Cold Frames and Hotbeds—Stretching the northern growing season

Photo Credit: Emily Bulmer

Cold Frames and Hotbeds—Stretching the northern growing season

👤Emily Bulmer 🕔Mar 27, 2015

When frost-free days are counted like gold in a treasury, gardeners search for ways to get the greens a little sooner. Cold frames, hotbeds, row covers and cloches help bridge the almost season and give the anxious green thumb something to do—other than obsessively check the overnight temperatures. Smaller and more adaptable than greenhouses, simple season-extension techniques can add weeks of growing without breaking the budget.

Cold frames to catch heat

Cold frames can be moveable, permanent, large or small. From a simple, temporary rectangle of straw bales with an old window on top to a permanent structure of dimensional lumber with hinges, they are highly adaptable. No matter how simple or sophisticated, good access to sun, drainage and ventilation are all necessary.

A location with good sun exposure is foremost. Find a place in your yard where the sun reaches in early spring and late fall, as shadows from fences, buildings, trees and other nearby structures are different than in the summer. A box that is lower in front will angle the clear cover to best capture solar energy when the sun is lower, though a flat lid on a box with very good exposure also works. Permanent frames are dug into the ground and provide more protection than portable frames; however, portable frames work well for 
small yards, as they can be stored when not in use.

Avid gardener Pauline Mahoney uses her “spinach house” to get a head start on spring greens. By planning ahead and using passive solar, she is able to eat fresh spinach in early April.

“I have a very low, permanent cold frame that I seed in September with claytonia (miner’s lettuce), spinach and corn salad—all very cold-hardy varieties,” she explains. “I water them in very well, which is important. Seedlings are more frost- and cold-tolerant than mature plants—you need to give them a certain amount of growth so they can handle no light and go into a dormancy. You can’t wait too late to germinate them because then it is too cold and you will miss the window.”

Mahoney’s spinach house is a four-by-eight raised bed about 16 inches high with a flat, corrugated plastic lid. She uses winter-weight floating row cover, also known as Reemay, pulled tightly over the bed to create a layer of air as insulation. Over the Reemay, the corrugated lid is fitted snugly, creating another layer of protection. As the snow piles up, the bed is well insulated from cold weather, and the little plants are protected. In early March, Mahoney clears the snow off the beds to allow the sun in and waters them with room-temperature water. By late March, the lid can be removed, depending on overnight temperatures.

“By April 15 we have huge leaves of spinach,” she says, and there’s enough to share with the neighbours.

Mahoney remarks that watering and watching the temperature inside are key to survival, as the plants can be just as susceptible to overheating as they are to freezing. Propping the lid open or partially removing it allows for ventilation so it does not become too hot. Keeping the inside at about 15 C is ideal. As temperatures rise, the lid can be removed for the year.  

Creating a heat sink is another trick to keep the frost away when temperatures take a dive overnight. Bricks, large rocks, pots of sand or jugs of water absorb heat during the day. At night, the heat is slowly released as they cool. With a lid on, this method can stave off frost inside the cold frame even when nighttime temperatures are consistently -5 C or colder.  

The cool thing about hotbeds

A hotbed is another season extender that uses an internal heat source other than passive solar, traditionally from composting manure. A very practical method when large machinery and ample manure are near by, this can be easily incorporated in a farm setting. To build a traditional hotbed, dig a pit about three feet deep and place fresh manure to a depth of two-and-a-half feet. The top six inches are left for the soil or planting medium. It is important to ensure this is deep enough that the seedling roots are not burned by the fresh manure. A hotbed can be be used in combination with a portable cold frame for extra heat retention.

For townies, a more practical approach is using heater coils. These can be buried under the soil in a greenhouse or coldframe and in spring can be instantly turned on with the flick of a switch. With either method, the heat source raises the soil temperature, so even when passive solar isn’t enough, plants can start growing far ahead of schedule. A soil thermometer is a useful tool for cold frame and hotbed monitoring. If the soil is too warm or too cold, the plants will not thrive. The directions on the seed packet will indicate the ideal soil temperature for starting any plant.

Raised beds, row covers and the cloche

Even a simple raised bed of 16 to 18 inches without a lid can warm the soil by about 10 C compared to the ground. Combining a raised bed with row covers can be a very low-tech and inexpensive way to gain more frost-free days. Three weights of Reemay are available, affording different levels of protection depending on the season.

A cloche is a season extender for a single plant. Originating in France, the first cloche was a bell-shaped glass dome placed over delicate plants to protect them from frost. A simple cloche can be made out of a four-litre plastic milk jug with the bottom cut out. The spout can be left open to allow ventilation during the day. Cloches can easily overheat, so it is important to monitor them carefully.

Cold frames and hotbeds are also useful for starting vegetables or flowers that will be planted in the garden. Head lettuce, squash, broccoli, cauliflower, brussels sprouts, cabbage, cucumbers and leeks can be started early in a cold frame in pots or plugs. Cooling overnight and natural light create plants that are sturdier than those started indoors under grow lights.

Establishing a consistent early-season regime requires experimentation and patience. Getting to know the finer points of the microclimate in your yard and using a thermometer and journal to track trends can help hone your system into a dependable early-season goldmine of greens.