Can I Mix Fertilizer and Grass Seed?
You can apply fertilizer and grass seed at the same time, but do not physically mix them together in the same spreader hopper. Apply starter fertilizer first, lightly rake it in, then broadcast seed separately. The different particle sizes of fertilizer granules and grass seed cause uneven distribution when mixed. Use a low-nitrogen starter fertilizer (high phosphorus) rather than regular lawn fertilizer, which can burn emerging seedlings with excessive nitrogen salt concentrations.
Short Answer: Keep Them Separate
Do not mix fertilizer granules and grass seed in the same spreader hopper. The two products have different particle sizes and densities, causing them to separate during spreading. The heavier fertilizer granules settle to the bottom while lighter seed floats to the top, resulting in uneven distribution of both.
Why Separate Application Works Better
Apply starter fertilizer first at the recommended rate, then make a second pass with your grass seed. This ensures each product is spread at its correct setting. Most broadcast spreaders have different dial settings for fertilizer versus seed. Using one setting for a mix guarantees that either the seed rate or fertilizer rate (or both) will be wrong.
Which Fertilizer to Use When Seeding
Use a starter fertilizer with high phosphorus, such as 10-18-10 or Scotts Starter (18-24-12). Phosphorus is critical for root development in new seedlings. Standard lawn fertilizers with high nitrogen (like 32-0-4) can burn tender new seedlings. The phosphorus exception to state fertilizer laws specifically allows phosphorus application when establishing new grass.
Application Order for New Lawns
Prepare the soil surface first by raking to a smooth, firm seedbed. Apply starter fertilizer and lightly rake it in. Spread grass seed at the recommended rate. Cover seed lightly with a thin layer of peat moss or straw mulch. Water gently to avoid displacing seed.
When to Start Regular Fertilizer
Wait until new grass has been mowed at least 3 times (about 6-8 weeks after germination) before switching to a standard lawn fertilizer. Young seedlings are sensitive to nitrogen burn. After the third mowing, the root system is established enough to handle regular fertilizer rates.
What About Pre-Mixed Seed and Fertilizer Products?
Products like Scotts Turf Builder Grass Seed with starter fertilizer built in are designed with the correct ratio and matching particle sizes. These work fine because the manufacturer engineered the mix for even distribution. DIY mixing of separate products does not achieve the same result.
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