Overseeding a Lawn with Weeds: Should You Kill Weeds First?
A lawn full of weeds can become even more problematic if you don’t overseed the right way. Overseeding is meant to renew your lawn, fill in any bare spots, and make the turf dense. But, when your lawn is full of weeds, should you go ahead and overseed without killing weeds first?
Overseed your lawn in the fall after killing weeds in late summer. If you’re late, scalp the weedy lawn, rake away all the debris, and spread grass seed. Some weeds will germinate and grow with the grass, but you can control them using a post-emergent herbicide before they spread further.
It is not a good idea to plant and grow new grass over weeds because weeds compete for nutrients and water in the soil, which leads to poor germination and growth of the newly planted grass.
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Should I Kill Weeds Before Overseeding?
Kill and remove weeds from your lawn at least 6 weeks before overseeding if you’re using a post-emergent herbicide. Alternatively, remove weeds by hand-pulling manually. Weed killers will affect the seed germination, so, allow enough time as indicated on the product label before planting grass seed.
Since the best time to overseed most lawns is fall, late summer is a good time to put down a post-emergent weed killer. By the time fall comes, it will be at least 4-6 weeks since you applied a weed killer, which is safe for planting grass seed.
How to Overseed a Weedy Lawn
It is best to kill weeds first before overseeding.
Weeds like bare spots and can continue to spread and invade your lawn if you don’t do anything about them. That’s why overseeding helps control them by making your lawn thicker and fuller, and able to choke out weeds.
Overseeding is time and cost-effective, as you don’t have to till and tear apart your lawn. Instead, you’re breathing life back by adding more grass seeds on top of an existing lawn. If done right, this technique lets you grow enough lush lawn that fills in the bare or thin patches in your yard, leaving no room for weeds to grow.
Here’s how to overseed a lawn with weeds:
1. Pull out the weeds manually
Remove weeds from your lawn manually using a weed puller. Pulling weeds out by hand is highly recommended especially if the weeds are grown and visible. Do not apply herbicides on your lawn before overseeding as this can cause poor germination of grass seed.
If you prefer using a weed killer before overseeding, do so early enough to allow time for the herbicide to break down completely before spreading grass seed. Most pre-emergents and post-emergent herbicides have indicated waiting periods before you plant grass seed.
Here’s how to pull out weeds by hand:
- Water your lawn deeply to make the soil soft.
- Pull out weeds by hand or using a weed puller.
- Throw the weeds away from your yard.
In some cases, you may not need to apply a weed killer as weeds like crabgrass usually die off as the weather becomes cooler. If you have annual weeds, just overseed your lawn with weeds late in the fall and allow the new grass to grow. By the time spring comes, your lawn will grow thick and full enough to choke most annual grassy weeds.
2. Mow the lawn on the lowest setting
Set your mower to the lowest setting possible in order to scalp the lawn you want to overseed. Scalping your lawn helps improve the seed-to-soil contact that in turn improves the rate of seed germination. Set your lawn mower’s deck as low as possible – though any setting between 1.5 and 2 inches would be a good option to consider if the lawn does not have too many weeds.
- If your lawn is level, you can set the mowing deck as low as 1 inch from the ground.
- If your lawn is bumpy, set the blades a little higher – up to 2 inches to prevent damaging the blades.
Pro tip: Always bag clippings if your lawn has weeds that have gone to seed to prevent spreading weed seeds all over your yard and worsening the problem.
It is also a good idea, in my experience, to mow right before the rainy week to keep the lawn moist. It makes my mowing job a lot easier as the grass is soft and the thatch easy to remove right before overseeding weedy lawns.
3. Remove grass clippings
Use a rake to remove grass clippings and other debris that’s covering the soil. The process of raking also helps loosen up the top soil in your yard.
If you mowed a little higher, you might have a lot of thatch remaining on the soil surface. Use a heavy duty rake to remove any debris that can prevent your grass seed from staying in contact with the soil for germination.
A good rake can also help you remove any moss that may be growing under the grass in your lawn.
4. Dethatch the lawn
A thick layer of thatch can also cause seed to soil contact problems. Use a dethatcher to break down the thick thatch and then collect the loosened-up trash bag and throw it away on your compost.
Alternatively, run a power rake all over the weedy lawn to loosen up any thatch and tangled-up grass.
When I have a large weedy lawn to overseed, I find it helpful to rent a slit seeder to complete the task in the shortest time possible.
5. Aerate the lawn
By the time it is fall, the soil is usually compacted already, which is why the grass in your yard is growing slowly while the weeds grow faster. After removing the weeds, mowing, and dethatching, core aerate the lawn to allow air, water, and nutrients to be easily accessed in the root zone.
The cores created in the yard will also allow for better seed-to-soil contact as soon as you overseed. This process is better than just sprinkling grass all over the lawn because it improves the rate of seed germination.
6: Spread the grass Seed
Spread grass seed over the prepared area using a lawn spreader. Follow the recommended spreader settings for the type and variety of grass seed you’re overseeding your lawn with to ensure you’ve put down enough make your turf thick and full the next season.
Pro tip: Spread a thin layer of straw to cover grass seed and prevent birds from causing damage. You can also use bird deterrents if you don’t want to cover the grass seed.
For the best results, plant one-half of the grass seeds in one direction and the other half in the opposite direction. I recommend using a lawn spreader for even distribution of the seeds.
Also, you need to make sure the seeds have a good contact with the soil because this helps with proper germination and enhances growth. You can use a water-filled roller or the back of metal rake to establish the recommended contact between the soil and your grass seeds.
7. Gently rake in the grass seed
Raking is important as it helps the seeds come into contact with the soil to germinate properly. If you used a spreader or simply broadcast the seeds with your hand, the grass seed isn’t in proper contact with the soil.
Lightly rake in the seed over the overseeded area to improve contact and promote faster germination. Raking will also prevent water from washing away seeds in your lawn when you irrigate.
8. Water your overseeded lawn lightly
Water the overseeded area lightly to keep the soil moist enough for the grass seeds to germinate and grow. Do not use a heavy sprinkler because it can easily erode the seeds away from the desired area.
Here’s a video on how to prepare and overseed a lawn with weeds:
How to Control Weeds after Overseeding
Weeds may sprout in your lawn soon after your grass seed starts to germinate. At this time, do not apply herbicides yet. Applying herbicides too soon can kill or weaken the young grass shoots.
Wait until the roots have established and the crown matured enough to withstand the strength and effect of the chemicals.
Also, instead of using equipment such as a power rake, which disrupts lawn surfaces, use less disruptive tools such as a hand rake, as they cause less disruption and don’t leave bare patches on the lawn.
Overseed your lawn at the right time to reduce weed competition and enhance the dramatic, successful growth of the overseeding process. You can overseed cool season turf grasses between mid-August and mid-September as a way to fix a weedy yard. These are favorable times for their germination and establishment.
Can I overseed after weed and feed?
It is not recommended to overseed your lawn after applying weed and feed fertilizer. This type of lawn food contains herbicides that can inhibit germination of both grass and weed seeds. If you’ve just put down weed and feed, wait at least 6 weeks before sowing grass seed.
Read the weed and feed product label for the recommended time to wait before reseeding your lawn. Follow the instructions to ensure you achieve a high germination rate when overseeding a weedy yard.
While fall is the perfect time to overseed your lawn and encourage weed control, there are other seasons to overseed provided you understand the risks.
You can overseed in the early spring, but make sure the lawn is well drained for the best results. Weed competition is more progressive and intense as spring progresses. So unless you time the overseeding period before weed germination in spring, doing so at this time may not be effective.
Summer is the least desirable time to overseed your lawn especially if it has weeds. In early summer, the conditions are so hot that there’s a huge demand for irrigation for proper overseeding to be possible. This is also the time when broadleaf weeds tend to thrive, and therefore your overseeding efforts are likely to fail.
Overseeding will make your lawn fuller, and thicker; and improve its pest, disease, and weed resistance. Remember, care for and maintain your lawn after reviving its look and density. Some tips include sufficient watering, proper fertilization, consistent weeding, foot traffic control, and proper mowing.
How Many Days Do You Have to Wait Before Seeding After Weed & Feed?
Weed and feed fertilizers are often used in combination with seeding. Weed and feed formulations consist of two components: a herbicide to kill weeds and a fertilizer to strengthen the turf. The herbicide will weaken the grass as well as the weeds and the fertilizer will strengthen the weeds as well as the grass. When applying seed over a weed and feed application, remember that some weed and feeds can prevent grass seeds from growing.
Types of Herbicide
It’s important to know a little about herbicides so you can make the best choice for when to apply seed in an area that has been treated for weeds. The most common types of herbicide in weed and feed products are selective and systemic. Selective herbicides target a species of plant to kill while systemic herbicides work by being absorbed though the roots and then transported throughout the plant, killing it from within. Read the bag label to see what kind of herbicide is used in the weed and feed you are considering using or have used. The bag label will tell you how many days you must wait before applying seed to a lawn that has been treated with that product.
Herbicides can target weeds before they germinate from seed – pre-emergent – or as developed plants – post-emergent. Before you seed, you can use a non-selective, post-emergent herbicide to control any weeds in the area to be seeded. Most of these can be applied up to two weeks before seeding to control any existing weeds. Herbicides should not be used after seeding until the new seedlings are established. Mowing and spot treatments can be used to control weeds until the seeded area is actively growing and requires only maintenance watering. Establishment times vary depending on the type of seed you use and your weather conditions.
Using Weed and Feed
Only use a weed and feed if the weed infestation is completely uniform over the entire lawn and all species of weeds targeted will be affected by the herbicide in the weed and feed. This scenario doesn’t occur often, so it is more likely the use of an herbicide and a fertilizer separately will be needed. If the weeds are uniformly spread over the area to be treated, match the appropriate weed and feed product to your grass, the seed you have recently applied or want to apply, and the time of year.
Know What You Grow
It is important to know what kind of grass you have growing or want to have growing. Certain chemicals act differently on different species of grass and weeds. For example, the common herbicide 2,4-D is toxic to some cultivars of St. Augustine grass (Stenotaphrum secundatum), which grows in the area roughly covered by U.S. Department of Agriculture plant hardiness zones 8 through 10. Another common herbicide, atrazine, is potentially lethal to grass when applied in temperatures above 85 degrees Fahrenheit. Use the instructions on the bag of each weed and feed product to determine how it will affect seeding.
- University of Florida IFAS Extension: Weed Management in Home Lawns
- Texas Agricultural Extension Service: Maintaining St. Augustinegrass Lawns
Sara DeBerry is a graduate of the University of Florida holding a masters degree in environmental horticulture and a minor in entomology and nematology. DeBerry has been writing for government agencies since 2004 and has published peer reviewed scientific articles during her studies at UF.
How to control weeds in a newly seeded lawn
When your new grass starts to grow, it can be quite alarming when weeds start to grow with it. Sometimes, despite the most thorough seedbed preparation, weeds can lay dormant in soil until the conditions are made perfect for them to make an appearance – such as a reseed.
However, you can get rid of these weeds just as fast as they have appeared.
The important thing to remember when new weeds appear in your newly sown lawn is not to act hastily – do not apply a Feed, Weed and Moss Killer type product of any kind on a newly sown lawn.
Although this can be frustrating and we can appreciate that a quick solution will be desired, the good news about these types of weeds is that they are largely shallow rooting and should come out with the first mow at the 6-8 week mark after sowing. If they don’t, they should be easy to pull out of the turf.
If you find that the weeds are recurring past the 6-8-week mark, you may wish to consider using a selective herbicide to spot spray your weeds. Some weed killers such as glyphosate (Roundup) kill more than just weeds, so it is important to not apply these as if they are not done precisely, they can kill your grass. Shop bought selective weed killers will recommend when to apply their product and how often and you should read the instructions thoroughly and adhere to them.
In short, here’s what you should do if you encounter weeds in your newly seeded lawn:
- Don’t panic – don’t use a Feed, Weed and Moss Killer product on a new lawn
- These weeds will be shallow rooting, you can pull them out by hand or wait until the 6-8 week mark to mow them out
- If the weeds are persistent and reoccurring, use a selective herbicide to spot treat them
If you find that when the lawn is at least six months old and has been taken over by weeds or moss, you can use a Feed, Weed and Moss Killer product.