When to Fertilize New Grass Seed for the Best Yard

There’s nothing worse than tackling the time-consuming task of laying a new lawn, only for it to turn out patchy and yellow. Getting your lawn off to a flying start is all about the prep, and using the right fertilizer for starting grass—and applying it at the right time—is key.

So, if you want thick, green grass that will make your neighbors green with envy, read on to learn about the best fertilizers for starting grass and when to use them. 

When to Fertilize Grass Seedlings or New Grass

Don’t take the slapdash approach when considering a new lawn’s canvas. As part of your game plan, you’ll likely have to apply a specific starter fertilizer around six to eight weeks before spreading grass seeds or sod. Rake the soil lightly before applying the fertilizer. This gives the soil the chance to accept all those valuable fertilizer nutrients, ready for the seeds to soak them up as they sprout. After four to eight weeks, when the grass reaches an inch tall, apply standard fertilizer to give it an extra boost.

So, if you’re seeding your grass in the spring (most common with warm-season grasses), you’ll want to put down a starter fertilizer sometimes in early to mid-spring. For cool-season grasses, often best sown in the fall, you’ll want to put the fertilizer down in late summer or early fall.

Timings vary depending on your local climate, the grass species, and the specific starter fertilizer you choose, so always check manufacturer instructions. When in doubt, a local lawn fertilizer service can make sure the job gets done right.

Why Select a Starter Fertilizer?

You might wonder why you can’t just use that bottle of standard lawn fertilizer tucked away in your garage when you're laying grass seed for a new lawn. Starter fertilizers are specially formulated with higher levels of the key nutrient phosphorus, which the limited root system of seedlings struggle to pull from the soil, and a quick-release form of nitrogen.  

Applying a starter promotes robust, rapid root growth. It also works effectively in the cooler, wet soils you’re more likely to experience in the early spring and late fall seeding times.

How to Select the Best Fertilizer for New Grass?

There isn’t one “best” starter fertilizer. When looking through the lineup, keep in mind the local climate, the grass species you're growing, and any soil test results. And steer clear of fertilizers containing herbicides, as these can slow or stunt germination.

Every bag of fertilizer has NPK numbers on it. It might look baffling, but this simply relates to the percentage of nitrogen, phosphorus, and potassium they contain. The P number should be high for a starter fertilizer with typical ratios of 1-2-1, 1-3-1, 1-4-2, or 1-5-0. Good starter fertilizers often have a phosphorus level of around 20%. 

If you put the fertilizer down well before sprinkling the seeds, you’ll save money and time by tool-spreading slow-release granular rather than liquid fertilizer.

Get a Soil Test

Because there’s such a wide range of starter fertilizers to choose from, take the risky guesswork out of things by organizing a soil test to find out its existing nutrient levels. Test your soil samples using an at-home kit or send them off to a lab. It takes up to two weeks to get results back, so factor this time into your schedule. If you’re feeling flush or not confident about analyzing the results, call in a soil testing pro near you.

Why You Shouldn’t Use Starter Fertilizer Year-Round

Don’t bulk-buy a starter fertilizer—it’s not suitable for use once your lawn is established. Too much phosphorus in the soil makes it tough for your grass to absorb other beneficial micronutrients. Plus, excess phosphorus from synthetic fertilizer runoff affects water quality. 

Many states have laws restricting high phosphorus fertilizer use, so it’s worth checking with your local extension office what the rules are in your region.

And, if your grass is looking lovely and lush, you can skip the standard fertilizer application after the grass has germinated. Too much nitrogen burns new grass, especially if you're lax with your lawn watering schedule.

When to Fertilize Established Grass

Once your new lawn is up and running, you only need to fertilize it once or twice a year to keep it green and in good shape. Timings, fertilizer type, and quantities vary depending on your hardiness zone, grass species, and soil conditions. Ideally, you want to apply a general lawn maintenance fertilizer when the roots are growing more actively than the blades. For warm-season grasses, this is during the late spring or early summer, and cool-season grasses appreciate a feed mid-fall and possibly early spring.

And don’t get carried away—too much fertilizer can burn your lawn, leaving it with ugly yellow or brown stripes and dead patches.

Gemma Johnstone contributed to this piece.