What is The Best Fertilizer to Use On New Grass Seed?

A lush lawn is an exciting prospect when you’re thinking about replanting your yard. You want the grass to grow in as thick and green as possible.  You want to know which type of fertilizer will do the job the best.

For newly planted grass seed, a starter fertilizer will work best for those early growth stages. As the grass grows, you can move on to stronger, normal-use fertilizers, but those won’t provide the proper nutrients for a budding lawn.

Compost is always an excellent fertilizing option, but again, the balance of nutrients for a new lawn might not be enough to ensure firm establishment and full coverage.

Soil Conditions

Before sowing grass seed, make sure your soil is prepared to nurture the new grass. Using a soil analysis kit, you can measure whether the grass is deficient or abundant in one or another nutrient. Then you’ll know precisely which nutrients need to be supplemented to stabilize the soil.

This doesn’t mean you won’t still need some kind of fertilizer, but knowing whether your soil is balanced, to begin with, is an excellent way to monitor your yard’s general health, as well as a starting point for planning your new lawn.

Daily watering will ensure that new seeds get enough moisture and that the soil has enough water to encourage deep root growth. Roots will stretch downwards as they absorb water and reach for nutrients and minerals, so keeping the soil moist constantly is vital in the first few weeks of germination.

Plant Nutrition

Grasses need three main nutrients to grow from seeds to lawn: nitrogen, phosphorus, and potassium. Each contributes an essential function or material for the grass’ growth process.

  • Nitrogen (N) – This element is the central part of any plant growth. It is a primary component of chlorophyll, the green building blocks of plants that metabolize light into energy. All plants use nitrogen, carbon, and water (oxygen and hydrogen) to build their leaves and grow foliage; for grasses, these are the blades.
  • Phosphorus (P) – This nutrient contributes to a plant’s energy usage, aiding in chemical reactions that make other nutrients and proteins available for the plant to grow. Phosphorus is also the main driver of root growth. Without healthy roots, you can’t have healthy grass.
  • Potassium (K) – Root growth is also supported by potassium, which helps build strong cell walls for all of the plant’s tissue structure. Potassium contributes to water retention and movement as well. It plays a part in metabolism and maintaining the grass’ immune system, supporting the plant’s resistance to disease, droughts, and other stressors.
Plant Nutrition

Do I Need Starter Fertilizer For New Grass Seed?

When you’re planting new grass seed, the type of fertilizer products you use will make a difference in how the grass grows. The fertilizer ratio will be different in starters than in normal-use products.

Following the formula N-P-K, products will list the nutrient content on the bag, which will be targeted at a certain kind of grass or desired outcome.

Starter fertilizers will contain more phosphorus than regular kinds, which usually doesn’t contain any at all (you’ll usually see a 0 in the center of the formula for regular fertilizers). This is because phosphorus supports root growth, and established systems don’t need that support.

Starter fertilizers also often contain quick-release nitrogen, which makes the nutrient readily available for absorption. This is important for the new growth of seedlings since the germination period is usually only up to three weeks long.

Will Regular Fertilizer Harm New Grass Seed?

Regular fertilizer will kill new grass since it doesn’t provide enough essential phosphorus to get the grass established and may be adding too much nitrogen or potassium for the seeds to use.

Conversely, a starter fertilizer won’t provide the right nutrients anymore for mature grass systems once the grass is established.

Some regular fertilizers contain herbicides, weed-killer that doesn’t harm established grass. During the germination process, however, the herbicide will disrupt the seeds from sprouting. Be sure not to apply herbicides for several weeks (up to 10) before or after planting grass seed. 

What To Do With Your Existing Lawn

You have two choices when planting a new lawn: overseed your existing lawn or till the grass and soil to plant anew. When you overseed your lawn, you simply spread grass seed and starter fertilizer over the existing yard.

Some people like this option to reinforce the existing lawn, but sometimes the grass system is older or strained and could use total replacement.

When you till over the lawn to expose the soil for new planting, you’re actually taking a step to fertilize the soil before adding any supplementary fertilizer. All of the nutrients grass needs to grow will be present in the blades, roots, and other organic material that will decompose into the soil, known as green mulching.

Still, it will be helpful to add starter fertilizer to make sure the seeds get the proper nutrient amounts, especially phosphorus, for root growth.

When Should I Fertilize New Grass Seed?

Fertilizer should be applied at the same time as, or just before, planting grass seed. This gives the seeds a chance to use the freshest nutrients during the first couple of months of life.

Should I Put Down Grass Seed or Fertilizer First?

How you plant your new grass seed will determine the order of application for the starter fertilizer. Remember, whether you overseed or plant in exposed soil, using fertilizer for mature grass will be too strong of a formula for germinating seedlings.

If you overseed your existing lawn, you can sow the seeds first (after raking any thatch buildup) and then apply the starter fertilizer across the lawn to lightly cover the seeds and deliver the proper nutrients. The new seeds will germinate and mix into the existing grass matrix.

If you’re planting in freshly tilled soil, apply the starter fertilizer first, then sow the seed. This way, seeds will get the amount of light they need to germinate across the entire yard, and they will settle into the soil with regular watering.

What If My Grass Seed Doesn’t Grow?

What If My Grass Seed Doesn’t Grow

Grass will generally germinate two or three weeks after planting. If you don’t notice sprouts and seedlings after that period of time, there are a few reasons why the grass may not have grown:

  • Seedlings need water daily during their germination period, but too much water can drown them. Check the soil moisture depth to see how wet the soil is below the surface.
  • New plants need sunlight to germinate, but too much sun will dry out the ground and the seeds. A good balance of shade and sun exposure, specific to the type of grass planted, will help the grass grow in healthy.
  • If you didn’t spread enough seeds, the new grass might not grow into the widespread, complex plan system that we want to cover our yard. Check the package as well to be sure the seeds weren’t too old to germinate.
  • It might be that you didn’t use enough fertilizer or that the soil balance was still off. Try another soil sample test to review the nutrient profile.

If your grass hasn’t grown, and it’s still early in the year, there may be time to replant and try again. Spring grasses should be planted in March, April, or May before the daily temperatures get too hot for newly sprouted grass.

Make sure to plant cool-weather grasses in September or October, several weeks before the first frost.

When grass seed doesn’t establish, planting sod (patches of already grown grass and roots) can be a good way to get your yard going. When pieces of sod are planted and watered, they’ll fill in the soil between pieces until the yard is covered.

Starter fertilizer can be mixed into the soil where the sod will fill in, and you can use regular fertilizer on the sod itself since the grass in them is well established.


Author: Matt Hagens

Hi, I’m Matt the owner of Obsessed Lawn. I love to be outside working on my lawn, planning my next project. I created this website to help people like you find the best products for yard care and great advice. Learn more about me and find me on Facebook.