Pre-emergent vs Post-emergent Herbicides

Dealing with weeds is an inescapable fact of garden life.

You can weed by hand. But it can be a frustrating and tiring job to keep on top of. So instead, many gardeners use herbicides to make it easier.

There are many different types of herbicides available. And one of the big distinctions is pre-emergent vs post-emergent herbicides.

When do you use a pre-emergent vs post-emergent?

I’ll give you the essential information you need to understand the difference between these herbicides and when to use them for the best results.

Let’s dive in!

Pre-emergent Herbicides

Pre-emergent herbicides prevent the development of weed seeds after germination. They work by inhibiting cell division, preventing the seedlings from establishing a root system.

For pre-emergent herbicide to work, you need to apply it to the ground before weed seeds germinate. Ideally, this will be a couple of weeks before. But at the latest, make sure you apply pre-emergent to the ground 36 to 72 hours before germination occurs.

The most common time of year to apply pre-emergent herbicide to the ground is early spring. This prevents summer annual weeds such as crabgrass, goosegrass, and foxtail.

Many gardeners also apply pre-emergent in late summer to early fall to prevent winter annual weeds such as annual bluegrass (Poa annua), henbit, and chickweed.

Pre-emergent herbicide comes in 2 types, granular and liquid. And once you apply either to the soil you need to water it in. This activates the pre-emergent and helps it to soak into the soil where it forms a chemical barrier that kills weed seedlings upon contact. You can water the pre-emergent yourself or you can wait for rainfall. But be careful, heavy rain can wash the pre-emergent away.

It’s also worth noting that pre-emergent herbicide is most effective at preventing weed seed development. So it doesn’t work well against perennial weeds that grow back each year from established root systems.

See our list of pre-emergent herbicide chemicals.

Post-emergent Herbicides

Do you have weeds already growing in your yard that you want to kill? If so, then you need a post-emergent herbicide.

You can use a post-emergent whenever weeds emerge. The earlier you deal with them the easier they are to kill. Mature plants often take multiple applications. And by killing them young you’ll prevent the weeds from spreading more seeds around your yard.

Post-emergent herbicides come in a variety of types:

  • Contact herbicides kill weeds on contact. But because they only affect the parts of the plant that you spray, the roots often survive. And sometimes the weeds can regrow.
  • Systemic herbicides are absorbed through the foliage of the plant. Then they’re transported around the plant’s circulatory system killing it from the inside. Because they kill the roots as well they prevent weed regrowth.
  • Non-selective herbicides kill all plants they come into contact with.
  • Selective herbicides target broadleaf weeds and certain species of grassy weeds. These are the post-emergents to use on your lawn as they don’t harm most common lawn grasses.


Should I use pre-emergent or post-emergent?

The main difference between pre-emergent and post-emergent herbicides is the stage at which they kill weeds. If you’re trying to prevent weeds from growing in your garden then you should use a pre-emergent. A pre-emergent herbicide will stop weed seeds from developing into mature plants.

If you have weeds already growing in your garden then you’ll need to use a post-emergent to kill them.

Some people like to use a pre-emergent and a post-emergent to keep their garden weed-free throughout the growing season.

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

Pre-emergent and post-emergent herbicides are usually used at different times. You should use a pre-emergent first before weed seed germination occurs to prevent weeds from growing. Then use a post-emergent to spot treat weeds that emerge.

Is Roundup a pre or post-emergent?

Roundup is a post-emergent weed killer. It contains glyphosate, a non-selective and systemic herbicide that kills all vegetation that you apply it to.

Roundup is a good choice for killing weeds in areas where you don’t want anything to grow. But it’s not suitable for weed control on lawns. And you should be careful using it around desirable plants in your garden.

Is prodiamine a post-emergent?

Prodiamine is not a post-emergent, it is a pre-emergent herbicide. You can use prodiamine to control many common grassy weeds and broadleaf weeds. You’ll find prodiamine in the popular brand ‘Barricade’.

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