Indoor seed sowing, greenhouses, cold frames, hot beds—you name it, northern gardeners have tried it to extend the season for vegetables and other plants. Today, gardeners typically use LED or flourescent lights to start seeds indoors, then move them to cold frames outdoors or low tunnels, temporary frames covered in plastic to create a cozy growing space.
Here are five tips from the archives of MSHS for eeking an extra week, two or more out of the vegetable season.
Place greenhouses for maximum light and protection. Way back in 1874, professional vegetable gardener William Cannon advised gardeners to use a greenhouse for early seed starting and to extend the season. He advised gardeners place it in a north-south orientation with some protection on the north side. For the absolute best greenhouse, build it with brick walls, 12 inches thick and 4 feet high at the sides. “As a rule, avoid cheap buildings for a greenhouse,” he wrote, “for they will become dear enough in the end.” For more on greenhouse building, refer to your copy of the November/December 2021 issue of Northern Gardener for an article on how even an unheated greenhouse extends the season.
Heat from the bottom. In the early years of the hort society, hot beds were a common tool to extend the season. The beds were heated from below with a layer of fresh (“hot”) horse manure. This was buried in the ground and straw and soil was added above it. The manure warmed the soil, on which a cold frame was placed. Two decades into the 20th century, horse manure was less available and horticulturists began experimenting with electric hot beds. In 1933, Dr. T.M. Currence of the University of Minnesota reported on his experiments with electrical hot beds. His experiments showed that a well-insulated cold frame with four 100 watt lamps could keep air temperatures above freezing (38 was its lowest temp) even when the outside temperatures were 9 below zero. Another experiment involved running an electrified lead cable through the soil under a cold frame. This device maintained similar air temperatures to the lamps, but warmer soil temps. It also used more electricity.
Insulate cold frames on the sides. Articles on how to build cold frames were common features in Minnesota Horticulturist from the 1910s on. Some recommended that gardeners surround their cold frame with soil to add extra insulation and to plant directly in the soil. Once the frame was set up, gardeners were advised to close it and allow the soil to heat up, which it will do very quickly. Use a soil thermometer to monitor temperatures.
Use a window frame for your cold frame. In the 1940s, old window frames were considered the gold standard for the clear element of a cold frame. Today, cold frame covers may be made of clear polycarbonate or fiberglass. Whatever you choose, it should be solid enough to handle the weight of snow and be sloped in such away as to draw in maxiumum light. At left is an old-fashioned guide for building a cold frame that would work today too.
Set posts for low tunnels in fall so you can use them in the spring. In a 2020 article on getting an early start in the garden, author Meg Cowden explained that she creates the frame for her low tunnels in the fall. She pounds rebar posts into the ground and sets flexible PVC hoops over them. The hoops are left bare until late February when they are covered with 6 mil poly from the home improvement store. The plastic covering helps melt the snow and warms the soil for setting out early vegetables.
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