To Use On New Lawns?

Can crab grass killer be applied to new grass that was seeded last fall?

Garden Counselor ANSWER:

Grass that was seeded last fall (2008) should be well enough established now (April, 2009) to withstand crab grass killer. If the grass has grown well, and looks healthy, it should be fine to go ahead.

Expect that the grass will show yellowing after application of this herbicide. This is quite typical of many chemicals on lawns. It should grow out of that after 1-2 mowings. (So don’t do it if you have a wedding or special event planned in the next few weeks!)

It is important that you do not mix crab grass killer any stronger than recommended on the label. It is better to have to do a repeat application than hope for a better weed kill ratio by using a stronger dose. A new lawn will be more susceptible to a higher concentration of active ingredient than will the weeds.

Also a critical concern is that you do not use any weed killing product when temperatures are too high, above 85 degrees. The heat pulls more moisture out of the grass blades, which means the chemical being absorbed sits at a higher percentage than if the grass was well hydrated.

As some of the articles state, you want to have watered the grass well prior to spraying (probably the day before, depending on your location and climate). That way the lawn can handle the stress of being sprayed and left unwatered for the required time.

I am assuming you have a typical bladed grass type, like fescue, rye or bluegrass. Crab grass killer will kill St. Augustine, zoyzia and some other grasses. So be sure to read the label thoroughly for any other cautions.

Editor’s add-on info:

Then what is the recommended interval you should wait to spray crab grass killer after planting new grass?
From seed, a new lawn needs at least two months to fill in well and handle traffic without negative impact. (That includes a period of 2-3 weeks to germinate.)

To undergo the additional stress of an herbicide, waiting one to two more months will put you into the safety zone. Longer is better, but It depends on the variety of grass, and on the weed killer you select also, as some versions could be more potent than others. Meanwhile, most of that time the weeds are flourishing. Ouch!

Some herbicide labels state that is safe to apply after the second mowing of a new lawn, or some similar point of maturity. Always read the label of the particular chemical you select. Do NOT Guess!

Use good judgment to evaluate how fast your grass crop has filled in and matured, which will vary greatly in different conditions. If you have any doubt, try spraying just a small area first and watch it for a couple of days for negative impact. (If you don’t have any concerns about going ahead, you probably are not reading this in the first place?)
Just remember how much work you went to, putting in a new lawn. Is it worth the risk now? Be cautious.

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Crab Grass Killer Is Different Than
Regular Weed Killer

Will new grass seed be harmed by using weed killer at the same time?

Complete Question: (April, 2014)
About two years back we stripped the yard of all grass, etc, and built up the soil layer with additional topsoil. Once we got the new growth going, a lot of weeds were present. They were killed off, but the grass was left thin due to the late time of the season.
Now we are getting ready to re-spray for weeds with probably the Spectracide Weed Stop. We are concerned if we will be able to over-seed the yard without harming the new grass seed due to the weed killer. Thoughts? Ideas? or recommendations? Time is a bit of concern as it is running late to plant new grass (per the wife's schedule!)

Garden Counselor ANSWER:

Spectracide products are good quality. If you have not already purchased, realize that they offer two versions of Weed Stop, one of them adding a crab grass killer.
Your choice will obviously depend on whether you merely have problems with broadleaf weeds, or if crab grass is also a concern.
Your approach to reseeding your lawn will be affected by which product is selected.

BROADLEAF WEEDS ONLY: Use Spectracide Weed Stop (the basic model).
From the label: "For new lawns, treat within 24 hours after seeding."
This means the product will not affect the seeds themselves, but once they start the germination process, the emerging sprout will be killed.

In your situation, if the lawn is in appropriate condition for reseeding and weeds have already appeared, then this is your best approach. You can reseed grass and spray broadleaf weeds, both at this time, with the following restriction:

Sow the new grass seed, apply a thin layer of top dressing if you choose, water the lawn thoroughly, and let it dry. Once the foliage is dry, but within 24 hours, apply the Weed Stop. Obviously, if you are having rainy weather, which is good for grass germination but bad for weed killing, you'll have to delay this process.

The theory/principle behind this recommendation is that you want:
- the new grass seed to get sufficient moisture to begin germinating;
- you want the mulch washed off the weeds;
- you want the weed killer to remain on the foliage of the weeds and not be diluted by existing wetness;
- and you want the weed killer to remain undisturbed on the weeds for 24 hours.

Note: had you already planted your grass seed you would have to wait until the grass is 2" high, or longer, before spraying Weed Stop.
Note: had you already sprayed Weed Stop, but not planted grass seed, you would wait 3 weeks before sowing grass.

Use Spectracide Weed Stop for Lawns Plus Crabgrass Killer.

This label does NOT permit the spraying immediately after the seed planting. (The crab grass killer, Quinclorac, must be more caustic to the seed, or have a stronger residual effect.)

Therefore you are limited to one of the following options, as interpreted from their label:

Note: If you plant grass seed first, you must wait until the grass is "well established" or at least 6 weeks after seedling emergence (which would be 8-9 weeks after sowing) before spraying Weed Stop plus Crab grass Killer.

Note: If you first spray Weed Stop plus Crabgrass Killer, (not having planted grass seed), you would wait 4 weeks before sowing grass.

Needing to use crab grass killer certainly limits your options as it will cause significant delay to either your weed spraying or your lawn reseeding.

Weeds are more thoroughly controlled when you can hit them early. So I would recommend spraying first. You have plenty of time for new grass to grow, even with waiting 3-4 weeks. (Unless you have an event or activity on the calendar for which the lawn must be pristine as soon as possible.)

Weeds don't cooperate with the best laid plans of man. The weeds you are seeing now will be reinforced by more of the same as the soil temperature increases, plus other varieties that pop up later in the season.

In other words, you will likely need to spray again, and your window for doing that will get pushed past the ideal stage as you wait for the new grass seedlings to mature. "And that's the way it was" and still is. Don't fret. Kill what you can, when you can. Know that everything you get rid of this season will have a positive impact on the next season.

Don't overlook the benefits of incorporating preemergents into your annual regimen. Look at the articles about weed preventers which might help you consider alternate options in the future.

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