How Long to Wait Before Reseeding After Weed Killer

Wait Times at a Glance

The golden rule: always check the product label. The wait times above are general guidelines. Manufacturer recommendations take precedence, especially for combination products containing multiple active ingredients.

If you’re planning a full lawn renovation — killing everything and starting fresh — glyphosate is the fastest route because it deactivates on contact with soil. You can reseed just a week after treatment. For selective lawn weed killers, the wait is much longer because the active ingredients persist in the soil.

Why the Wait Matters

Different weed killers behave very differently in soil:

• Glyphosate binds to soil particles almost immediately and is broken down by soil microbes within days. That’s why you can reseed so quickly after using it — by the time seeds germinate, the glyphosate is gone
• Selective lawn herbicides (MCPA, 2,4-D, mecoprop-P) are designed to persist long enough to be fully absorbed by weeds. This means they also persist long enough to damage grass seedlings if you sow too soon
• Residual herbicides (sometimes found in path weed killers) are specifically designed to stay active in soil for months. These are completely incompatible with reseeding — check the label carefully

Temperature affects breakdown too. Soil microbes are most active in warm, moist conditions (spring and early autumn). In cold winter soil, herbicide residues break down much more slowly.

Step-by-Step: Reseeding After Glyphosate

If you’ve used glyphosate to clear an area completely — the most common scenario for lawn renovation — here’s the process:

1. Spray and wait 7-14 days. Give the glyphosate time to work fully. You’ll see weeds yellowing and dying back. For tough perennial weeds like couch grass, a second application after 4-6 weeks is usually needed before you start
2. Remove dead vegetation. Rake off the dead growth. For small areas, hand-rake. For larger lawns, a scarifier on its lowest setting removes dead material quickly
3. Prepare the soil. Loosen the top 2-3cm with a rake or rotavator. Break up any compacted areas. A fine, crumbly surface (called a tilth) gives seeds the best chance of making contact with soil
4. Level and firm. Rake level, then walk over the area to gently firm the surface. You want it firm but not compacted — your feet should leave shallow impressions, not deep footprints
5. Sow seed. Scatter evenly at the rate stated on the bag. For most lawn seed, that’s 30-50g per square metre. Sow half in one direction and half at right angles for even coverage
6. Lightly rake in. Just enough to cover most seeds with a thin layer of soil. Seeds need light to germinate, so don’t bury them deeply
7. Water gently. Use a fine spray or oscillating sprinkler. Keep the surface moist (not waterlogged) until germination, which takes 7-21 days depending on temperature

Reseeding After Selective Lawn Weed Killer

This is the more common situation: you’ve treated broadleaf weeds in your lawn with a selective product, the weeds have died, and now there are bare patches where they used to be.

The approach is simpler than a full renovation:

1. Wait the full period. Check the table above for your product. Most selective lawn weed killers need 6-8 weeks minimum
2. Rake out dead material. Use a spring-tine rake to scratch out the dead weed remnants and open up the soil surface
3. Overseed the bare patches. Scatter seed into the gaps at the recommended rate. Press it into the soil with your feet or the back of a rake
4. Top-dress lightly. A thin layer (2-3mm) of compost or fine soil over the seed improves contact and retains moisture
5. Keep moist. Water lightly every day if it doesn’t rain, until the new grass is established (3-4 weeks)

The best windows for overseeding are April-May and September-October. Soil temperatures are warm enough for germination and there’s usually enough rain to keep things moist. If you treated weeds in summer, you may need to wait until autumn before conditions are right for sowing. For advice on seasonal timing, check our application guide.

Aftercare: Getting New Grass Established

New grass is vulnerable. Here’s how to look after it:

• Watering: Keep the soil surface moist but not soggy. In dry spells, water lightly morning and evening. Once grass reaches 5cm, reduce watering to encourage roots to grow deeper
• First mow: Wait until the grass reaches 6-8cm, then mow on the highest setting. Only remove the top third. Cutting too short too early weakens new plants
• No weed killer: Don’t apply any herbicide to newly seeded areas for at least 6-8 weeks after germination, or until the grass has been mowed 3-4 times. New grass is as vulnerable to selective herbicides as broadleaf weeds
• Foot traffic: Keep off newly seeded areas as much as possible for the first 6-8 weeks. The seedlings need time to root firmly
• Feeding: A light application of lawn starter fertiliser at sowing time helps, but avoid strong feeds until the grass is well established

If your weed killer didn’t work properly the first time and weeds are already regrowing in the bare patches, treat them again before reseeding. There’s no point sowing into ground that’s already being reclaimed by weeds. For persistent problems with buttercup or creeping cinquefoil, treat in early autumn and overseed in the same window once the wait period is up.

Common Mistakes to Avoid

• Sowing immediately after spraying. Even glyphosate needs 7 days. Selective products need 6-12 weeks. Check the label
• Using a weed-and-feed product then reseeding. Weed-and-feed contains selective herbicide. If you apply it and then overseed a few days later, the herbicide will kill the new grass. Wait the full 6-8 weeks
• Reseeding in summer. July and August are too hot and dry for reliable germination. Even with regular watering, seedlings often fail in summer heat. Wait for September
• Sowing too thickly. More seed doesn’t mean better results. Overcrowded seedlings compete with each other for light and water, resulting in thin, weak grass. Follow the bag rate
• Skipping soil preparation. Scattering seed onto compacted soil or thick thatch gives poor results. Even 5 minutes of raking makes a significant difference

Frequently Asked Questions

Can I reseed the same day I use glyphosate?

Technically, glyphosate deactivates on soil contact, but the dying weeds take 7-14 days to die back fully. You need to remove them before preparing the seed bed. In practice, 7 days is the minimum sensible wait, and 14 days is better because it confirms the weeds are fully dead.

How long after weed and feed can I reseed?

At least 6-8 weeks. Weed-and-feed products contain selective herbicides (usually MCPA or mecoprop-P) that remain active in the soil. Sowing within this period will likely kill the seedlings. The feed component is fine — it’s the herbicide that’s the problem.

What type of grass seed should I use?

For overseeding existing lawns, use a mix that matches your current grass. A general-purpose lawn mix works for most situations. For shady areas or high-traffic lawns, choose a specialist blend. Sow at the rate stated on the bag — typically 30-50g per square metre for new sowing, less for overseeding.

Should I use topsoil when reseeding?

A thin layer of fine compost or topsoil over the seed helps, but it’s not essential. What matters more is good seed-to-soil contact. If your existing soil is reasonably fine and not compacted, raking the seed in is sufficient.

What if weeds come back in the newly seeded area?

Don’t treat them until the new grass has been mowed at least 3-4 times and is well established (usually 8-12 weeks after germination). At that point, you can use a selective weed killer at half strength for the first application. Hand-weeding is the safest option for young grass. For persistent weed problems in renovated areas, a thick, healthy sward is your best long-term defence.

Can I use a pre-emergent weed killer before reseeding?

No. Pre-emergent herbicides prevent all seeds from germinating, including grass seed. They’re useful for established lawns to prevent annual weed seeds from sprouting, but they’re completely incompatible with reseeding. Wait until your new grass is fully established before considering any pre-emergent product.