Can I use pre-emergent on weeds that are already growing?

No — that is the classic mistake. A pre-emergent works by creating a barrier that stops germinating seeds from establishing, so it has essentially no effect on a weed that has already sprouted and is visibly growing. If you already see the weeds, you need a post-emergent. Some products combine both actives to cover both situations, but a straight pre-emergent applied to an existing weed simply will not kill it, and you will have missed the germination window it was meant for.

When do you apply pre-emergent herbicide?

Before the target weed seeds germinate, which is the whole point. For summer annuals like crabgrass, that generally means early spring, before soil temperatures at the surface warm to roughly the mid-50s Fahrenheit; for winter annuals, late summer to fall. Because the barrier degrades over weeks, timing is tight — too early and it breaks down before the seeds sprout, too late and the seeds are already up. Many gardeners tie crabgrass pre-emergent timing to soil temperature or local bloom indicators like forsythia.

Are post-emergent herbicides selective or non-selective?

Both types exist, and the distinction matters. Selective post-emergents kill certain plants while leaving others unharmed — a broadleaf weed killer that spares lawn grass, for example. Non-selective post-emergents like glyphosate kill essentially any green plant they contact, which makes them useful for driveways and clearing beds but dangerous to spray in a lawn you want to keep. Read the label to confirm which you have and what it is safe to apply over.

Do you water in pre-emergent and post-emergent the same way?

No. Most granular pre-emergents need to be watered in shortly after application, because the water activates them and moves the barrier into the top layer of soil where seeds germinate. Post-emergents, especially foliar-applied liquids, are the opposite — they need time on the leaf to be absorbed, so you want a dry window after spraying and should check the label’s rainfast period. Always follow the specific product’s directions, since formulations vary.

Can you apply pre-emergent and post-emergent at the same time?

Yes, and it is a common strategy: a combined or tank-mixed application can kill the weeds already present (post-emergent) while laying down a barrier against the next flush of germinating seeds (pre-emergent). Some retail products are formulated to do both. The key is that each active is still doing its own job — the post-emergent handles the visible weeds and the pre-emergent handles the seeds — so you still have to respect each one’s timing and watering needs. Follow the label.

Pre-Emergent vs Post-Emergent Herbicide: The Difference

Written & researched by Elena Marsh Gardening & Landscaping Editor

Updated October 2026

Close-up detail of the material being worked in a pest & weed control project, in soft early-morning light

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A pre-emergent herbicide stops weed seeds from establishing before they sprout, forming a barrier in the top layer of soil; a post-emergent herbicide kills weeds that are already up and growing. The difference is entirely about timing relative to the weed’s life stage: pre-emergent is prevention applied before you see anything, post-emergent is treatment applied to visible weeds. Use the wrong one at the wrong time — a pre-emergent on grown weeds, or a post-emergent after seeds have already germinated unchecked — and it fails. Many programs use both.

How does a pre-emergent work?

A pre-emergent does not kill seeds outright and does not stop germination itself; it creates a chemical barrier in the surface soil that disrupts the seedling as it tries to establish, so the weed dies right as it emerges. That mechanism has two consequences. First, it must be in place before the seeds germinate — apply it after the weeds are visible and you have missed the entire point. Second, most granular pre-emergents need to be watered in to activate and settle the barrier into the germination zone. The barrier degrades over several weeks to a few months, which is why timing to the target weed’s germination window is so important.

Wide environmental view of a person spraying weed treatment along a garden edge with a pump sprayer, in soft early-morning light

How does a post-emergent work?

A post-emergent acts on the living weed you can already see. It is usually absorbed through the foliage — sometimes the roots — and either burns the tissue it contacts or, if systemic, moves through the plant to kill it more completely, roots included. Because uptake happens through the leaves, post-emergents generally need a dry window after application and work best on young, actively growing weeds. They come in selective forms that spare desirable plants and non-selective forms like glyphosate that kill any green plant they touch, so matching the product to where you are spraying is critical.

Three-quarter view of a person spraying weed treatment along a garden edge with a pump sprayer, in warm late-afternoon light

Pre-emergent vs post-emergent at a glance

	Pre-emergent	Post-emergent
Targets	Germinating seeds	Established, visible weeds
When applied	Before weeds appear	After weeds are growing
How it acts	Soil barrier at germination	Absorbed through leaves/roots
After application	Usually water in to activate	Keep dry; needs foliar uptake
Best on	Known problem seeds (crabgrass)	Weeds you can see

Why is timing the whole game?

For pre-emergents, the window is narrow. Summer annuals like crabgrass want an early-spring application before surface soil temperatures climb into the mid-50s Fahrenheit, while winter annuals call for a late-summer or fall application — miss it and the seeds germinate through the gap. Many gardeners use soil-temperature readings or bloom indicators, such as forsythia flowering, to time crabgrass pre-emergent. Post-emergents are less calendar-bound but still work best on young, unstressed weeds and need a rainfast window. Get either one’s timing wrong and even the right chemistry underperforms.

Overhead view of a person spraying weed treatment along a garden edge with a pump sprayer, in bright midday sun

Using them together

The two are complementary, not competing. A common approach is to hit the weeds already present with a post-emergent while laying down a pre-emergent barrier against the next wave of germinating seeds, either as separate applications or with a combination product formulated to do both. Each active still does its own job and still needs its own timing and watering, so read the label and follow it — in the United States the label is the legally binding instruction. For a chemical-free route, see killing weeds without chemicals.

Where to go next

The non-selective versus selective question deserves its own read: selective versus non-selective herbicides explains which products are safe over a lawn. To pick a product, the best pre-emergent herbicide and the best weed killer for lawns rank the options. Everything sits under the pest and weed control hub.

We never buy, apply or physically handle the products we rank — recommendations come from aggregated owner reviews and expert consensus, and the method is on how we rank. Follow the label and check your local extension service for regional timing.

What Is the Difference Between Pre-Emergent and Post-Emergent Herbicide?