If you have a thin or patchy lawn, the instinct is to throw seed down the first warm weekend in April. It feels like the right time. In Central New Jersey, it usually is not.
The short answer
The best window to seed a lawn here is late August through early October. Spring, from April to mid-May, is a distant second.
Why fall works better
The grasses that do well in New Jersey are cool-season types: tall fescue, Kentucky bluegrass, perennial ryegrass and fine fescue. They grow hardest when days are mild and nights are cool. Early fall gives them everything they want at once:
- Warm soil. After a full summer, the soil is warm enough for seed to germinate quickly.
- Cool air. Seedlings are not being cooked by 90 degree afternoons.
- Fewer weeds. Crabgrass and other summer weeds are finishing their life cycle rather than starting it.
- Two growing seasons before summer. Grass seeded in September gets all of fall and all of the following spring to build roots before it faces its first July.
What goes wrong in spring
Spring seeding can work, but the new grass is up against a lot. Soil is still cold, so germination is slow. Crabgrass sprouts at the same time and competes hard. And the crabgrass preventer most people apply in spring also stops grass seed from sprouting, so you usually have to choose one or the other unless you use a product made for new seedings. Then summer arrives when the new lawn is barely two months old.
If you missed the fall window, spring is still better than leaving bare soil for weeds. Just expect to do some overseeding the following fall.
Seed is only half of it
Timing gets seed to sprout. Soil is what keeps it alive. Before any seed goes down, look at what it is going into:
- Loosen compacted ground. Seed sitting on hard soil dries out and washes away.
- Add topsoil where it is thin. Many lawns in our area have only an inch or two of topsoil over clay or sand.
- Fix low spots. Anywhere water sits, grass will struggle.
- Get good seed-to-soil contact. Rake seed in lightly. Seed lying on top of thatch rarely takes.
Watering new seed
For the first two to three weeks, keep the top inch of soil consistently moist. That usually means light watering once or twice a day rather than one long soak. Once the grass is up and you have mowed it a couple of times, switch to deeper, less frequent watering to push roots down.
When to call for help
If more than about half the lawn is weeds or bare soil, or if the ground is uneven and holds water, overseeding alone will not fix it. That is when a full lawn renovation with grading and new topsoil makes sense. Fall slots fill early, so the sooner you get on the schedule the better.