In Missoula, the growing season feels short because it is. Missoula’s median last spring frost is mid May (about May 16 at the airport, per NOAA’s 1991 to 2020 normals), but in about 1 year in 10 it comes after June 1. The median first fall frost is late September (about September 26). That makes for a typical frost-free season of about 130 days, under about 110 in 1 year in 10, and low spots often run shorter. The warm-season crops want every day of it. The good news is that you do not have to accept those dates as hard limits. With a few simple tools and some planning, you can start earlier in spring and keep harvesting weeks later in fall.
Extending the season is not about expensive greenhouses. Most of it comes down to trapping warmth, protecting plants from frost, and timing your plantings well. Here are the approaches that work in our climate.
Get a Head Start in Spring
The simplest way to lengthen your season is to start plants before the weather is ready for them outside. Starting seeds indoors gives heat-loving crops like tomatoes and peppers a head start of several weeks, so they are already established when the soil finally warms. This matters a lot here, where those crops need all the warm time they can get.
Out in the garden, a few tools let you plant sooner. Row covers, which are lightweight fabric draped over the bed, trap warmth and protect young plants from light frosts. How much protection you get depends on the fabric weight. Manufacturers rate lightweight covers (about 0.5 ounce per square yard) at roughly 2 to 4°F of protection and 1.5-ounce fabric at about 6 to 8°F, with less light getting through the heavier grades. MSU Extension suggests a cover rated for about 4°F as a good balance of warmth and light, and recommends taking covers off tomatoes, peppers, and squash once they flower. Cloches, basically individual covers set over single plants, do the same on a small scale. For tomatoes, peppers, and eggplant, water-filled protectors that circle the plant store daytime heat and release it at night, and University of Minnesota Extension says they let you transplant about three weeks earlier than normal. All of these buy you time on the front end of the season.
Warm the Soil
Cold soil holds plants back even when the air feels warm, so warming the ground is part of extending the season. Raised beds naturally warm earlier in spring because they drain better and the sun reaches more of the soil. Covering a bed with a row cover or cloche for a week or two before planting raises the soil temperature several degrees, which gets seeds germinating and transplants growing sooner.
This early warmth is especially useful for the warm-season crops that sulk in cold ground. Plastic mulch is another good soil-warming tool. Lay it over moist soil a week or two before planting and set plants through slits. Clear plastic warms soil the most, up to about 14°F according to University of Minnesota Extension, though weeds can grow under it. In MSU Extension’s Bozeman tomato trials, red plastic film sped soil warming, while black plastic controlled weeds and held moisture but did not significantly raise soil temperature. Save straw and other organic mulches for later in the season, since they keep soil cooler. A little soil warming up front can mean the difference between a crop that ripens and one that runs out of time.
Stretch the Fall With Covers and Cold Frames
The back end of the season is where you can gain a lot, because many cool-season crops keep going past the first frost, especially with protection. A row cover thrown over a bed of greens on a frosty night can keep them producing for weeks longer, depending on the weather and the crop.
A cold frame takes this further. It is a simple bottomless box with a clear lid, set over a bed, that acts like a tiny greenhouse. Vent it on sunny days so it does not overheat. Crops like lettuce, spinach, kale, and other hardy greens keep growing inside one well into fall. Once the days get very short in early winter, growth nearly stops, and a cold frame mostly holds mature cold-hardy crops for harvest rather than growing new ones, so get fall crops close to full size before then. Low tunnels, which are hoops covered with row cover fabric or clear plastic over a bed, do the same thing on a larger scale and are easy to put up and take down, though MSU Extension notes they give the least frost protection of these options. None of these are complicated or expensive, and they can noticeably extend how long your garden produces.
Plan Your Plantings Around the Season
Tools help, but timing is just as important. Succession planting, sowing small batches of fast crops every couple of weeks, keeps fresh harvests coming instead of everything maturing at once. And the fall garden is one of the most overlooked opportunities here. In late July and into August, as summer crops finish, plant another round of cold-hardy greens and root crops. They mature in the cooling weather and, with a little cover, produce long after the first frost.
Choosing short-season and cold-tolerant varieties ties it all together. A variety that matures in 55 days instead of 75 effectively extends your season by giving the crop time to finish. Seed companies count days to maturity differently, from sowing or from transplanting, so use the numbers to compare varieties rather than as a calendar. Combined with the warmth-trapping tools above, the right varieties let you grow more in our short window than the calendar would suggest.
For frost dates and crop-by-crop planting windows, see the Missoula Planting Calendar, and for fall protection, Protecting Your Missoula Garden From the First Frost.
Sources
- Montana State University Extension, Hotbeds, Cold Frames, and Low Tunnels for Montana Gardeners
- Montana State University Extension, Growing Tomatoes in Montana
- University of Minnesota Extension, Extending the Growing Season
- NOAA NCEI, U.S. Climate Normals 1991 to 2020 (Missoula International Airport)