Will pre emergent kill weeds that have already come up?

No. A pre-emergent forms a chemical barrier in the top layer of soil that interferes with cell division in a germinating seedling as it pushes through. A weed that has already emerged has roots below that layer and a functioning shoot above it, so the barrier never touches the growing point. Spraying pre-emergent on a visible weed does essentially nothing. You need a post-emergent product for anything you can already see.

What kills weeds that are already growing?

Post-emergent herbicides. For broadleaf weeds in a lawn, selective products based on 2,4-D, dicamba, MCPP or triclopyr kill the weed without harming most turfgrass. For grassy weeds like crabgrass in turf, quinclorac and similar actives are used. For everything on a driveway or bed edge, non-selective glyphosate kills any green plant it touches. Hand pulling remains effective on small infestations and costs nothing.

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

Often yes, and many combination products exist for exactly this. Some labels permit tank mixing a pre-emergent with a selective post-emergent so you block new germination and kill existing weeds in one pass. Check both labels before mixing, since not every combination is approved and some can injure turf under heat stress. Applying them separately a few days apart is the low-risk alternative.

What happens if you apply pre emergent too late?

The application is largely wasted for that season's target weed. Once crabgrass or another annual has germinated, the barrier cannot affect it, and the residual will run out before the next germination window. It is not harmful, and it will still block any seeds that germinate later in the season, but the main flush is gone. Treat the visible weeds with a post-emergent, then set a soil-temperature reminder for next year.

Does Pre Emergent Kill Existing Weeds? No

Q: Does weed and feed work as a pre emergent?

It depends on the product. Some weed and feed formulations contain a pre-emergent such as pendimethalin or dithiopyr plus fertilizer, and prevent germination. Others contain a post-emergent broadleaf herbicide plus fertilizer, and only kill weeds already present. The bag looks similar either way, so read the active ingredients panel rather than the front label to know which job the product actually does.

Written & researched by Elena Marsh Gardening & Landscaping Editor

Updated October 2026

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

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No. Pre-emergent herbicide does not kill weeds that have already sprouted. It forms a thin chemical barrier in the top layer of soil that disrupts cell division in a germinating seedling as it pushes through — so it only ever affects seeds in the act of sprouting. A weed you can already see has roots below the barrier and a growing point above it, and the product never reaches either. For anything visible you need a post-emergent herbicide, or hand weeding. The two products do genuinely different jobs.

How does a pre-emergent actually work?

Actives such as prodiamine, pendimethalin and dithiopyr are watered into roughly the top inch of soil, where they stay bound to soil particles as a continuous layer. When a weed seed germinates and the shoot and root push through that zone, the herbicide interferes with cell division in the growing tissue and the seedling dies before it ever reaches daylight. Nothing about that mechanism can help you with an established plant: the barrier acts on emerging tissue at a specific depth, and a growing weed simply has no tissue in that zone. This is also why breaking the barrier matters — raking, digging or core aerating after application opens gaps where seeds germinate freely.

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

Which product does which job?

Situation	Product type	Common actives
Weeds not yet germinated	Pre-emergent	Prodiamine, pendimethalin, dithiopyr
Visible broadleaf weeds in turf	Selective post-emergent	2,4-D, dicamba, MCPP, triclopyr
Visible crabgrass in turf	Selective post-emergent	Quinclorac
Anything on a driveway or path	Non-selective post-emergent	Glyphosate, pelargonic acid
Small patch, any weed	Hand pulling	None

The full contrast is set out in pre-emergent versus post-emergent herbicide.

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

Can you use both together?

Yes, and it is a common approach. Many lawn products are sold as combinations that pair a pre-emergent with a selective broadleaf post-emergent, so a single spring pass blocks new germination and kills the dandelions and clover already up. Some labels also permit tank mixing the two separately. Two cautions apply. First, check both labels — not every combination is approved, and some mixes carry a higher risk of turf injury in heat. Second, remember the timing conflict: post-emergents work best on young, actively growing weeds, while pre-emergent timing is driven by soil temperature, and those two windows do not always coincide. Applying a few days apart is the low-risk version.

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

What about weed and feed?

This is where the confusion usually starts, because bags look alike. Some weed and feed products contain a pre-emergent plus fertilizer and prevent germination; others contain a post-emergent broadleaf herbicide plus fertilizer and kill only what is already growing. There is no way to tell from the front of the bag. Read the active ingredients panel: pendimethalin or dithiopyr means pre-emergent, while 2,4-D or dicamba means post-emergent. Our spring picks are in best weed and feed for spring.

What if you already missed the window?

Nothing is harmed, but the main flush is gone for that season. Once crabgrass has emerged, a pre-emergent cannot touch it, and the residual — typically 6 to 12 weeks — will expire before the next germination window opens. The sensible sequence is to treat what is up with an appropriate post-emergent, keep mowing high so the turf shades the soil surface and suppresses further germination, and then set a reminder based on soil temperature rather than the calendar for next year. Timing detail is in when to apply pre-emergent herbicide, and the crabgrass-specific plan in getting rid of crabgrass.

Follow the product label, which is legally binding in the United States, and check your state extension service for regional timing. We never buy, apply or physically handle the products we rank — our method is on how we rank. Product options are in the best pre-emergent herbicides.

Does Pre Emergent Kill Existing Weeds?