When to Put In a New Lawn in Western Montana

When is the best time to seed a lawn in Montana?

Mid-August to mid-September. MSU Extension's planting MontGuide says you can seed in late April and May, but seeding from mid-August to mid-September (around Labor Day) is better. Two reasons: annual weeds do not have time to produce a crop of seed, and the grass gets the entire fall and the early spring to establish before the heat and stress of summer.

Colorado State University Extension, writing for a similar semi-arid, cool-season climate, says the same: it is easier to establish cool-season grasses such as Kentucky bluegrass in late summer, and for a bluegrass lawn the best time to sow is late August to early September. CSU's PlantTalk on tall fescue gives late August into September as the best seeding time too, with late April into early June also workable.

Why does late summer beat spring?

Think about what a spring seedling faces. It germinates alongside a flush of weed seedlings, then meets July and August with roots only a few weeks old. MSU's maintenance guide notes that water use for a bluegrass lawn in most of Montana runs about 1 to 1.5 inches a week early and late in the season but can reach 2.5 inches a week in midsummer. A young lawn has to survive that peak before it has a root system to handle it.

A late-summer seedling does the reverse. Days cool, nights lengthen, weed pressure drops, and the grass builds roots through fall, rests under snow, and gets a second cool growing period in spring before its first summer. The lawn that faces July has had two seasons of rooting instead of six weeks.

Can I still seed in spring?

Yes, and sometimes you should, for example after construction finishes in April or when bare soil would otherwise erode all summer. MSU names late April and May as workable. Go in knowing the costs: more weed competition, more watering, and seedlings facing summer heat early. Seed as early as the soil can be worked and kept moist, and plan to water through the whole first summer.

One conflict to avoid: CSU warns that a spring crabgrass preventer will kill seedling grass, and that pre-emergent herbicides cannot be used on newly seeded or sodded lawns. If you seed in spring, skip the pre-emergent on that area; if you applied one, CSU advises waiting until August to seed.

How late in the fall is too late to seed?

The limit is how much growth the seedlings get before the ground freezes, and that varies by year and elevation. CSU's renovation note puts bluegrass germination at about 21 days, and the young plants need weeks more after that to be sturdy enough for winter. MSU's window ends around mid-September for that reason. Our sibling guide to frost dates in Missoula and the Bitterroot helps you judge your own site; higher benches and side valleys run out of season sooner.

Seed sown well after MSU's window is a gamble. Some of it may germinate in a warm fall; some may sit until spring. If the calendar has slipped, it is often better to prepare the soil now and seed at the right time, or to switch to sod.

When can sod go down?

Through most of the growing season. CSU says sod can be installed anytime good quality sod is available and the soil is prepared, as long as roots are well established before freezing temperatures. CSU's note on fall sod adds that cool-season sod can be laid as long as the soil is not frozen.

The catch is winter. CSU says sod planted late in fall must be irrigated during winter to prevent drying, because it has little time to develop roots. Water only when conditions let water soak into the sod and soil, and only enough that no puddles or ice form; in an especially dry winter that may mean every two to three weeks. With sprinklers already blown out for the season, that is hose work. Our sprinkler winterization timing should be planned around any late sod.

What about summer installs?

Sod can go down in summer if water is reliable, but it is the hardest season for it: the soil must be moistened six to eight inches deep before the sod arrives (CSU), and the sod kept moist until it roots in two to three weeks. Seeding in midsummer is the worst option; seedlings need constant surface moisture through germination of up to two weeks (CSU), and that is hardest to provide in July heat and wind. In Missoula, the city's conservation page recommends watering from May 1 to September 30 during 6 to 11 a.m. and 5 to 10 p.m., which also suits establishment watering.

What should happen in the weeks before planting?

Work backward from the planting date. A soil test first, since MSU says it tells you nutrients, pH and soluble salts, and that phosphorus should be worked in before seeding if needed. Then perennial weed control: CSU says to control perennial weeds before disturbing the soil, using an herbicide with no residual activity if you use one. Then grading (MSU asks for at least a 1 percent fall away from the house), any irrigation work, amendment and a firm, fine seedbed. MSU suggests a roller light enough that only your sole prints show.

If any of that will take a while, start it in early summer for a late-summer seeding. Our guide to sod or seed covers the prep in more detail.

When does a new lawn get its first mowing and fertilizer?

Mow when the grass reaches about a third above the target height, since MSU's rule is to remove no more than a third of the blade at a time, with sharp blades. For fertilizer, MSU says that if the soil has enough nitrogen and you plant in fall, fertilizer may not be needed until spring; for spring plantings it recommends a starter fertilizer before seeding. After that, the established-lawn schedule applies, which our guide to fertilizer rates and timing explains.

Is overseeding an existing lawn on the same schedule?

Largely, yes. MSU's maintenance MontGuide says aeration should happen when turf is vigorously growing, in spring and fall, never when it is dormant, and that you can overseed after aerating to improve seed-to-soil contact. CSU names September as a good overseeding month and says overseeding in both spring and fall speeds the process. The same logic as new seeding applies: fall seed meets less weed competition and gets two cool seasons before its first summer. Our guide to when to aerate and overseed covers that job in detail.

Do native and low-water grasses follow a different calendar?

Yes. MSU's native plants MontGuide says late fall is the optimal time to seed most native forbs and grasses, because many need a cold period (cold stratification) before they will germinate. It cautions that fall seeding may not be the best option on exposed sites with little snow cover and high wind, and that spring-seeded natives that need stratification may not come up until the following year.

Warm-season grasses are different again. CSU says warm-season grasses such as buffalograss are usually seeded in May or June, and MSU notes buffalograss and blue grama green up slowly in spring and brown soon after the first fall cold. If you are converting part of a yard away from lawn, see our guide to replacing some lawn with native plants.

A western Montana calendar at a glance

Late April to May: workable for seed, good for sod once soil can be worked. June to July: sod only, with reliable irrigation. Mid-August to mid-September: the best window for seed, and good for sod. Late September onward: sod while the soil is unfrozen, with a plan for winter watering. Through all of it: soil test, weed control and grading first. The broader Montana landscape maintenance calendar shows how this fits with everything else in the yard.

Planning a new lawn with HJ

We schedule lawn installations, aeration and overseeding and irrigation work so the planting lands in the right window rather than whenever the crew is free. Once the lawn is in, our licensed fertilizer and weed control program and recurring lawn care take it from there. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call (406) 215-7319 or request a free estimate.

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