How To Thicken a Newly Planted Thin Lawn

Q. I started a new lawn last fall and I am very happy with the results.  I want to re-seed the areas that came up a bit thin. When and how should I do that so I don't damage the rest of it? I was also considering doing an aeration soon, as I always did every winter at my previous home with great results. Should I aerate this younger lawn?

It is common for newly seeded lawns to be a bit
"thin" at first, but larger open areas should be reseeded.

A. I understand your concern about the lawn being too thin but this can oftentimes be the case in a new lawn, particularly with cool season grasses like fescues. Fescues fill in voids between seedlings primarily by "tillering".

            They do not have the capacity to fill voids with strong rhizomes (underground stems) and stolons (above ground stems that lay on the soil surface). Frequently these are more associated with the warm season grasses such as bermudagrass, zoysia and St. Augustine.

            Tillering is the "sprouting" of side shoots from the base of the plant. As a grass plant emerges from the seed it shoots up one stem with associated leaf (‘mono’ ‘cotyledon’ or one leaf). This single stem with one leaf that eventually sprouts side leaves along the stem will grow very tall before it sends up side shoots unless the tip of this single stem is cut off. This is done by mowing (prior to lawn mowers it was done by animals eating the grass or grazing).

            Once the single stem has been cut off, the plant kicks into a survival mechanism and sends up side shoots (tillers) around the central single shoot. The side shoots occupy space around the central shoot and begin to fill in the voids between germinating seeds.

Tillers are side shoots that come from the base of the grass plant such as
this Poa annua bunchgrass.

            It is very natural to have these voids or open spaces around the seedlings for quite a long time unless we "graze" on the plants with our lawn mowers. So just by cutting or mowing the lawn we will encourage the lawn to get thicker by individual grasses "tillering".

            This is further sped up with a little bit of nitrogen fertilizer applied every few weeks, warm temperatures and regular waterings.

            Most lawn weed seeds respond to sunlight (they germinate when they "see" light). So quickly filling the spaces between grasses by tillering helps to reduce weed invasion in new lawns.

            Weed seeds like to germinate when soil temperatures hit about 50 to 55 F which would be in early to mid-February or perhaps sooner in real warm microclimates. So it is important to get the grass up and shade the soil surface as soon as possible in the spring.

            In fact, I prefer fall lawn seeding because of weed problems and fall and the following spring weather gives the seed a double dose of good weather for establishment.

            However, if you feel the distance is still too far apart between these seedlings then I would use the exact same seed and lightly seed in these areas and lightly cover the seeds with topdressing.

            If you want faster germination, soak the seed in cool water for several hours (not overnight or you will kill them) and then dry them long enough to dry their surface so they can be spread easily.

            Do not dry the entire seed out or you will kill the seed. Once they are surface dry you can spread the seed and you will save yourself a day or two of germination time. Fertilize lightly every couple of weeks and mow as soon as you can cut off 1/4 of the length of grass. Do not pick up the clippings but let them compost back into the soil unless it is loaded with weeds.