Fertilize or Overseed First? (Good Question)

A common question I get is if a lawn should be fertilized before overseeding or if overseeding before fertilizing is a better idea. Being a fan of low-effort lawn care that offers good results, I often prefer to do both steps at the same time. However, there are cases where there is a “right” answer to which step needs to come first. The time of year and type of fertilizer can change the answer, too. If you want to know the specific answer for your situation, you should be able to find it below.

Fertilize or Overseed First: Is There a Right or Wrong Answer?

Generally, no. Whether you overseed or fertilize a well-prepped lawn first will not make a difference in the success you have with your newly seeded lawn. This is the reason many grass seed varieties or overseeding products are sold with fertilizers mixed in.

That doesn’t mean there aren’t a few exceptional circumstances, though. Sometimes your soil may have all the nutrients it needs to start, or you may want to rely on the nutrients your cover mulch can provide for a lawn that was seeded in the spring.

What Will Happen If You Fertilize First?

“Should I fertilize before overseeding?” is the question we’ll answer first. In most cases, it will either be “it doesn’t matter” or “no”. What happens if you fertilize your lawn before seeding depends on when the last time you fertilized your lawn was, what state your lawn is in at the moment, the type of fertilizer you have to use, and the time of year. Below are some specific scenarios about how fertilizing before you overseed will turn out.

Fertilizing Before Overseeding in the Winter

If the first killing frost has passed or temperatures are cold enough to send your intended grass variety into dormancy, fertilizing first will make little to no difference. That is, as long as you’re using a good slow-release fertilizer. This could include commercial fertilizer granules or pellets as well as fine, scattered compost. Compost teas and other liquid fertilizers would be wasted at this time.

Fertilizing Before Overseeding Within a Month of Another Fertilizer Application

If you fertilize your lawn too frequently, your established grass may be able to withstand this treatment, but many new sprouts will be overwhelmed with the availability of nutrients and may die. If you have recently fertilized but are planning to overseed anyway, you should wait the appropriate amount of time, of a few weeks longer, before adding more fertilizer to your lawn for best results.

Fertilizing Before Overseeding With a Liquid or “Fast-Acting” Fertilizer

Fertilizing your lawn before overseeding it by using a liquid fertilizer may help boost the health of an established lawn, but it will do little or nothing for new sprouts and seedlings as they are just beginning to grow. These types of fertilizers are best used once the sprouts reach at least 2 inches in height, otherwise, the nutrients may be washed away before they can be absorbed.

What Will Happen If You Overseed First?

If it’s a choice between one or the other, I would always pick overseeding before applying fertilizer. It’s just a bit less risky if you don’t want to consider the effects certain factors, like those mentioned in the scenarios above, could have on your lawn’s outcome. Like in the “fertilizing before overseeding” examples above, below are a few scenarios you may encounter if you choose to overseed before applying fertilizer.

Overseeding Before Fertilizing in the Spring

If you choose to overseed in the spring and you have a flexible schedule, you’re giving yourself the most options possible. If you seed first, you can either apply a slow-release fertilizer immediately or wait for a few weeks and apply a fast-acting, liquid fertilizer. A mild fertilizer, such as scattered compost can also be followed by an application of compost tea as the new grass begins to reach a height of 2 inches or more.

Overseeding Before Fertilizing in the Fall or Winter

In the fall, overseeding should only be done once temperatures are below freezing or the high dormancy temperature for your chosen grass type. Seeding above these temperatures will lead to death after germination in most cases. In this type of weather, you’ll want to use a slow-release fertilizer that will deposit nutrients into the soil to be absorbed later, in the spring, when the ground thaws. Liquid fertilizers applied at this point will not benefit your plants as much as they would at another time.

Factors that May Affect Your Decision

When you’re deciding whether to fertilize or overseed your lawn first, you should consider the following 5 factors to make the best decision.

Time of Year

Do you live in a region that gets below the dormancy temperature for your selected grass type? If you are expecting a freeze in the next few weeks, for example, you should only apply slow-release fertilizers or wait to fertilize until spring, even if you choose to seed now.

Fertilizer Type

Liquid fertilizer, slow-release fertilizer, compost, or mulch-based fertilizers can all lead to different results. As mentioned above, slow-release fertilizers are best for the winter while liquid fertilizers are best used after things are greened up and growing in the spring.

Grass Type

The more nitrogen your grass needs, the more often it will need fertilizer. Some grass seed types must have fertilizer applied around the same time they are planted if they’re going to thrive. Other types, those that have lower requirements, may not need any extra fertilizer at all, beyond what you have already added to your lawn.

Cover or Mulch

If you are overseeding to get rid of large bare patches in your lawn, you will probably use some sort of cover. Fine, well-sifted compost is the best option for this and it doubles as a post-seeding fertilizer. No additional fertilizer will be necessary.

State of Your Current Lawn

If fertilizer was applied to your lawn in the last month, you should wait to add more fertilizer, even if you are overseeding. If your lawn is dormant, a slow-release fertilizer applied either before or after seeding is the best option.

What I’ve Found Through Experience

In my experience, there are two main options. You can overseed your lawn in the late fall or early winter and apply a slow-release fertilizer at the same time, before, or after seeding. If you are overseeding your lawn in the spring, do so just as things are starting to green up but wait until you see strong sprouts before applying a strong compost tea over the entire lawn.