Can You Use Pre-Emergents & Overseed at the Same Time?

By Brian Mounts | Mar 06, 2020
Can You Use Pre-Emergents & Overseed at the Same Time

The two most popular methods for maintaining a high quality lawn year after year are applying a pre-emergent and overseeding.  These both lead to that beautiful perfect lawn that is desired by most homeowners.  Many novices think if both strategies are great then doing both should make it even easier.

They couldn’t be more wrong.

The simple answer to the question can you use a pre-emergent & overseed at the same time is no.  Do not do it!

However, as with anything there are a few special situations that a homeowner may consider for their exact lawn and current situation if they have a problem that needs to be fixed.  There is also one specific product that can be used with overseeding, Tenacity, but it has several very specific use cases and limitations to be aware of.

Let’s get into the details of exactly why you should never apply a pre-emergent and overseed at the exact same time and when the rules may be bent with specific techniques.

The General Rule for Pre-Emergents & Overseeding

As mentioned, the general rule is to never use a pre-emergent at the same time as overseeding.

A pre-emergent when applied correctly creates a chemical vapor barrier in the uppermost layer of the soil.  This vapor barrier is designed to eliminate germinating seeds from being able to establish a tap root in the soil and begin to grow.

The pre-emergent you selected may be perfectly fine for use with the turf grasses in your lawn, but that doesn’t mean it will allow grass seed germinate.  A side effect of the most popular pre-emergents on the market is that they make it difficult for almost any seed to get established in the area that is applied.

This is why they are so effective at eliminating weeds like crabgrass and other unsightly weeds that may pop up in your yard in the early spring and throughout the summer.

Why so much confusion regarding pre-emergents and overseeding?

When researching how to have a perfect lawn you are going to encounter articles with lists of best practices for homeowners to do to grow a beautiful lawn.  Included in those lists will be applying a pre-emergent and overseeding.

You may also find some herbicide products on the market that market themselves in a manner that implies they will not kill turf grasses.

There are also tons of lawn and garden forum posts with homeowners talking about how they had no problem using a pre-emergent and overseeding at the same time and see no reason why you shouldn’t just go for it.

This causes massive confusion for the first time homeowner trying to figure out the right regimen for their lawns care.  The general rule of thumb is to use a pre-emergent in the spring and overseed in the fall.

The spring is when the most invasive weeds germinate and grow so applying the herbicide to coincide with their development stops them before they ever get started.  The fall is generally slightly cooler and wetter the conditions that are perfect for turf grass seeds to get established and grow.

More information on the difficulty of overseeding in the spring and when is the best time of year to put down grass seed.

There are a limited number of herbicide pre-emergents that have a short duration that can be used in conjunction with an overseeding strategy.

The only one that I am willing to approve of is Tenacity. You can see the product description and pricing for Tenacity here on Amazon.

The active ingredient in Tenacity is Mesotrione and only lasts for approximately 4 weeks, but really only reliable for 21 days.  The short duration does not make this the best option for pre-emergents for most homeowners looking for a season lasting herbicide and thus should not be part of any beginner strategy for lawn care.

Tenacity and Overseeding

Tenacity is both a pre-emergent and a post-emergent herbicide and it will not affect newly planted grass seed from properly germinating.  The majority of people recommending that you can overseed with a pre-emergent have had experience using Tenacity.

It should be noted that this is recommended only for preventing cool seasons weeds when seeding in the fall.

Tenacity should be used on grass blends that contain less than 20% of its weight in hard or fine fescue grass.  Apply at the exact same time as seeding in order to be most effective.  Do not spray it on seed that has just recently germinated, if you forgot to apply directly at the time of seeding you should wait until the new grass has been mowed a few times before applying.

Strategies for Pre-Emergents and Overseeding in the Same Year

The best strategy to use to get a lawn that all your neighbors will envy is to use a pre-emergent in the spring and overseed in the fall.  You are using both methods in the same year, not at the same time, this gives seeds the time to germinate and become established prior to applying the herbicide vapor barrier.

Homeowners get the most bang for their buck with this plan as the spring and summer weeds are the most difficult to deal with and when people want to entertain and have friends and family over for summer BBQ’s and want the backyard looking perfect while they socialize and the kids play on the backyard trampoline.

How to spot seed after using a pre-emergent?

If you have laid down your pre-emergent in the late winter or early spring your lawn is now going to prevent germination of seed for the next 2 – 6 months.  So what do you do if your lawn gets spotty or you have a construction project and the yard gets damaged.  If you spread out some seed nothing is going to germinate.

The following 5 simple steps will fix that bare spot.

1. Break up the soil

Use a hand rake or cultivator or even just a shovel and break up the soil.

You want to disturb the vapor barrier that was created by the pre-emergent.  You only need to disturb the top inch of the ground.

2. Add a soil mix to the area

Add about a half inch to one inch layer of soil over the area that has been disturbed by the hand rake cultivator.

This will give the seeds pure soil to germinate in and avoid encountering the herbicide the second the roots emerge from the seed.

3. Add your preferred grass seed

Go heavy with the seed and put down a good amount.  Use the cultivator to trun the seed and soil into each other.

4. Add peat moss or Seed Mulch

Add a layer of peat moss or seed mulch which is made from recycled paper and cover the entire area that you seeded.

This does three things, it will protect the seed from being exposed, the seed will stay in place avoiding being blown in the wind, and provide extra moisture retention for the grass seed to germinate.  This is very important with spring seedings as he temps warm up quickly and if there is no rain the seed will not all germinate leaving spots behind that you will not be able to fix until the fall.

This covering will also keep the birds from eating the grass seeds.

5. Water Water Water

Water the area in well and make sure to keep the area very moist until the seed has germinated.

If the new seed bed dries out the seedlings will die, resulting in a bare spot in your otherwise perfect lawn. Keep the area wet, be careful of overwatering grass seed.

How to decide whether to overseed or use a pre-emergent?

If you have lots of bare spots in your yard and you know the weed pressure will be heavy as the spring and summer unfold how do you decide which method you should do this spring.

I also lean towards a spring pre-emergent plan knowing that the yard may not be perfect for this year’s summer, but with a solid fall season overseeding by next summer you will have that perfect lawn you always wanted.

If you only have a few bare spots try and avoid getting the pre-emergent on those areas and following the above 5 steps for seeding the bare areas.

I recommend using the liquid Prodiamine 65 WDG for this plan as it is easier to apply than the granular versions that are applied with a mechanical spreader.  If you are really careful with where you spray this can be very effective.

In Conclusion

Make a yearly plan for your lawn including a spring pre-emergent, fertilizers, soil conditioners, aeration, and fall seedings to get the best yard on the block.  Use each strategy during the best time of year for their application instead doing it all at once.

Maintaining a perfect green lawn with no spots is attainable without too much work when properly planned.  Try and force an issue if you are out of season and expect to be spending a lot of extra time doing work that a solid plan would allow you to avoid.