Root of a good lawn? Planting seed now.

“People care about their lawns,’’ said Clayton Heims of Mahoney’s Garden Center, who believes that September seeding makes for landscaping bliss. “I call it the second wife,’’ he said.

The reason? With its ups and downs through good seasons and bad, a lawn can take on the character of a life partner - and sometimes it’s time to start over. Tired of killing weeds with chemicals and looking for an alternative, Adrian Poole of Belmont put in a new lawn last September.

“We had a front lawn in very bad shape,’’ Poole said. “I said, let’s start fresh.’’

She had the right idea. Whether you’re putting in a new lawn, patching a bare spot, or overseeding an existing lawn to thicken it up, September gives you the cool nights, warm soil, and ample rain you need to get started, according to lawn experts. The warm soil helps seed germinate, the rainfall gives it the needed moisture, and September’s cool sends a wake-up call to grasses after slumberous mid-summer heat.

“Basically it’s what nature wants you to do,’’ said lawn-care expert Jackson Madnick of Wayland. Grasses naturally go to seed at the end of August and new seeds sprout in September. “We humans know better,’’ he remarks, wryly. “We do it in the spring.’’

Seeds started in September grow deeper roots by the time next year’s hot, dry weather drives most homeowners to turn on the sprinklers, the experts say. (Be advised that this year’s wet July was an anomaly; most lawns are stressed by midsummer.) Deeper roots mean grass won’t brown on the surface and cry for water.

Seeding in September also gives grass a head start on the weeds. If you want to keep crabgrass out of your lawn next summer, Madnick said, put more seed on your lawn now. Unlike turf grass, crabgrass is not a perennial. It starts over each spring from seeds produced the previous year. If your grass is thick enough, those weed seeds won’t get started. A thick, deeply rooted mat of grass blocks the light crabgrass needs to grow like a sheet of black plastic - but it looks a lot better.

Landscapers, contractors and - judging from seed sales - a growing number of homeowners are catching on to fall lawn planting; garden centers sell plenty of grass seed in September.

“We’re selling tons of it,’’ said Heims, the barn manager of Mahoney’s in Winchester. “I have to replenish something on the shelves every day.’’

Heims recommends a step-by-step program for planting grass in the fall. “Thatch’’ your lawn; i.e., remove the “crowns’’ of old dead grass. (New grass clippings are nitrogen, food for the soil, and should be allowed to remain.) Aerate the soil, because good soil needs oxygen. Add lime. Add a starter fertilizer. Seed thickly. Put down a thin layer of peat moss. Water daily.

As for the seed itself - lay it on thick. Ideally each seed should be touching its neighbor. “The best way to stop weeds growing organically is to thicken your lawn,’’ Heims said.

Mahoney’s has been pushing the organic approach to grass-growing - improving soil and using native seed rather than relying on synthetic products to stimulate growth and kill weeds - because of health and environmental concerns over chemical lawn products, Heims said.

Conservation concerns have also led to publicly funded programs to educate homeowners on how to keep lawns looking good without heavy watering or using chemical products, said Debbie Cook, who heads up the South Shore’s Greenscapes program.

Cook agreed that September is the natural season to plant grass.

Grass goes dormant in midsummer heat and doesn’t grow, she said. Rather than pump water onto your lawn all summer, put your energy into building up your soil and planting the right kind of grass.

“Crabgrass only really grows in compacted or distressed soil,’’ Cook said. “The good stuff crowds out the crabgrass.’’ You can see weeds growing in places where people walk a lot, hardening the soil, she said.

She urges avoiding chemical treatments and fertilizers. “[They] only treat [grass] from the top down. . . . They’re finer, they run off and you lose them,’’ Cook said. “And they end up where we don’t want them, in the streams and everywhere.’’

Whether you’re putting down seed this month over your existing grass to thicken it up or starting over, Cook recommends using a native grass, such as Pearl’s Premium, the product Madnick developed from native fescue grasses and sells through Whole Foods Markets. “It’s much more acclimated to our area,’’ Cook said.

Poole said her experience backs up that claim. Her grass came in thick, needed very little water, and was green all winter. “We were the envy of the neighborhood,’’ she said.

Robert Knox can be reached at rc.knox2@gmail.com.