The Spruce / Adelyn Duchala
Key Points
- Don't fertilize and overseed together-grass seedlings and mature grass need different care.
- Fertilize either before overseeding or after seedlings have matured.
- Proper timing and technique are key for both fertilizing and overseeding.
Fertilizing and overseeding the lawn at once is, technically, possible, but that does not mean it's a good idea for the average homeowner.
Why not? Most lawn fertilizers are intended for mature grass. The grass seedlings that result from overseeding have different needs from mature grass and can actually be harmed by fertilizing before they become established.
In this article, we will learn when you should fertilize (and the consequences of your timing), how to fertilize correctly, and how to overseed correctly.
What Is Overseeding?
Overseeding is the practice of sowing grass seed on top of an existing lawn to achieve thicker grass and maintain the vitality of the lawn. It can be considered one of the preventative tasks of lawn care, preventing future issues such as weed invasions.
Can You Fertilize and Overseed at the Same Time?
While you can fertilize and overseed at the same time, it's not recommended. Most lawn fertilizers are made for mature grass plants, which have different nutritional needs from those of new grass plants.
Looked at in terms of NPK ratios (nitrogen, phosphorus, and potassium), mature grass plants need more nitrogen and potassium than do the seedlings that result from overseeding. The nitrogen is what is responsible for that wonderful green color of the blades of your grass, and the potassium helps fight disease. While nitrogen is great for established lawns, too much of it can burn grass seedlings.
By contrast, new grass plants need more phosphorus. Phosphorus helps plants build root systems, which is what's most important when plants are young. So-called "starter fertilizers" contain more phosphorus for precisely this reason. This makes them a good choice when you are planting seeds to start a new lawn.
However, starter fertilizer is not intended for established grass plants, so it's not recommended for use when overseeding.
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When to Fertilize-Before, During, or After Overseeding
Let's break down the reasons behind the different timing options for fertilizing:
Fertilizing before overseeding: This is an acceptable option, as long as you wait about a month before you overseed. This time period will give the existing grass time to use up the fertilizer and put on a growth spurt, after which you should mow, then overseed.
Fertilizing during overseeding: Avoid this, as it could harm your seedlings.
Fertilizing after overseeding: This is another acceptable option, as long as you wait about 8 weeks before fertilizing. After this waiting period, the seedlings will have matured enough to be able to gain some benefit from the fertilizer.
How to Overseed and Fertilize Safely for Best Results
For both overseed and fertilizing, you will most likely be using a fertilizer spreader.
To apply the right setting on your spreader, find out the recommended spreading rate on the product and cut it in half. The number is intended for two-directional spreading, but four-directional spreading gives better coverage and uses less seeds. Push the spreader north to south and back until you've covered the entire lawn and then follow that up with east to west and back rows.
To overseed for the best results, kill any weeds in the lawn first. If you were to kill them afterward, you would risk killing the grass seedlings at the same time. Do not apply pre-emergent herbicides before overseeding: They will suppress your grass seedlings as well as weed seedlings.
Mow the lawn and rake it (which will remove thatch) to give your grass seed better access to the soil. It also helps to sprinkle the lawn with 1/2 inch of compost for the seeds to root in. Late summer is a great time to overseed.
Timing is also important for applying fertilizer. There are bad times to fertilize, including in the heat of the day in summer and just before a downpour. Your best bet is to get on a yearly fertilizing schedule and stick to it. For example, in the North, this will mean fertilizing three times annually:
In late fall with a winterizer
In late spring
In late summer or early fall
Frequently asked Questions
Will fertilizer burn new grass seedlings?
Yes, regular lawn fertilizer is too strong for young seedlings and will likely burn them.
When is the best time to apply starter fertilizer?
Apply a starter fertilizer either just before or just after planting seed for a brand new lawn or when filling in bare patches; about a month after, apply starter fertilizer a second time. Don't use starter fertilizer for overseeding.
How soon should I apply regular fertilizer after seeding?
Once the seedlings are established, you can get back to a regular fertilizing regimen. It typically takes six to eight weeks for seedlings to become established.
Read the original article on The Spruce
