When to Fertilize New Grass for Best Results

Growing grass generally doesn’t take an agonizing amount of effort. But cultivating a new lawn requires a certain level of diligence to give grass seed the best chance to germinate and thrive. It is also important for roots to grow deep into the soil in order to form a well-established lawn. In this article I’ll explain when to fertilize new grass so that you can enjoy the best results.

The key benefit of a well-established lawn is that it will be hardy and more resistant to inclement conditions. 

Fertilizer provides grass seed or newly germinated grass with concentrated nutrients. While you could introduce fertilizer at any time (or not at all), fertilizing grass at just the right times in the growth cycle can put your grass into “hulk mode” – if you will.

The nutrients available in fertilizers also come in varying percentages that can be more beneficial for different stages of growth. Choosing a fertilizer that is too highly concentrated can actually burn your lawn!

Here are my best tips to fertilize new grass at the right times and get great results:

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I’ll expand and elaborate on this short summary below, so keep reading to learn my best tips and tricks for a successful seeding project.

And I also recommend that you check out my free lawn measuring tool and grass seed calculator to make sure you’re ordering the perfect amount of fertilizer and seed for your project.

About Lawn Fertilizers Containing Phosphorus

One quick note on the use of lawn fertilizer containing phosphorus – some states and communities have passed legislation which bans or limits the use of phosphorus fertilizer due to eutrophication of ponds and streams.

In my state (Maine), you are allowed to use it when establishing a new lawn or re-seeding/overseeding an existing lawn, or when a lab-based soil test indicates that phosphorus is needed.

I recommend that you check for local restrictions.

Why Should I Fertilize New Grass?

Fertilizers contain essential nutrients that can improve overall soil health in your area. The main nutrients in fertilizer are Nitrogen (N), Phosphorus (P), and Potassium (K). Healthy soil is more resistant to weeds, pests, fungus, erosion, runoff, and patchy grass. Soil that lacks the essential nutrients can be difficult growing medium. 

That being said, too much can lead to burning and using the wrong fertilizer can have far-reaching effects on the soil in your area.

When the ground is saturated with the nutrients found in fertilizer, it can end up leaking through to the water table and lead to runoff. Runoff of fertilizer chemicals has been found to be responsible for toxic algae blooms in local ponds and lakes that are harmful to people and pets. For this reason, fertilizers are often highly regulated and often only certain amounts can be purchased at a time. 

A Tip from An Expert

Peter Landschoot is a Professor of Turfgrass Science at Pennsylvania State University Extension. He says that “Proper fertilization practices require amounts of nutrients be delivered to turfgrass areas. Small mistakes in area measurements or fertilizer rate calculations can produce poor results, and, sometimes, serious turf injury.”

We recommend that you measure your lawn accurately using our free tool before you spread fertilizer.

How to Choose the Best Fertilizer for New Grass

There are two main types of fertilizer to start with: regular (or slow-release) fertilizer and starter (or quick release) fertilizer. Consider the dietary needs of humans in different age groups: the needs of a baby are far different from an adult. 

“Weed and Feed” fertilizers contain herbicides such as corn gluten to prevent weeds from germinating. This is an important fertilizer to take note of and avoid when planting new grass seed because most of these will also prevent your grass seed from germinating!

My Recommended Starter Fertilizer for New Grass

Crabgrass is everywhere in my area, so my favorite fertilizer to use when seeding a new section of lawn is Scott’s Turf Builder Triple Action Built for Seeding.

Unlike many other weed and feed products, this fertilizer does not harm new grass as it germinates, but it does (at least in my experience) successfully block crabgrass and other common weeds for 4-6 weeks to give your new grass time to establish itself. One bag goes a long way too.

Understanding The Nutrients in Lawn Fertilizer

There are three main nutrients in lawn fertilizer. Every fertilizer has a different ratio of these nutrients, and these ratios are on fertilizer packaging as a set of three numbers separated by dashes.

The sequence of numbers indicates the amount of nitrogen, phosphorus, and potassium, respectively and are thus known as NPK ratios.

If you see three numbers on a bag of lawn fertilizer, those numbers will be listed in this order:

Nitrogen – Phosphorus – Potassium

A soil test can help you determine what type of fertilizer will best support your lawn’s health, just make sure you buy a kit that goes beyond simple PH levels and measures these three nutrient levels like this one.

What These 3 Key Nutrients Do to Support New Grass

• Nitrogen is important for the leaf growth you see above ground and helps grass look greener.
• Phosphorus is responsible for promoting root growth below the ground and is important for getting a lawn established.
• Potassium prevents disease and makes the grass more resilient.

New grass seeds need a starter fertilizer that has a higher level of phosphorus and nitrogen that is quick-release, thus readily available for the seeds to absorb.

Quick-release nitrogen also helps seeds absorb more potassium. Some areas actually restrict phosphorus usage exclusively to those starting new lawns.

Fertilizer Ratios for Established Lawns

An established lawn thrives best with a fertilizer that is high in nitrogen. Mature lawns don’t really need much potassium or phosphorus, so you will look for a ratio with a large first number and smaller second and third number. For example, a 30-0-0 or a 27-3-3 ratio would be most appropriate for an established lawn that you want to green up and look beautiful.

Starter Fertilizer Ratios for New Grass Seed

A good starter fertilizer for new lawns should be closer to a 21 – 22 – 4 ratio of nitrogen, phosphorus, and potassium, and those nutrients should be quick-release so they’re accessible to your seedlings right away to help your new lawn establish itself as quickly as possible.

Potash commonly found in soil is a source of potassium, so it is common for the levels of potassium in fertilizer to be very low. 

There’s More To it Than Just Those Three Nutrients

According to Turfgrass Specialist Peter Landschoot, Ph.D. at PennState, “Turfgrasses require at least 16 nutrients for normal growth and development. Some nutrients are needed in large amounts, other nutrients only in minute quantities.” 

He continues, “Regardless of the amount required, a deficiency of any of these nutrients will limit the growth and development of your turf.”

This means that, for example, “a calcium deficiency can be just as detrimental to the plant as a lack of nitrogen, even though turfgrasses use more nitrogen than calcium.” 

Three of the necessary nutrients for your lawn are primary nutrients that are needed in larger amounts. Peter explains that “Calcium, magnesium, and sulfur [are] the secondary nutrients.” These “are needed only occasionally in the form of fertilizer or lime.” 

When to Fertilize New Grass

It is important to make sure that your soil has the appropriate nutrients for new grass seed prior to dispersing the seed itself.

So, after preparing your soil for seed or sod, the last step before planting is to fertilize the soil with a starter fertilizer. This can be done before you lay seed or sod, or at the same time.

After you apply starter fertilizer, don’t reapply it. The ratios of nutrients can actually be harmful and burn established grass. I recommend using a traditional, nitrogen-rich organic fertilizer 6-8 weeks after planting new grass. 

While you may be eager to fertilize again to encourage growth, fertilizing too often is harmful. It can burn your grass, leach into the water table, and more. It’s important to wait a minimum of four to six weeks before another application of fertilizer, and I recommend 6-8 weeks. 

My Process for Seeding a New Lawn

1. Dethatch and Aerate the lawn if needed.
2. If lawn does not need to be dethatched, use an iron rake to loosen the soil and remove dead grass.
3. Remove any dead grass from the area you will be seeding.
4. Apply starter fertilizer evenly across the area you will be seeding.
5. Apply a generous amount of grass seed that is appropriate for your area and the growing conditions of your lot.
6. Use the back of a leaf rake to gently work the seed into contact with the loosened soil
7. Apply 1/4″ – 1/2″ of compost loosely over the grass seed to retain moisture and provide nutrients to the new grass seedlings
8. Water to keep compost and seedlings moist until well established.
9. After grass seedlings are established, water less frequently and more deeply to promote root growth.
10. Mow once grass seedlings are about 3″ tall, removing 1/2″ – 3/4″ of grass blades with a sharp mower blade. Bag these clippings and remove.
11. Mow again once grass gets to 3″ again, removing no more than 1″.
12. Apply nitrogen-rich organic fertilizer 6-8 weeks after seeding (optional).

Should You Fertilize Again in the Fall?

An application of a fertilizer that has a modest amount of slow-release nitrogen in the fall can help to bolster your grass before the coming winter. It’s important to make sure that this is done well before the first frost, so no later than November 1st for southern states and an even earlier cutoff for northern states. 

I recommend fall lawn fertilizer high in potassium, which can ease the transition to cold weather, but be careful with high nitrogen in the fall because it can cause new tender growth that will die with the first frost because it never had time to “harden off.”

Fertilizing with nitrogen before snow can create snow mold and kill your lawn which landscaper Roger Cooke discusses in the video below:

When spring rolls around again, if you already have an established lawn, then the best time to fertilize will be when grass has greened up and you’ve been able to mow a couple of times. Do this about 6 weeks after overseeding. Use regular fertilizer that has a higher ratio of nitrogen. 

In late spring and early summer, if your lawn has been a little bit neglected and needs a boost then you can apply a slow-release fertilizer in 45 to 60 day intervals.

My Preferred Slow-Release Organic Lawn Fertilizers

I use either Purely Organic Lawn Food or Milorganite on my lawn – both are effective organic options.

Compost is the best and most natural fertilizer that you can have available at your fingertips, and I try to apply a thin layer of compost to my entire lawn at least once every two years.

Using a dark, rich, and loose compost at least once every three or four years in the early fall can increase the nutrients in your soil naturally.

My town has an organic composting center where residents bring leaf and grass clippings, and residents are able to enjoy free screened compost in whatever quantity they need.

If you don’t have access to this, contact your local nursery – they can probably deliver screened compost to you. If you have a small yard, split a delivery with your neighbors.

How Do I Fertilize New Grass Seed?

Start by weeding the area that you will be planting in, then gently rake the top layer of soil to loosen it.

This is when to fertilize new grass seed. You can apply fertilizer to the soil, or you can do it at the same time as while you spread grass seed.

Spread your grass seed; a popular method is using a broadcast spreader.

Cover the seeds with a very fine layer of soil either by raking in one direction or sprinkling a little layer of soil using the same broadcast spreader. A very light watering is okay, just make sure not to uncover the seeds from their blanket of soil.

Growing, Growing, Gone

In summary, a lawn starter fertilizer high in phosphorus and quick-releasing nitrogen is ideal for starting a lawn from seed.

Regular slow-release fertilizer that is rich in nitrogen is best for planting sod or giving your existing lawn a boost. It is best to apply starter fertilizer just before, or at the same time as planting grass seed. Follow-up at least four to six weeks later with a regular fertilizer.

More frequent application can be harmful to your lawn and the environment, so don’t overdo it.

A final application of fertilizer in the fall (well before the first frost) can also provide a beneficial boost for your grass through the winter and lead to more growth come spring.

Just be careful about large doses of Nitrogen – the tender growth that results can die off with the first frost, which is the opposite of what you want.

I like to use a potassium-heavy fertilizer in the fall to support plant health and brace against the stress of extreme temperatures. This way my lawn is ready for an organic nitrogen treatment in the spring to green up beautifully.

Answered: When to Fertilize New Grass

Growing a thick and lush lawn that will be the envy of your neighborhood isn’t as complicated as it may seem.

Most grasses require only a small amount of maintenance to grow quite robust.

Properly timing the application of fertilizer can give you the most “bang for your buck.” It can also give your grass a boost in growth without burning your lawn or leading to harmful runoff.

Additional Resources

• When Should You Fertilize Your Lawn? by Jackie Jordan, Clemson University Cooperative Extension – Home & Garden Information Center (link)
• Time to Fertilize Your Lawn by Tom Kalb, North Dakota State University Extension (link)
• Turfgrass Fertilization: A Basic Guide for Professional Turfgrass Managers by Peter Landschoot, Ph.D., PennState Extension (link)