Extending the Gardening Season

One of the biggest challenges to gardening in Montana is the short growing season, but there are some simple ways to extend it.

by Mac Burgess
Montana State University Plant Sciences & Plant Pathology

One of the biggest challenges to gardening in Montana is the short growing season. Fortunately, there are simple ways for gardeners to get a jump on the season and enjoy the fruits of the garden earlier. The first step to extending the season is to learn about the factors that limit the duration of the growing season at your location. Online maps of the first and the last frost dates are a good place to start, but local microclimates mean frost may nip plants at one location and not another nearby. The historical average date of the last spring frost ranges from early May in eastern Montana to mid-June in Butte, but higher elevation locations have even shorter growing seasons.
 
Montana State University Extension has a MontGuide, Can I grow that here?, which is a good reference for when to start various vegetable plants relative to your local first frost date. Realize, though, that many frost hardy garden plants that are less limited by the threat of below-freezing air temperatures are more susceptible to disease or slow growth in cold or windy conditions, or in poorly drained soils. Recommended planting dates can be pushed earlier with the innovative techniques outlined below.
 
Raised beds
 
Raised garden beds can be formed simply by mounding soil up with a rake and leveling the top of the mound or by constructing above-ground boxes from wood and filling them with a combination of topsoil and organic material, such as compost or peat moss potting mix. Soil temperatures are important to plant growth and raised beds are generally quicker to warm in the spring. Raised beds with heavily amended soil tend to drain water more quickly, allowing for earlier seeding than wet spring conditions would otherwise permit. Raised beds with sides constructed of wood also provide a convenient structure for the attachment of hoops to support coverings for additional protection.
 
Cloches
 
The word “cloche,” (pronounced klo ? SH), is French for “bell” and is associated with the bell-shaped glass domes used in 19th century French market   gardens. Modern versions made from paper or plastic can be purchased or constructed from reused plastic containers. A challenge with these small protective structures is that some may block too much light. In contrast, others allow temperatures to quickly get too hot inside during the day, so they need to be removed and replaced daily. This can become an arduous chore in wet spring conditions. The lighter-weight cloches made of paper and plastic can also be a challenge to keep from blowing away in the wind.
 
Another modern version of the cloche is the Wall O’ Water®, a ring of vertical plastic tubes filled with water. The water is heated by the sun during the day and releases that heat during the evening. Perhaps even more importantly, the water in the tubes would need to freeze before the temperature inside could drop substantially below freezing. The Wall O’ Water is advertised as providing frost protection down to 16° F.
 
Plastic Mulch
 
Dark-colored plastic film stretched tightly over the soil surface warms the soil beneath it by absorbing the sun’s energy and transferring the heat into the soil by conduction, which means the plastic must be in contact with the soil. In addition Extending the Gardening Season One of the biggest challenges to gardening in Montana is the short growing season, but there are some simple ways to extend it. In addition to warming the soil by many degrees, plastic mulches also provide weed control. Weed control from plastic mulches is especially effective for plants that are transplanted a good distance from one another, leaving most of the surface covered by plastic. The most common plastic mulch is black, but red, green, and other colored films are also used to reflect light up into the plant canopy, promoting growth and fruiting of some plant species.
 
Row Covers
 
Fabric made of various weights of spun, bond polypropylene is available from several manufacturers for use in the garden. The lightest grades are used primarily for excluding insect pests; they transmit upwards of 90% of sunlight and provide minimal temperature moderation. Light grades can be pretty fragile, especially in windy conditions. Heavier grades are more durable. They transmit less sunlight during the day, but trap more heat both day and night. While permeable to air and water, row covers do help shield plants from wind and help maintain surface soil moisture, helpful for emergence of slow
germinating seeds like carrots.
 
While I haven’t measured a significant difference between air temperatures under row covers and outside the cover, they are advertised as providing as much as 4 to 6 degrees of frost protection. Row covers can reduce radiant heat loss from leaves, which is important for consistent plant development. Mid-weight row covers are the most versatile and can be left floating on the soil surface over many young plants. When covering plants with growth points on top (like tomatoes and peppers), install wire or tubular hoops to support the row cover.
 
Row covers work well in increasing plant growth even during the height of the growing season. A big advantage of row covers is their protection from drying winds, but the wind is also a major challenge for keeping them in place. Row covers can be secured to supporting hoops with plastic clips and held to the ground with large rocks, bags full of smaller rocks, or shovel-scoops of soil. While row covers can be very effective at excluding common insect pests like flea beetles, they also exclude pollinators, so open up the covers on your cantaloupes once you see flowers.
 
High Tunnels
 
High tunnels are essentially simple greenhouses without supplemental heating, lighting, or forced ventilation. Metal, plastic, or wood structures are typically covered with a single layer of clear polyethylene plastic film. A system of passive ventilation needs to be provided, as daytime temperatures will quickly become too warm, even on cool, sunny days. Roll up side-walls or end wall vents are typically used to provide ventilation. While high tunnels reliably increase daytime temperatures, I have often measured nighttime temperatures inside single-wall, polyethylene, covered structures slightly colder than outside air temperatures. This effect is well-documented by others. Despite this counterintuitive temperature effect, tunnels provide a great environment for getting started with gardening as early as mid-March, or even year-round growing of especially hardy plants like spinach.
 
One of my favorite benefits of gardening inside a high tunnel is that it never rains indoors, so you don’t have to worry about soil that is too wet for planting. Subsequently, the high tunnel creates the challenge of providing plants all the water they will need through irrigation.
 
Combining Season Extension Technologies
 
Can you use plastic mulch with row covers inside of a high tunnel? Absolutely! Indeed, many small-scale commercial growers routinely use row covers inside high tunnels to combine the benefits of both technologies.