crabgrass control and overseeing

Asked October 2, 2018, 12:10 PM EDT

an area of my lawn has a lot of crabgrass. I have dethatched the area in preparation for overseeding late this Fall or early next spring. I also hope to rent a core aereator this Fall. My question is, how can a spray the area next Spring with a pre-emergent crabgrass control product and not affect the germination of the grass seed? When should I fertilize the lawn? This Fall, late, early Spring before seeding (if I wait for spring overselling), late Spring/early summer? Do I have to be careful about what fertilizer type I use? Thank you for your help John Olson

Brown County Minnesota crabgrass

3 Responses

You have asked many good questions. Before answering them, you may want to look at this lawn care calendar that the University of Minnesota has prepared:

https://extension.umn.edu/planting-and-growing-guides/lawn-care-calendar

From the above you will see that you are pretty much on track.

Crabgrass is an annual plant and the plants that are now present will soon die. However, they probably have already spread seeds that will germinate next spring. Using a pre-emergent spray next spring should prevent, hopefully, most of these seeds from germinating. However, such a pre-emergent spray will also prevent grass seed from germinating. What this means is that if you wait until next spring to seed your grass, you will need to wait at least four weeks or longer after applying the pre-emergent spray. See:

https://homeguides.sfgate.com/can-plant-grass-seed-after-crabgrass-killer-101637.html

By waiting this long in the spring before reseeding, you may encounter elevated temperatures that will not be conducive to good grass seed germination. My suggestion would be to apply grass seed this fall as soon as possible. While fall is a good time to aerate the soil, given your situation if you want to reseed grass this fall, wait until spring to do this aeration. Aerate before you apply your pre-emergent spray. The strategy here is to seed this fall, water well, and hope that the grass seed will germinate yet this fall and establish itself. This will occur if we have warm temperatures this October. Before seeding this fall, apply your fertilizer and then reseed. Try to avoid applying the fertilizer to areas that you will be reseeding. If you wait until the spring to apply fertilizer, that would be OK as well. If you go this route, apply the fertilizer after aeration and the pre-emergent application.

With respect to what grass fertilizer to use, you probably will experience the fact that few if any product sold in Minnesota contains phosphorus (the middle number of the three on fertilizer labels). As most Minnesota soils already have a high level of phosphorous, look for lawn fertilizer that contains nitrogen and potassium (the first and third numbers of the three on fertilizer labels)

The following may help you in this regard:

https://extension.umn.edu/lawn-care/fertilizing-lawns#what-is-in-fertilizer%3F-1256811

Good Luck!!

Thank you for your help Steve. One concern. I am afraid if I reseed now (rather than late oct or nov) and the grass seed does germinate, these fragile seedlings won't have a good chance to survive the Winter. Can I reseed late October, then apply the crabgrass pre-emergent in early May. Will that give the grass seed time enough to germinate in late March and April?
Thanks again for your prompt response!
John Olson

Hi John,
You have a valid point about young grass shoots not making it through the winter. It could certainly occur. If you do the grass seeding in late October to early November, the seed will just sit there and germinate in the spring. Then as you pointed out, the new grass may not have sufficiently grown by the time you do the pre-emergent treatment in early May.

So let me propose a third option. Do the aeration and fertilizer application this fall. As crabgrass grows best in compacted soils, this aeration will be step one in battling the crabgrass. Then next spring, seed your grass. But instead of doing a pre-emergent treatment, if and when you first see crabgrass appearing apply a post-emergent herbicide that is directed against crabgrass. Since such herbicides can affect very young shoots of grass, be sure that your young grass is reasonably well established before using this herbicide. Repeat this post-emergent herbicide treatment if and when you see further crabgrass appearing later in the summer but, hopefully, before they have had a chance to disperse their seeds. Also by that time your newly seeded turf will have had a chance to firmly establish itself, which will also limit the growth of crabgrass. This is not to say that some crabgrass may yet release seeds so assuming that there is no need for further grass seeding, do your pre-emergent treatment in the spring of 2020. Completely eliminating crabgrass rarely occurs in a single season so you need to be as persistent as it is. Weather conditions are a huge variable in all of this.

The following should give you some further guidelines:

http://blog-yard-garden-news.extension.umn.edu/2018/08/lots-of-weeds-in-grass-make-your-lawn.html

https://plantscience.psu.edu/research/centers/turf/extension/factsheets/annual-weeds

Good Luck!!