Will Fertilizer Kill New Grass?

A person applying fertilizer to a grass

New lawns are vulnerable to fertilizer damage in more than one way. Excess fertilizer can kill new grass before it germinates, and while it struggles to grow proper roots and leaves. But don’t worry — fertilizer damage is easy enough to avoid with attention and the right information.

Read this guide and learn the best fertilizer to use on a new lawn, how to apply it correctly, where the risk of fertilizer damage comes from, and how to protect the new turfgrass while still providing proper nutrients for growth. 

Three ways fertilizers can kill new grass

New grass needs nutrients close to its roots to grow healthy and strong. But when you apply too much fertilizer or the wrong type, fertilizers can turn against you and leave the yard patchy and thin. Here’s why.

Having too much (fertilizer) salt in the soil

Commercial lawn fertilizers are rich in salts like ammonium sulfate, potassium chloride, and sodium nitrate. When they accumulate around grass seeds and roots, fertilizer salts draw water away. Grass seeds dry out and fail to germinate, and sprouted plants die because their roots can’t absorb water.

In high amounts, salts are damaging for new and established lawns alike, covering the turf in signs resembling drought stress:

  • Grass blades take a discolored, straw-like appearance
  • Patches of lawn look scorched, covered in yellow and brown grass
  • If you touch the grass, it feels crunchy and crispy
  • Checkerboard-like shapes with scorched turf in the middle appear where the fertilizer spreader path overlaps

Nevertheless, new grass is the most vulnerable. If you apply too much fertilizer when seeding, you’ll see many bare spots where grass should have germinated but didn’t. Newly installed sod is also sensitive to overfertilization because its roots are not yet established and are still shallow and weak.

Applying weed and feed

Another way to kill grass by applying fertilizer is to use a weed-and-feed when seeding or installing sod. 

Weed and feed are 2-in-1 products that mix nutrients with a herbicide. These fertilizers are meant to solve two problems — feed the grass and kill the weeds — in one pass of a spreader. Instead, they often cause fertilizer burn and toxic chemicals to leach into the soil and water. 

For new grass, a weed-and-feed can be deadly. 

Some weed-and-feed products contain a pre-emergent herbicide that prevents seed germination for weeds and grass. If you spread it before or right after seeding, it will stop grass seeds from growing their first roots and leave your lawn bare or highly thinned. 

Others use a selective post-emergent herbicide that kills only weeds and not grass. Selective herbicides are safe in well-established lawns. However, in a new lawn, they can be toxic for young plants that have barely put some shallow roots into the soil.

Spreading fertilizer on dry soil

Think about it like this: Mineral salts from the fertilizer you’re spreading compete for water with your grass. If the soil is moist when you apply fertilizer and there’s plenty of water to go around, all is well. If you spread fertilizer on dry soil, salts will draw whatever moisture is left in the ground, desiccating seeds and depriving grass plants of water.

Why does new grass need fertilizer?

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The seeds you’re spreading across your lawn contain enough food and energy to get your grass to germinate (grow its first roots and leaves). After this stage, new sprouted grass must find and absorb nutrients from the soil. When the soil lacks proper nutrients, grass grows slower, thinner, and weaker. You need to apply a grass fertilizer with the proper nutrients to enjoy a thick, healthy lawn that turns your yard green in a few weeks. 

Phosphorus is especially important because it supports grassroots, and at this stage, you’ll want to help the grass grow a robust, deep root system. Phosphorus is also less mobile in soil (it doesn’t travel much with water) and must be close to the root system for the grass to access it.

Applying a proper lawn fertilizer provides new grass with all the needed nutrients directly in the top layer of the soil, where their small, shallow young roots can easily absorb them.

Starter fertilizer: The best fertilizer for new grass

What is a starter fertilizer? It’s a special lawn fertilizer required to feed seedlings and grass that has not yet been established. Starter fertilizers contain more phosphorus than regular fertilizers and a balanced mix of potassium and nitrogen. 

Nitrogen, phosphorus, and potassium are essential for grass growth:

  • Nitrogen (N) is essential for green growth and supports stem and leaf development. You need a lot in established lawns because established grass already has strong roots, and it’s focused on growing lush, green blades. If you’re feeding new grass, you’ll need to tone down nitrogen a notch and allow your turf to focus on growing its roots.
  • Phosphorus (P) is essential for root growth. It is also a common source of nutrient pollution, so some states only allow it when installing new lawns. Before choosing your fertilizer, check the city or state’s limitations regarding phosphorus fertilizer use.
  • Potassium (P), or potash, is a nutrient for whole-plant development. It supports grass metabolic functions and helps plants fight diseases and stress. 

Starter fertilizers are balanced mixes that include all three macronutrients. When looking for the best lawn fertilizer for your new grass, read the NPK ratio to determine the amount of nitrogen, phosphorus, and potash in your product. 

For example, an NPK ratio of 18-3-8 means 10 parts nitrogen, 2 parts phosphorus, and 8 parts potassium. Suitable NPK ratios for a starter fertilizer are 10-10-10, 16-8-8, and 20-10-10. 

Here are some examples of starter fertilizers to consider:

Can you use regular fertilizer for new grass seeds? 

If applied correctly, regular fertilizer won’t kill new grass, but it’s not the best type of fertilizer for a newly planted lawn. You should avoid it, and here’s why. 

Regular fertilizers contain a higher concentration of nitrogen and potassium and little to no phosphorus. They’re meant to boost leaf growth and support mature plants, and they will do so in your grass seedlings at the expense of the root system. The result is growing wobbly, thin, weak grass that is more exposed to diseases and drought stress. 

Should you choose liquid or granular fertilizer? 

Liquid and granular fertilizers are both good options for a starter application. Liquid fertilizers give grass plants better access to phosphorus, are easier to spray, and have a more uniform concentration drop-by-drop. On the other hand, they’re more expensive and also a quick-release fertilizer, so they don’t last as long.

Granular fertilizers are more popular because they are more affordable. They’re slow-release fertilizers, so they last longer in the soil. On the other hand, they contain more salt, and if misused, they come with a higher risk of killing the turf. 

Do you need a soil test?

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You might wonder why there are so many NPK ratios out there. Setting aside the grass development stage, you also choose the fertilizer and its NPK ratio depending on the state of your soil. For example, if your soil is already rich in potassium, you’ll look for a fertilizer with a lower amount. 

Take a soil test to determine what nutrients your soil has and lacks. Soil test kits are easy to buy online. You also can send a soil sample to the local Extension Office and get a more detailed, professional soil test done in a lab (we recommend it).

Testing the soil also lets you know if the pH is suitable for grass growth. Turfgrass thrives in slightly acidic soil with a pH of 6.5 to 7.0, which allows proper nutrient absorption. Your fertilizer won’t have the desired effect if the soil is too acidic or alkaline, and you’ll need to amend it:

  • Apply lime to reduce soil acidity.
  • Use sulfur if the soil is too alkaline. 

Check the lime/sulfur application rate on the product package.

Note: When planting seeds in poor soil lacking in organic matter, consider organic starter fertilizers. Organic fertilizers release nitrogen slowly, provide organic matter and valuable micronutrients, and have a lower risk of nitrogen burn.

How to apply fertilizer on new grass

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Lawn care pros recommend applying a starter fertilizer before or right after spreading new grass seeds or installing new sod. Here are the steps to make it a successful and safe fertilizer application.

Prepare the soil

You’re preparing the soil for seeding, but all the tasks you need to do also help the fertilizer disperse better and more evenly into the soil. 

  • Clear the area of debris (garden furniture, flower pots, dry leaves, sticks, branches, toys, stones, etc.).
  • Remove unwanted grass and weeds. If you need to use a herbicide to kill nasty lawn weeds like crabgrass or nutsedge, wait two to four weeks until seeding or installing sod. Check the waiting period on the package.
  • Remove the thatch layer to provide a clean soil surface for the new grass. Thatch is a layer of dead organic matter that forms a mat over the soil and prevents seeds, water, and fertilizer from reaching it. 
  • Consider aerating if the soil is compacted. Soil compaction limits fertilizer absorption and also prevents seeds and sod from properly setting roots. Core aeration solves the problem by making tiny holes in the soil and loosening the ground.
  • Water the soil a few days before seeding or laying sod. A proper moisture level deep into the soil helps grass develop roots more easily and prevents fertilizer burn.

Determine how much fertilizer you need

Overfertilization is one sure way to kill newly sprouted grass. To ensure your grass grows strong and healthy, measure the fertilized area and determine how much starter fertilizer you have to spread. 

Measure the seeded area. Approximate the area you plan to seed or lay sod on to a rectangle (or multiple rectangles if it’s an irregular shape). Calculate the rectangle area by multiplying its length by its width. For example, if your future lawn is 30 feet wide and 70 feet long, the area is 2,100 square feet. 

Check the recommended application rate. Typically, you’d spread 0.5 to 1 lb. of fertilizer per 1,000 square feet of seeded soil. However, the safest way to determine the correct amount is to check the recommended application rate on the fertilizer package.

Calculate the amount of fertilizer. For example, if the lawn is 2,100 square feet and the application rate is 0.5 lbs per 1,000 square feet, you’ll need about one pound of starter fertilizer (2,100 square feet divided by 1,000 square feet, then multiplied by 0.5 lbs.)

If you’re using a liquid fertilizer that you must dilute, follow the instructions carefully. Using a starter fertilizer that is too concentrated can kill new grass before it has a chance to grow. 

Note: Always use a scale or a measuring cup when adding fertilizer from a package to the spreader. Don’t approximate the amount.

Apply the starter fertilizer

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  • Begin spreading the fertilizer from one corner of your lawn. Walk the spreader straight to the other edge of the lawn, then turn and walk back on a parallel strip that slightly overlaps with the previous one. Continue until you cover the entire area.
  • Use a broadcast spreader to apply the fertilizer even across the lawn.
  • Turn off the spreader every time you stop walking or make a turn at the lawn’s edge. 
  • Work the fertilizer granules into the topsoil 4 to 6 inches deep. 
  • Avoid spilling fertilizer on your driveway and walkways. If it happens, clean the area to prevent runoff. 
  • Immediately remove any fertilizer accidentally spilled on the lawn. If liquid, water the soil to disperse it. 

To learn more about how to spread fertilizer and take it to a professional level, read our guide on how to spread fertilizer.

Plant the grass

Once you spread the fertilizer, the soil is all set to receive the grass seeds or the sod strips you plan to install. We’ve got detailed guides for both strategies, so go ahead and read all about them:

Irrigate the lawn

Properly watering the lawn is necessary for seeds to germinate and sod to set its roots into the ground. It also ensures the fertilizer you’ve spread starts to decompose and release nutrients into the soil. Here’s how to water your newly seeded lawn:

  • Week 1: Apply ⅛ to ¼ inches of water per day in 2 to 4 sessions about 10 minutes long. Keep the top 1-2 inches of soil moist (not soggy) to prevent grass seed from drying out.
  • Weeks 2 and 3: Meet your new grass seedlings! Once the seeds sprout and the lawn is almost entirely covered in short, thin grass blades, reduce watering to one or two sessions daily and then to one session every other day.
  • Weeks 4 and 5: By this time, you should start training the grassroots for more robust, deep growth. In lawn irrigation rules, this means watering deeply and less often: one or twice a week, long enough to moisten the top 6 to 8 inches of soil.

Note: Avoid evening irrigations; they expose your grass to fungal infections. Water early in the morning, preferably between 6 a.m. and 10 a.m. 

When to fertilize new grass

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New grass fertilization refers primarily to the starter fertilizer you’re adding when seeding a new lawn, laying sod, or overseeding an existing lawn. The best time for starter fertilizer application is the same as the best time of year for planting grass:

  • Late spring to early summer is an excellent time for planting warm-season grasses. Also known as summer grasses, turf types like Zoysia and Bermuda thrive in the summer heat.
  • Plant cool-season grasses from late summer to early fall. These turf types (Kentucky bluegrass, tall fescue, ryegrass) grow best in the mild weather of fall and spring. Early spring is another good moment to seed winter grasses if you missed the fall.

How often should you fertilize new grass? Lawn care experts recommend applying the second fertilizer only after six to eight weeks of spreading the starter. 

Most starter fertilizers feed the grass for at least a month, but this waiting time differs from product to product. The best way to plan your next fertilizer application is to read the indications on the starter fertilizer package.

After six to eight months, your grass is established, and what your lawn needs as nutrients changes. You’re no longer using a starter fertilizer but a regular one, and the application is slightly different. We cover these particularities in our guide, Fertilizer 101: How to Fertilize Your Lawn.

Lawn maintenance for new grass

You’ve prepared the soil, applied the fertilizer, seeded the grass, and watered the lawn, and now your yard is covered in beautiful green grass. But the care for young turf is not yet over. To ensure it grows properly, follow these instructions:

  • Avoid mowing too soon. We know it’s a unique joy to lead the lawn mower over the grass for the first time but be patient. If you mow new grass too soon, you risk damaging it. Wait until it grows at least half an inch taller than the recommended mowing height before cutting it.
  • Use the grass clippings as mulch. Once you start mowing, use the grass clippings as a mulch. They make for an excellent organic fertilizer rich in slow-release nitrogen that feeds the grass naturally.
  • Keep your feet off the grass. New grass is sensitive to foot traffic, so keep pets, kids, and heavy lawn care tools away if possible.
  • Consider organic fertilizers. Spreading topdressings like compost and decomposed manure is a healthy, safer way to feed the lawn with a lower risk of nitrogen burn.

FAQ about fertilizers and new grass

Can I put grass seed and fertilizer down at the same time?

You can put grass seed and fertilizer down at the same time when overseeding, but it’s not recommended when seeding a new lawn or patching up bare spots. 

While it might save time and effort, it can ruin seeding and fertilizer applications because you can’t control their application rates. You’ll get unevenly spread grass and possibly fertilizer burn.

Will grass grow back after fertilizer burn?

If the turfgrass still has some functional roots, it will recover with proper treatment: 

  • Watering the area to flush out the excess fertilizer
  • Aerating to improve airflow
  • Correcting irrigation – prevent the soil from drying out, but also avoid keeping it too moist

If the roots are badly damaged, the grass will not grow back, and you’ll need to reseed or resod. 

How long does it take for granular fertilizer to dissolve?

Depending on the product, granular fertilizer can take days to weeks to dissolve into the soil and release its nutrients.

Leave the fertilizer problems to the pros!

Planting new grass and watching it green up your yard is a unique experience all homeowners enjoy. But you don’t need to endure the DIY seeding and fertilizing process to rip off the rewards. Why worry about killing your new grass with fertilizers when a pro can seed your lawn without risks? Find a local lawn care company with LawnLove and treat yourself to a lush, green lawn you can admire every morning!

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Sinziana Spiridon

Sinziana Spiridon is an outdoorsy blog writer with a green thumb and a passion for organic gardening. When not writing about weeds, pests, soil, and growing plants, she's tending to her veggie garden and the lovely turf strip in her front yard.