Seeding After Slow-Release Fertilizer: Timing and Weed Management

Can You Seed in September After August Fertilizing?

Your lawn company applied granular slow-release fertilizer on August 23rd but skipped the promised weed spray. Now you're wondering if September seeding is safe. The answer depends entirely on what was actually in that spreader.

Critical First Check: Weed-and-Feed vs. Straight Fertilizer

If it was weed-and-feed: You must wait. The pre-emergent herbicide component prevents grass seed germination—often for 8-12 weeks. Check the invoice or ask for the product label.

If it was straight fertilizer: You can seed immediately, but you'll inherit the weed problem.

The Weed-Seeding Dilemma

New grass seed requires frequent, light watering for germination. Unfortunately, this same moisture supercharges existing weeds. You have three paths forward:

Option 1: Spot-Treat Bad Patches

Use glyphosate (RoundUp®) on concentrated weed areas, wait 7-10 days, then reseed those spots.

Option 2: Pre-Emergent + Delayed Seeding

Apply pre-emergent herbicide, wait the label-specified interval (usually 6-8 weeks), then seed. Crucial: Pre-emergents don't distinguish between weed seeds and grass seeds—they stop all root development from seeds.

Option 3: Seed Through Light Weed Pressure

If weeds are sparse, seed anyway and manage weeds later with selective post-emergents once new grass is established (usually after 3-4 mowings).

September Seeding Success Tips

• Water daily (sometimes twice) until germination
• Use starter fertilizer at seeding (phosphorus for root development)
• Mow high (3-4") once grass reaches 4" height
• Apply selective broadleaf weed control after 3rd mowing

Fall is actually ideal for cool-season grasses. Just ensure the fertilizer wasn't a weed-and-feed product first.