How Long After Spraying Roundup Can You Plant Grass Seed?

If you fall behind on manually removing weeds from your lawn, the situation will quickly get out of hand.

Before you realize it, your whole lawn will be covered in weeds.

The easiest solution to this problem is to use an herbicide, such as Roundup.

However, if you spray your whole lawn with Roundup, it will also kill your grass. 

How long do you need to wait before you plant grass seed after using Roundup?

After using Roundup on your entire lawn, it is recommended to wait anywhere from three days before planting grass seed. This gives the chemicals time to be washed away. Stronger Roundup formulae may require a longer wait time, so check the packaging.

Using Roundup at the right time of the year will also significantly impact how well your grass seed grows.

Keep reading to learn more about planting grass seed after Roundup, including how the herbicide works and the best time to use it.

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How Does Roundup Work?

Roundup is a glyphosate weed killer, which will kill any type of plant because it is a non-selective weed killer.

The glyphosate herbicide works by blocking certain enzyme paths, which prevents essential plant proteins from being produced.

It only takes 30 minutes for the Roundup to become absorbed by the plant tissues.

Once the herbicide is absorbed, it will quickly spread throughout the plant and stop new growth in the roots.

Roundup is sold in different formulas, and it is usually concentrated. 

Always mix the concentrate with the correct amount of water according to the instructions on the label.

For most Roundup products, you will be able to plant lawn grasses three days after application. 

Wait at least 24 hours after Roundup application to plant ornamental flowers, shrubs, or trees.

Some Roundup products take up to one week to reach the roots of a plant. 

These products are highly recommended because they are generally cheaper and more effective than the 3-day formulas.

The most potent formula of Roundup contains an herbicide called diquat.

Diquat is a pre-emergence weed killer, and it not only kills plants and delays the growth of new seeds.

Roundup formulated with diquat does not kill the weed roots. 

This makes diquat less effective than other herbicides because grass may sprout from any viable roots, and you may have to make several applications for the best results.

The Roundup product containing diquat is also not recommended if you would like to plant a new lawn right away.

Diquat is a pre-emergent herbicide, which means it will prevent new seeds from sprouting, and it will remain in the soil for up to four months or longer.

When Is The Best Time To Use Roundup?

The best time to use Roundup depends on what type of grass you have and the weather.

If you are planting warm-season grass such as Zoysia, Bermuda, or Centipede grass, it is best to do it in the fall.

Plant your grass seeds no later than August or September. 

By the time the spring season comes, your new grass will begin sprouting, and you will have a vibrant, green lawn for the summer.

If you want to plant a cold-season type of grass like Kentucky bluegrass, perennial ryegrass, or fescue, it is best to wait until the end of warm weather.

Plant the seeds for cool-season grasses as soon as the weather cools down, no later than the end of October.

Weather is important when it comes to deciding when to use Roundup.

When it is windy outside, spraying Roundup may spread the herbicide to areas you do not want to be affected, such as a nearby flower or vegetable garden.

Wait until conditions are calmer to avoid accidentally spraying in irregular patterns in unwanted areas.

Rain will dilute your herbicide, making it less effective. 

It is best to wait until the grass is dry before spraying Roundup.

The Roundup will soak into the weeds within 30 minutes. 

Once the herbicide has completely penetrated the weeds, it will not be washed away by the rain.

Planting Grass Seed After Using Roundup

Before planting your grass seeds, there are a few things you need to do to ensure your grass grows successfully.

First, you will need to wait until the Roundup has had plenty of time to do its job. 

You will know your lawn is ready when all of the grass turns brown.

It usually takes up to 48 hours for the Roundup to completely kill tougher weeds. 

Larger weeds will take longer to die, and you may need to add another application of herbicide to get the job done.

Carefully read the instructions on your specific Roundup formula to know when to plant the grass seed safely.

Then, you will prepare your lawn for mowing by removing any stray rocks, branches, or other debris which may cause damage to your mower.

Mow your lawn to the shortest possible length and till 3” inches of the soil surface. 

Check out our helpful tables on how often to mow your lawn.

This works the dead weeds into the ground and gives you properly aerated soil ready for planting new grass seed.

Use a metal rake to level out your lawn, and be sure to fill in any areas prone to pooling. 

If you plant grass seed in areas where there is standing water, your grass seed may be washed away before it has a chance to germinate.

Once your lawn is nice and level, use a sower to spread grass fertilizer and use a tiller to ensure thoroughly mixed into the soil.

Choose the best type of grass seed for your climate.

Planting warm-season grass in a cool weather climate, such as St. Augustine, will only lead to disappointment. 

The grass will not be able to survive harsh winter conditions, and you will not have the lush green lawn you had hoped for.

Plant your grass seed with a seed planter, or sow by hand for more precision.

Use a lawn roller or the backside of a rake to work the grass seed into the soil gently. 

Use light pressure to avoid compacting the seeds into the ground.

When all of the grass seed is planted, water the entire lawn. 

The goal is to moisten the soil, so avoid soaking the ground with too much water.

Maintaining Your New Lawn

After your grass seed is planted, you will need to water your lawn every day to keep the soil moist.

Avoid overwatering to prevent washing the grass seed away or creating muddy spots on your lawn.

Once the seeds start sprouting in around three weeks, reduce the watering frequency but ensure the sprouts do not become too dry.

When the grass reaches a height of around 3” inches, it is safe to begin mowing your lawn.

Be sure to follow the one-third rule when mowing and only cut one-third of the grass height at one time.

Cutting your grass too short will inhibit its ability to absorb vital nutrients, and your lawn may turn brown and die.

When the grass is well established on your lawn, you will only need to water it 2-3 times per week for the rest of the growing season. 

You may need to water more often during the hottest months of the year to keep the grass from drying out.

To keep your lawn lush and green, use preventative measures to control weeds and prevent them from overtaking your yard.

Since using an herbicide is likely to kill your grass along with the weeds, it is best to use Roundup as a last resort for weed control.

Instead, use a hand tool such as a small hand shovel to dig the weeds out of your lawn.

A garden hoe may also be used for weed removal, but it works by cutting the weeds. 

If the roots are left in the ground, there is a chance of them sprouting back up again.

You will need to pull the roots out of the ground to prevent any new growth of weeds.

A weekly maintenance schedule will keep your lawn looking its best throughout the growing season.

To help your grass survive during the winter months, spread a layer of mulched grass clippings or leaves over your lawn at the end of the growing season.

This mulch layer provides some protection against frost and ensures your lawn comes back green and healthy at the start of the next growing season.

Commonly Asked Questions

Is Roundup safe for dogs after drying?

To avoid any danger to your dogs, keep them off the lawn for at least 24 hours after applying Roundup.

This waiting period gives the herbicide time to soak into the ground and reach the roots of the weeds.

Once the grass is dry, it is safe for your pets and children to walk on.

It is advisable to monitor your dog to ensure the animal does not dig into the ground where the chemicals have seeped into the soil.

Does Roundup need the sun to work?

Roundup does not require sunlight to work.

The herbicide will still be effective if there is no rain, even on a cloudy day.

What is the best temperature to spray Roundup?

The best time to use Roundup is during the spring and summer months, in temperatures ranging from 45-90° degrees Fahrenheit (7-32° C).

Choose a day with calm weather, as wind and rain will reduce the effectiveness of any herbicide.