When planting new grass seed, a common question that comes up is whether you should fertilize at the same time as seeding. There are pros and cons to fertilizing at seeding, and the answer depends on several factors.
In most cases, it is recommended not to fertilize when seeding a new lawn. Fertilizer can encourage weed growth which can outcompete new grass seedlings. Additionally, new grass plants are small and do not need a lot of nutrients right away. It is better to wait until the new grass is established (3-4 mowings) before fertilizing.
When to Fertilize a New Lawn
Here are some general guidelines on fertilizing a newly seeded lawn:
- Do not fertilize at time of seeding – This can lead to weed growth.
- Wait until new grass has been mowed 3-4 times before fertilizing.
- Use a starter fertilizer when sowing grass seed – Starter fertilizers are lower in nitrogen to prevent weed growth.
- Fertilize newly seeded grass 6-8 weeks after germination – After several mowings, the grass will be established enough to benefit from fertilizer.
- Use a balanced fertilizer with lower nitrogen – Look for fertilizers in a ratio like 10-10-10 or 12-12-12.
- Follow up with regular fertilization – Fertilize established lawns at least 1-2 times per year in the spring and fall.
Should You Fertilize When Seeding Grass?
Fertilizing at the same time as seeding is generally not recommended. Here’s why:
Fertilizer Causes Weed Growth
When you apply fertilizer, you are giving nutrients not just to grass seeds but also to any weed seeds that are present in the soil. Weed seeds will thrive with the boost from fertilizer and can outcompete new grass seedlings.
New Grass is Small and Doesn’t Need Much Nitrogen
Grass seedlings are small initally and have limited ability to take up nutrients. Flooding the lawn with nitrogen fertilizer provides excess nutrients that the young grass plants do not need. This leads to fertilizer being wasted.
Risk of Burning New Grass
Some soluble nitrogen fertilizers can burn or damage tender new grass shoots. It’s best to wait until the grass plants are more mature before fertilizing.
Wait 3-4 Mowings Before Fertilizing
A good general guideline is to wait until the new grass has been mowed 3-4 times before fertilizing. This means the grass has gone through initial establishment and has a deeper root system to take up nutrients. Fertilizing at 3-4 weeks post-germination is a good timing.
When is it OK to Fertilize at Seeding?
In some cases, applying fertilizer at seeding may be beneficial:
- If using a starter fertilizer – Starter fertilizers have a higher phosphorus ratio which helps with root establishment.
- When planting on poor, infertile soils – Adding a small amount of balanced fertilizer can help give seeds nutrients to germinate.
- When seeding in the fall – The cooler weather can help reduce the risk of burning new grass.
In any case, use half the normal application rate if fertilizing at seeding. This will provide some nutrients, while minimizing risk to young grass plants.
Tips for Fertilizing a New Lawn
Here are some key tips to keep in mind when fertilizing a newly seeded lawn:
Wait Until Established
It’s always safest to wait until new grass has gone through 3-4 mowings before fertilizing. This allows the grass to mature and build a deeper root system before receiving nutrients.
Use a Starter Fertilizer at Seeding
Starter fertilizers like 10-20-10 or 5-10-5 are a better choice at seeding time if you choose to fertilize. The lower nitrogen and higher phosphorus benefit new transplants and seeds.
Fertilize in Fall for Cool Season Grass
The cooler temperatures in fall allow for safer fertilization of newly seeded cool season grasses like fescue and bluegrass. Avoid fertilizing in hot summer months.
Use a Balanced Fertilizer
Choose a balanced fertilizer with a ratio similar to the nutrients grasses need, such as 10-10-10 or 12-12-12. Avoid high nitrogen formulas which can burn seedlings.
Follow Directions Closely
Carefully follow the application rate and instructions on the fertilizer bag. Never apply excessive amounts which can damage or burn new grass.
Water in Fertilizer
Lightly water after applying fertilizer to help move it into the soil. Avoid heavy watering which can lead to runoff.
Reapply as Needed
Once the lawn is established, fertilize again in 6-8 weeks if grass is slow to green up or fill in. Mature lawns should be fertilized 1-2 times per year.
Starter vs Regular Fertilizer at Seeding
Starter fertilizers are specially formulated for use when planting new grass seed. Regular lawn fertilizers can also be used at seeding time, but some adjustments are needed.
- Lower nitrogen (ex. 10-20-10)
- Higher phosphorus for root growth
- Safer for young tender grass plants
- Less chance of burning new grass
- Lessens weed competition
- Enhances seedling establishment
- Usually higher nitrogen (ex. 20-10-10)
- Higher burn risk on new grass
- Can lead to weed competition
- Best to cut application rate in half
- Wait until grass is 3-4 inches tall
- Use extreme care to avoid burning new lawn
While starter fertilizer is ideal for use at seeding time, a regular balanced fertilizer can also work if applied carefully at a reduced rate.
How Much Fertilizer Per 1000 sq ft at Seeding?
When fertilizing at the time of seeding, it is important to use reduced rates to prevent burning new grass.
Starter Fertilizer Rate
- 5-10 lbs per 1000 sq ft
- Read bag for exact rate – varies by brand
- Spread evenly over seeded area
- Rake lightly into top 1/4″ of soil
Regular Fertilizer Rate
- 3-5 lbs per 1000 sq ft
- Cut normal rate in half
- Apply when grass is 3-4 inches tall
- water gently after application
Regardless of fertilizer type, it is vital to use conservative rates and follow label directions to avoid burning newly germinated grass.
Should I Use Weed Killer When Seeding Grass?
When seeding a new lawn, weed killer is generally not recommended.
Reasons to Avoid Weed Killer
- Weed killers often prevent new grass seed from germinating
- Residual weed killers can remain in soil for weeks or months
- Hard to find selective weed killers safe for grass seed
- Safer to manually remove existing weeds
- New grass will crowd out weeds in time
When Weed Killer May Work
- Spot treating isolated weeds after germination
- Using very short-term, selective weed killers
- Applying before seeding and waiting several weeks before seeding
The best approach is to eliminate existing weeds through manual weeding, tillage or non-selective herbicides (like glyphosate) at least 2-3 weeks before seeding the new lawn. Avoid weed killers at time of seeding.
Should I Use Lime When Seeding Grass?
Lime is often beneficial when establishing a new lawn from seed. However, timing of the application depends on your conditions.
Reasons to Use Lime
- Raises pH of acidic soils for better seed germination
- Provides calcium for healthy plant growth
- Counters soil acidity caused by fertilizers
- Improves microbial activity in soil
- Enhances nutrient availability for plants
When to Apply Lime
- If soil test shows low pH, lime before seeding
- Allow several weeks for lime to react before seeding
- Only use lime if soil is acidic (pH below 6.0)
- Lime after seeding risks damage to young plants
Lime is an important soil amendment when establishing lawns on acidic soils. Just be sure to test pH and allow time for the lime to react before seeding.
How Long After Seeding to Fertilize?
It’s best to wait 3-4 weeks after seeding before fertilizing a new lawn. Here are some guidelines on timing:
- Wait until new grass has been mowed 3-4 times
- Allow 2-3 weeks after germination before fertilizing
- Fertilize cool season grass 6-8 weeks after seeding
- Warm season grasses may need 8-10 weeks to establish before fertilizing
- If using starter fertilizer, 2-3 weeks is OK
The most important thing is to allow enough time for the new grass plants to mature and develop an established root system before applying fertilizer.
6 Key Tips on Timing
- Wait until 3-4 mowings to fertilize
- Allow at least 3 weeks after grass emerges
- Don’t fertilize right at seeding time
- Let grass get 3-4 inches tall before fertilizing
- Water lawn gently after fertilizing
- If unsure, wait longer until lawn is thicker
Following these guidelines will give new grass the best opportunity to flourish when fertilizer is eventually applied.
New Grass Seed Fertilizing Schedule
Here is a typical fertilizer schedule for a newly seeded lawn:
|Use starter fertilizer or no fertilizer
|2-4 weeks after seeding
|6-8 weeks after seeding
|Light application of balanced fertilizer
|Fertilize established lawn
|Fertilize established lawn
This schedule allows new grass to mature before fertilizing while providing nutrients on a regular basis once the lawn is established.
Tips for Fertilizing New Grass Seed
- Read and follow fertilizer label rates
- Apply starter fertilizer at seeding or wait 2-3 weeks
- Allow 3-4 mowings before regular fertilizer
- Use balanced fertilizer with low nitrogen
- Spread evenly and water gently after
- Repeat fertilization per lawn’s needs
Following proper timing and rates will allow new grass to thrive while avoiding potential fertilizer damage.
What to Look for When Buying Fertilizer for New Grass
When purchasing fertilizer for newly seeded grass, look for products with:
- Balanced NPK ratios like 10-10-10 or 15-15-15
- Lower nitrogen (first number)
- Higher phosphorus (middle number) – aids rooting
- Moderate potassium (last number)
- Starter fertilizer label for seeding
- Slow-release nitrogen
- Natural or organic sources preferred
Avoid fertilizers with very high nitrogen levels as this can easily burn young grass. Read labels closely to choose an appropriate product.
When to Use Starter Fertilizer
Starter fertilizers are specifically designed for use when establishing a new lawn from seed. Use starter fertilizer in these situations:
- At time of seeding in spring or fall
- Overseeding existing lawn
- Seeding poor, infertile soils
- Laying new sod
- Repairing bare spots and patches
The lower nitrogen and higher phosphorus in starter fertilizer helps new grass seeds germinate and get established.
Fertilizing a newly seeded lawn requires careful timing. It’s usually best to wait until new grass has established before fertilizing. Starter fertilizers can be beneficial at seeding time in some cases. But for most situations, waiting 3-4 weeks after germination and allowing 3-4 mowings before fertilizing will give new grass the greatest benefit while avoiding potential damage from fertilizer.
Following proper timing, using lower nitrogen products, reducing application rates, and fertilizing appropriately for the grass species and season will give new lawns the healthiest start.