won't do anything for them — that ship has sailed. Post-emergent is the tool for that job.

Selective post-emergents target specific weed categories — broadleaf weeds or grassy weeds — without harming your turfgrass. Non-selective products (like glyphosate) kill everything they touch, including your lawn, and should only be used for spot-treating isolated patches you're willing to reseed.

Match the Product to the Weed

This is the step that trips people up most. Broadleaf herbicides (2,4-D, dicamba, triclopyr) kill dandelions, clover, and ground ivy — but they won't touch grassy weeds like crabgrass or nutsedge at all. For those, you need a product specifically labeled for that species: Drive XLR8 for crabgrass, Sedgehammer for nutsedge.

Identify the weed before you buy anything. A broadleaf product on a grassy weed problem is a wasted application.

Timing by Season

Avoid applying above 85°F — heat makes herbicide more volatile, increases the chance of drift onto desirable plants, and stresses grass that's already dealing with summer heat. Early morning on a calm day is the safest window.

Application Best Practices

• Apply to actively growing weeds. Wilted or drought-stressed weeds won't absorb herbicide effectively — water the day before if the lawn is dry.
• Don't mow for 2–3 days before or after. Weeds need leaf surface area to take in the herbicide; mowing too close to application cuts that surface away.
• Check grass type compatibility on the label. Some products aren't safe for St. Augustine, centipede, or newly seeded lawns — this varies by product, not just by weed target.
• Watch the forecast. Rain within 24 hours washes the herbicide off leaf surfaces before it's absorbed.

Common Mistakes

Applying in high heat. Above 85°F, herbicide becomes more prone to drift and the grass is more likely to show damage. Stick to a cooler, calmer application window.

Reaching for a non-selective herbicide out of convenience. Glyphosate kills everything, including your lawn. Save it for spot treatments where you're prepared to reseed the area.

Expecting one application to finish off perennial weeds. Dandelions, ground ivy, and other perennials typically need 2–3 applications spaced 2–3 weeks apart before they're actually gone.

Applying right before rain. Give the herbicide at least 24 hours of dry weather to be absorbed before rain has a chance to rinse it away.

Frequently Asked Questions

When is the best time to apply post-emergent herbicide?

Apply when weeds are young and actively growing, in mild temperatures between 60–80°F — spring (April–May) or fall (September–October) for cool-season lawns, late spring or early summer for warm-season lawns. Actively growing weeds absorb herbicide far better than drought-stressed or dormant ones.

What's the difference between pre-emergent and post-emergent herbicide?

Pre-emergent prevents weed seeds from germinating and has to go down before weeds appear. Post-emergent kills weeds that have already sprouted and are visibly growing. If weeds are already up in your lawn, pre-emergent won't touch them — you need post-emergent instead.

Can I apply post-emergent and fertilizer at the same time?

Combined "weed and feed" post-emergent products exist, but check the label — the fertilizer portion may not line up with your grass type's actual fertilization calendar. Applying them separately usually gives you more control over both the weed control and feeding schedule.

How long after applying post-emergent can I mow?

Wait 2–3 days after application before mowing, and avoid mowing for 2–3 days beforehand too. Weeds need leaf surface area to absorb the herbicide, and mowing right before or after application reduces how much gets taken up.

Do I need to apply post-emergent more than once?

Perennial weeds like dandelions and ground ivy often need 2–3 applications spaced 2–3 weeks apart for complete control. A single application is more likely to fully work on annual weeds caught while still young.

Is post-emergent safe to use on St. Augustine or centipede grass?

Not always — some post-emergent products aren't labeled safe for St. Augustine, centipede, or newly seeded lawns of any grass type. Always check the label for grass type compatibility before applying, since the wrong product can damage the turf you're trying to protect.

Missed the pre-emergent window and now you're looking at weeds already up in the lawn? That's exactly what post-emergent is for — see the pre-emergent timing guide to be ready earlier next season.

Get a personalized schedule with treatment timing built around your zip code and grass type, or browse lawn care schedules by city for a zone-specific calendar.