Applying Pre Emergent weed killer to a lawn is a common Spring chore. Unfortunately this occurs in the same time frame when we also want to over-seed thin lawn areas, and plant new grass seed in bare areas, or lay down new grass sod. 

Weed Preventers ought to be called “Seed Preventers” because they don’t exempt your friendly grass sprouts from the warfare they throw at enemy weed seeds. 
In fact, many weeds can be immune to various types of preemergents, but few grass varieties can withstand any of these chemicals. 

So what do you do? You can hope for the best by working around the unchangeable.  Every yard situation is different, but knowing a few basic principles should point you in the right direction. The Garden Counselor's PreEmergent User's Guide will lay out those steps for you.

Many gardeners have inquired about their options in specific situations. Let me share some of the approaches we recommend for them. 
See if your lawn crisis resembles any of those and offers good news or bad news for your immediate or long-term prognosis. 

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  1. When Should I Apply PreEmergent and Reseed?
  2. PreEmergent Already Applied. Can I reseed?
  3. Can I Apply PreEmergent, then Lay New Sod?
  4. PreEmergent is Applied. How Long to Wait before Reseeding?

QUESTION 1:  When Should I Apply Pre Emergent and Reseed?

I live in Kentucky and last year I power seeded my yard but I think that I was a bit too late.  I did get some success, but want to know what is the best time to get a pre-emergent down?  I don't want to miss this window and I desperately want to apply it at the very very best time.  

Also, I do have an extra big bag of seed that I have left over from last year.  When can I apply that in my yard before it goes weak?  It’s been kept in my house for 4 1/2 months. I have about .75 acres and would like to know the kind and amount of pre-emergent to buy.  


First, are you mostly concerned about crabgrass? If so, you want to get the pre-emergent down specifically before the crabgrass germinates. In your area, that should be awhile, (written in February) but the only way to tell is by soil temperature. 

When soil temps get up to about 55 degrees for 1-2 days, the crabgrass will start to germinate. A few seeds may seem to start before then, because temps vary widely even within a small area.
Soil temps are affected by shade, moisture and closeness to paved surfaces or walls that soak up heat from the sun. Note: air temps are not a good indication of the soil condition. 
(Soil thermometers cost about $10.)

If broad-leafed weeds also concern you, be aware that there is some overlap among types of pre emergents. Those tailored for killing crabgrass also kill some broadleaf weeds. Varieties that prevent the largest number of weed species probably won’t work on crabgrass. For all of them, however, the application must occur before germination

Check with your County Farm Agent or State University Extension program, as they usually provide charts that will predict the typical arrival dates for different types of weeds in your area. The Master Gardeners program may also be active in your locale.

Here is the tricky part: 
Most pre-emergents for weeds also will
grass seed from germinating. 

The one exception to this is “Tupersan”, also known as “Siduron”. 
It can be applied to turf areas of bluegrass, fescue, redtop, smooth brome, perennial ryegrass, orchard grass, zoysia, and some bent-grass varieties. It is not for bermuda grass areas. 

The negative sides of Tupersan are a short active life, the price, and limited availability. 

  • It is only good for 4-6 weeks, which means you must time your application carefully to get the most benefit. 
  • As a specialized chemical, it warrants a higher price. There is no generic version of siduron. 
  • When found, it is usually mixed with fertilizer, so you are forced to buy two products. This makes it less practical for a large area because of the extra cost. (And your lawn may not need those particular nutrients at that time). 
  • It is hard to find, not being the herbicide of choice in the common garden products that manufacturers send to local stores.  Retailers would sell small quantities because they don’t have knowledgeable salespeople to promote the product, and the higher price would let the bags collect dust.
  • The concentrated product (straight siduron, not a combination) may be stocked at professional landscape suppliers or agricultural stores, but in some states they cannot sell professional quality products to "just a homeowner". You can find it on-line; sometimes at Amazon; usually at DoYourOwnPestControl. 

If you go this route, you can plant your grass anytime that is typical for your gardeners in your area. This will also depend on the grass type you use. 

If you seed a new, bare area of soil, as opposed to just over-seeding to thicken the lawn, be advised: Do NOT Disturb the soil AFTER you spray the pre emergent. 
So you must calculate the timing of your application with that in mind if the area will be tilled or cultivated before planting.

Another “Dimension” To Consider 

An Alternate  Product, “Dimension” is an excellent pre-emergent for lawns with a crabgrass problem. It has less of an impact on the roots of grass seedling than many other products. 

It is a good choice for a follow up application if your crabgrass season is stretching out and you need more protection after the Tupersan wears off. 

Dimension is more accommodating with the timing of application. It can also kill young crabgrass seedlings if you were late getting to them (Often effective until the four-leaf stage of maturity). 
But the newly planted grass must already be up and growing for a while before you use this product. 

* * *

I cannot advise the amount you need to buy or apply, as this will vary according to the concentration of the product, which can vary a lot.  Read the label carefully to determine this. 

A final note, any pre emergent needs to be watered into the soil, (best within 3 days), to get it activated in the soil. Schedule your application to take advantage of a light rain (not a gully-washer) if you are not able to irrigate. About ½ inch of precipitation is usually required. 

Finally, don’t worry about your grass seed. It should remain viable without a significant loss of germination rate for about a year. (Much of it will last even longer.) A cool, dry location (maybe a closet, or some garages) is better than a warm storage area for long term storage, but yours should be fine.

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QUESTION 2: Already Applied
Pre Emergent.  Can I Reseed?

I used Sta-Green Crab Grass Preventer and Fertilizer on my lawn 2 weeks ago. I would like to reseed my lawn, so I bought Pennington Starter Fertilizer and Pennington Tall Fescue Grass Seed to start. Can I do this since I already applied the crabgrass preventer?


The crab grass Pre Emergent is non-selective, for the most part. More than likely it will inhibit the growth of your grass seed if you put it down prior to the end of the time limit listed on the Sta-Green label.

To clarify, do NOT reseed until you are certain that the weed preventer is no longer active. 

  • If you put down the tall fescue seed, it will start its germination process as soon as the proper soil temperatures have been reached. 
  • The pre-emergent then interferes with the growth process and the grass will not develop past the sprout stage. 
  • But it will be dead, not just waiting for a more opportune time to grow. 
  • The current status of your lawn requires that you wait before applying the new grass seed so that you don’t waste the seed or your efforts at planting new grass.

Sta-Green Crab Grass Preventer specifies that the product can be effective up to 4 months, and recommends that you wait at least 12 weeks after application before reseeding. 

There are a few variables that could affect the actual down time. How heavily the pre emergent is applied, how much irrigation it undergoes, and whether or not the soil surface is disturbed, all can lessen or increase the duration of its effectiveness. 

Don’t forget Murphy’s Law — the gardener who wants the crabgrass pre-emergent to last as long as possible won’t be that fortunate… AND…  the benefit of a never ending weed preventer will likely go to the one who is most anxious to start planting! 

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QUESTION 3:  Can I Apply Pre Emergent,
then Lay New Sod?

My lawn has several medium size crab grass patches. I was considering pulling these patches out and then putting down a pre emergent weed killer (can you recommend any?)
Then immediately I want to put down new sod. What do you think? I live in Ft. Myers, FL. The temps in the evenings is right at 60. 


A Pre Emergent weed killer can be an important weapon in your arsenal to fight weeds, especially the persistent problem of crabgrass. However, your question points out how important it is to get the right tool for the right job. 

Let's look at the overall nature of the project as you have laid out the steps. Then we'll identify specific herbicides that are appropriate to your needs, and which products would cause a new problem.

I infer that you are actually planning to put down pre-grown sod, as opposed to planting grass seed (some folks use the terms differently, so I like to confirm). The two different approaches have a major point of difference. 

Laying sod will require that you remove about an inch of soil to accommodate the depth of the sod and keep it level with the surrounding area. This will probably remove with it the majority of crabgrass seeds that might be present in that location. Those that will remain will be not likely to sprout, with the thickness of the sod above them.

For this reason, you could do this project without using a pre emergent herbicide

Yes, you can argue that it is better to be safe than sorry, and you may not want to let a single weed get a chance to live.
Yes, I have seen it happen that the seams where the sod pieces butt together is the likely place where any crabgrass sprouts might pop through. 

You must weigh the potential benefit against any potential risk. There is a possibility that the herbicide could interfere with the grass roots pushing out from the cut sod. The new grass plants would not root into the lower soil layers as quickly as they would grow under ideal conditions. This would reduce their heat or drought resistance and overall health.

This hindrance would be a very definite concern with some varieties of pre emergent, and a minimal risk with other types. Your new sod will have the best opportunity for getting established quickly if it is not exposed to any chemicals that can restrict growth. 

The other consideration is if you were planning to broadcast the pre-emergent weed killer over the entire lawn area, or just in the bare spots that you need to replant.  

Treating the whole lawn is the smart approach for ultimate control, since you can expect that the crab grass seeds are widely dispersed everywhere. They will try to pop up anywhere they can, leaving you with the need to repeat this process time after time. 

If this is your intent, then go ahead. Just remember that you must make concessions for the young grass that would not be a concern for gardeners treating a well-established existing lawn. 

Which pre-emergent weed killer will be effective and safe for your new sod? Tupersan… (…remainder of paragraph deleted, as it duplicates information from previous questions about this product.)

An alternative would be to wait and observe the area after you sod it, for new crabgrass coming up in the grass and hit it with the pre-emergent at that time. 

But Wait! ...Isn’t it too late then? 

Thankfully there is a pre emergent that also works post-emergent on crabgrass usually up to the 4- 5 leaf stage. Not all pre-emergents will work this way. 

You want to get one with the active ingredient "dithiopyr", usually found under the trade name "Dimension". 

Dimension has less of an negative influence on root establishment.

Be sure to read the label completely to make sure it is acceptable to use on your type of turf, at that time of year, and follow the directions carefully.

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QUESTION 4:  Pre Emergent Is Now Applied.
How Long to Wait before Reseeding?

I applied to my lawn the Crab Grass Preventer by Vigoro.
How long do I need to wait before I can sow some new grass seed on the lawn? I was planning on using a slit seeder to apply the grass seed.
(Editor’s note: a slit-seeder is a machine with multiple blades that slice into the soil; and it may also dispense seeds.)


Let me give you some general concerns to consider for your situation. I cannot be exact since I do not know which Vigoro product you used. 

  • It is possible that the active ingredient was Dimension or Benefin, but manufacturers are known to change chemicals without changing the packaging. 
  • It is essential that you check the label of the bag you used or go back to the store where you purchased to see the criteria on it.  

Different pre emergent chemicals can have a residual effect that ranges from 8 − 12 weeks after application, and even longer depending on the strength of the concentration. 

This duration varies according to the amount of moisture that has been received, also the temperature range and the type of soil, and even different varieties of weeds which awaken at different times.

So you wait and wait, and then you might see weeds popping out sooner than the advertised effective date. 
You could expect that your grass seed should be able to sprout then, right? 
Maybe NOT! Read the full answer, before drawing that conclusion.

An interesting Catch-22/Murphy’s Law factor should be considered in your situation. 

A pre emergent herbicide forms a barrier in the top layer of soil where seeds normally germinate. As the sprout grows into the barrier area, the chemical affects the seed or sprout according to the mode of action it is designed for, such as stopping cell division for example. 

All preemergents emphasize that you get maximum protection against weed development when that barrier zone remains undisturbed. Any disruption of that barrier (any cultivation of the ground) can furnish a ‘chemical-free’ path for some sprouts to pass through to the surface without getting zinged. 

This suggests, in my opinion, that the slit seeder could be a blessing or a curse, depending on your timing or your needs. 

  • If you plant too soon using that machine, you risk losing the benefit of the pre emergent barrier and face the possibility of a substantial crop of crab grass which will have the advantage over your grass seed.
  • There is also the concern that some buried weed seeds will be dislodged by the blades and repositioned to a place where they are more prone to germinate. 
  • After you slit-seed, if some chemical is still active, the remaining residue in the soil could have a killing effect on a high percentage of your grass seed. 

The odd thing to consider is whether this gives you the possibility of breaking the barrier to shorten the waiting period. 

  • If you figure that most of the crabgrass has been killed by a certain point, then why not go ahead and plant? 
  • If breaking the barrier allows the grass seeds to germinate and develop, then could you get an earlier start on the reseeding, right? 

I wouldn’t risk that, simply because most desirable seeds, whether grass, flowers or vegetables, are usually hybrids that are more delicate and more sensitive to chemicals or any adverse growing conditions. 

Weeds on the other hand, tend to be naturally tough, more resistant to assault, and more prone to find a way to survive. Plus, crab grass seeds will be at different depths and prone to germinate over a long season. 

So if you try to short-circuit the designed traits of the pre emergent earlier than recommended, Mr. Murphy would likely kill the grass and let the weeds through. 

So my suggestion is that you’ll get the best results with minimum aggravation by being patient for the entire waiting period listed by Vigoro.  

BUT… maybe you’re an Action type of guy, chewing your fingernails while waiting…..

Then try a small test plot first. That will keep you in check so you don’t lose the investment of your seed, your financial investment, and your labor, if the preventer is still preventing. 

EDITOR'S NOTE TO NEW READERS:  I realize that doesn't give an exact answer the question for you. How Long Do I Wait before Reseeding?
Is there a single answer? Yes.

READ THE LABEL of the product you used/plan to use. Most of the time it states a specific interval to wait, perhaps hidden in the fine print.
If you don't see it, contact the manufacturer at their website.
(If they do not have an FAQ section which gives this info, send them an e-mail.)
Don't guess or assume. Even the same active ingredient used by two products can have different active lives because of concentration or method or season. Be safe. Avoid disappointment. 

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Go to PreEmergent User's Guide 

Go to My Garden Needs: Weed Preventer for a selection of Pre Emergent products

Go to When To Apply Crabgrass Preventer

Go to Garden Counselor Home Page

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