Planning a Landscape Around a Short Growing Season

How short is the growing season in western Montana?

The Western Regional Climate Center's freeze tables for four long-running stations in our service area give median freeze-free seasons (between the last spring and first fall 32 degree freeze) of 119 days at Missoula airport, 120 days at Hamilton, 104 days at Darby and 101 days at Stevensville. At Missoula airport, about one year in ten had a season shorter than 97 days.

Freeze-free days are not the whole story. Cool-season grasses and many hardy shrubs keep growing in the cool weeks either side of the frost-free window, so the working season for a landscape is longer than the number suggests. But for anything frost tender, and for plants that need a long warm stretch to flower or ripen, that 100 to 120 day window is the limit. Our frost dates guide has the station-by-station dates.

Does a short season change which plants I should choose?

Yes, in two ways. First, hardiness: perennials, shrubs and trees have to survive the winter, and the 2023 USDA map puts the whole Missoula and Bitterroot valley floor in zone 5b. Our hardiness zone guide explains why we shop a zone colder for anything important. Second, timing: a plant also has to complete its year in the time it gets. Late-blooming perennials and shrubs that flower in October in warmer places may be frozen before they open here.

MSU Extension makes the same point for vegetables, advising gardeners to count the days between the average last spring and first fall frost and choose varieties whose days to maturity fit, noting for example that snap beans need 52 to 70 days and potatoes 80 to 140. For ornamentals, the practical version is to favor plants proven in Montana, buy from nurseries that grow or source for our climate, and treat plant tags written for a national market with suspicion. Our guides to perennials and shrubs for western Montana focus on plants that fit the season.

When should new trees and shrubs go in?

There are two good windows: spring, once the ground can be worked, and early fall, while the soil is still warm and the air is cooling. Hardy, nursery grown trees and shrubs are not frost tender the way annuals are, so spring planting does not have to wait for the median last frost date. What it does need is enough time for roots to grow before the dry heat of July and August, and enough water to get through it.

Fall planting gives roots time to grow in cool, moist soil before spring growth, but it ends earlier here than in milder places, because the ground freezes. New plantings also go into their first winter with small root systems. Colorado State University Extension notes that newly planted trees are the most susceptible to winter drought injury and that trees take roughly one year to establish for each inch of trunk diameter. Our guide to fall planting covers the fall window, and our how to plant a tree guide the steps.

When is the best time to seed or sod a lawn here?

Late summer into early fall is the strongest window for seeding cool-season grasses in western Montana: the soil is warm enough for germination, the air is cooling, weeds are slowing down, and there is still time to establish before winter. Spring seeding works but puts young grass into the hottest, driest part of the year. Our aeration and overseeding service is built around the early fall window for that reason.

The short season makes the cutoff real. Seed sown too late in fall may germinate and then be killed by freezing before it is established, or not germinate until spring. Our guides to timing a new lawn and sod or seed go into the details.

How does a short season affect irrigation?

The irrigation season is compressed between spring startup and fall blowout, and most of the water a landscape uses goes on in a few dry summer months. That makes system design matter more, not less: zones that match plant needs, heads that put water where it is needed, and a controller adjusted as the season changes. The EPA's WaterSense program recommends cycle-and-soak scheduling on clay soils and slopes so water soaks in instead of running off.

New plantings also need water outside the irrigation season. CSU recommends that in dry spells without snow cover, trees and shrubs be watered when air and soil temperatures are above 40 degrees, at midday, about once or twice a month. Our spring sprinkler startup and winterization guides cover both ends of the season, and our irrigation systems team handles design, repair and seasonal service.

When should landscape construction be scheduled?

Hardscape, grading and irrigation installs need workable, unfrozen ground, and planting needs the time after them. On most projects that means the construction has to fit between spring thaw and the first hard freezes of fall, with planting at the front or back of that window. A project that starts in late September has very little margin if the weather turns early.

The practical answer is to plan in winter, book early, and sequence the work so each phase leaves the next one in the right season. Large projects often split naturally into a spring hardscape and irrigation phase and an early fall planting and seeding phase. Our guides to the landscape design process and phasing a project over several seasons cover how that works.

Can I stretch the season for annuals and containers?

A little. MSU's vegetable guidance relies on starting seeds indoors and transplanting after the danger of frost has passed, and the same idea applies to annual flowers: buy or grow transplants rather than seeding in place, and set them out after the median last frost for your area. Containers on a patio or against a warm south wall can be moved or covered on cold nights in early June and September.

Plants in containers and hanging baskets dry out quickly in a dry, windy summer, so a drip line to containers on a separate zone saves a lot of hand watering. Our drip irrigation guide explains how.

What if I live in a colder side valley?

Then plan for far less. WRCC station tables give a median freeze-free season of just 64 days at Seeley Lake, 42 days at Potomac and 28 days at Sula, and those records run to 2005. In places like that, frost tender plants are a project rather than a default, and the backbone of the landscape should be plants that shrug off a summer frost: native conifers, hardy shrubs and grasses, and perennials proven at elevation. Reserve annual color for containers you can carry indoors.

Note, though, that the 28 degree tables are less severe. Sula's median hard-freeze-free season is 78 days, and Seeley Lake's is 117. Hardy plants are judged by that colder number, not by the first light frost.

How dry is the growing season here?

Dry enough that the short season is also a thirsty one. The Western Regional Climate Center's monthly summary for Missoula airport shows about 13.6 inches of precipitation in an average year, with June the wettest month at about 2 inches, then a drop to about 0.9 inch in July and 1 inch in August. Hamilton averages about 12.3 inches a year, with about 1.7 inches in June and about 0.8 and 0.9 inch in July and August.

So the warmest weeks of the season, when plants are growing hardest, are also close to the driest. Plan on new plantings depending on irrigation through their first summers, and time planting so roots have had the wetter weeks of May and June to get started. Our summer watering guide covers how much lawns need in that stretch.

Which months matter most for a Montana landscape?

Spring sets up the season: cleanup, irrigation startup, planting and the first fertilizer. Late summer and early fall are the second big window, for seeding, aeration, planting and the last fertilizer of the year. Late fall is about getting ready for winter: blowouts, cleanups and protecting young plants. Our month-by-month landscape maintenance calendar lays out the whole year.

Planning your project around the season

We plan landscape projects across Missoula and Ravalli counties with the season in mind from the first site visit, so the install, the irrigation and the planting each land in the right window. That planning is part of our landscape design and installation service, and this guide is part of our Montana Landscaping, Lawn & Irrigation Guide.

If you are thinking about a project for next year, winter is the time to start. Call HJ Property Care & Tree Service at (406) 215-7319 or request a free estimate.

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