Every spring somebody hands us a bag of grass seed they bought in March and asks us to put it down. We usually talk them out of it. Strange way to turn down work, so it is worth explaining why.
What cool-season grass wants
Lawns in northern New Jersey are tall fescue, Kentucky bluegrass, perennial ryegrass, or some blend of the three. All of them are cool-season grasses. They grow hardest when soil temperature sits between about 50 and 65 degrees, and they stall out above 80.
That gives you two windows a year. Spring, when the soil is warming up through that range, and early autumn, when it is cooling back down through it.
The two windows are not equal.
Why spring seeding usually fails here
Seed put down in April germinates into a race it cannot win.
- Crabgrass germinates in the same soil temperature window, and it is far more aggressive as a seedling.
- You cannot apply pre-emergent crabgrass control, because pre-emergent does not know the difference between crabgrass seed and your seed. So you either protect the lawn you have and lose the new seed, or seed and let the crabgrass in.
- New grass in April has roughly eight weeks of root development before its first July. Eight weeks of root is not enough to survive a dry August in this soil.
We have seen spring seeding work. It works on an irrigated lawn, in a cool wet spring, when somebody waters twice a day for three weeks. That is a lot of conditions to line up.
Why September wins
In the first three weeks of September the soil is still warm from summer, so seed germinates fast. Nights are cooling, so the seedling is not stressed. Crabgrass is done for the year and is not competing. And the new grass gets autumn, winter and the whole following spring to root before it ever sees real heat.
Nine months of root development against eight weeks.
What we do in September
Core aeration first, on the same day. A machine pulls finger-sized plugs out of the lawn and drops them on the surface. That does two things. It breaks up the compaction that heavy clay develops under foot traffic and mower weight, and it opens thousands of holes for seed to drop into. Short of tearing the whole lawn up, nothing else gets seed and soil in contact as reliably.
Then seed, then starter fertilizer, then water. Light and frequent for the first two weeks, then longer and less often once it is up.
If you have already missed it
The window closes around the first week of October in this area. After that the seedling does not have enough time to establish before the ground freezes, and you lose most of it over winter.
If it is late October and your lawn is thin, wait. Put the money into a fall feeding instead, get through winter, and do the repair properly next September. Grass seed is cheap. Doing the job twice is not.