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.
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.
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.
Using Weed and Feed
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.
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.
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.
Types of Herbicide
You can fertilize your lawn before or after overseeding. Both tactics work to feed your new grass seed. It’s best to fertilize within 3 days of seeding. This means you can spread your starter fertilizer a few days before you lay down your seed or a few days after. Both are far more beneficial for a healthy lawn than going without fertilizer.
The herbicide in weed and feed is designed to linger in the soil for up to 3 months. If you have already applied weed and feed, don’t overseed your lawn for at least 12 weeks. The herbicide in the soil will attack every type of grass seed.
Weed and feed products are a death knell for grass seed. This is due to the fact that most weed and feed combines fertilizer with pre-emergent herbicide. This type of herbicide attacks all types of plant seeds, killing them as they germinate. If you spread weed and feed on a lawn that has been overseeded in the past 8 weeks, it can kill all the grass seedlings there.
A lawn starter fertilizer is formulated so that of the three main ingredients in fertilizer (nitrogen, phosphorus, and potassium) phosphorus is the most abundant nutrient. This is important because phosphorus drives grass root growth. A lawn starter fertilizer will encourage your grass seedlings to take root quickly. This will make them hardy and established, whether you overseed in late spring or fall.
Can You Put Fertilizer Down With Grass Seed?
If you’ve recently overseeded your lawn, there’s no reason to wait to apply fertilizer. The sooner you spread a lawn starter fertilizer on your yard, the sooner it will begin to feed your grass seedlings. If you like, you apply fertilizer the minute you’re done spreading your grass seed.
After overseeding your lawn, apply a lawn starter fertilizer to provide grass seedlings with the nutrients they need to establish themselves. Do not substitute lawn starter with a fertilizer meant for mature grass. Mature grass fertilizer doesn’t contain the nutrients new grass needs to develop roots. Similarly, avoid using “weed and feed” fertilizers. These products will kill your grass seedlings.
In addition to preventing your grass seedlings from taking root, thick thatch also acts like a sponge, absorbing water and fertilizer before it can penetrate the soil. Fertilizing on thick thatch will be far less effective. Removing thatch prior to overseeding helps you deliver more lawn starter fertilizer to your grass seeds.
Aerate Your Lawn
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Compost is an excellent natural fertilizer, but there’s one catch—the nitrogen in compost is contained in uric acid, and uric acid evaporates quickly when exposed to light. So, a thin layer of compost spread over your lawn will quickly lose most of its fertilizing power. There is a solution though. If your lawn was recently aerated it has thousands of tiny holes. You can spread compost over a recently aerated lawn to fill these holes and inject nitrogen-rich fertilizer into the soil.