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Season extenders for Minnesota gardens

Beth R. Jarvis

Minnesota's short growing season and the chance of unexpected spring cold snaps make gardening a challenge. Coldframes and cloches will help you get a head start on spring.

Starting garden plants from seed is a popular activity that helps break winter's monotony and increases your choices of plants. After seedlings are well established, harden them off before transplanting them into the garden. "Hardening off" means adapting plants to cooler outdoor temperatures, more intense light, and drying winds.

Hardening off takes about seven days. If you have only a few pots, put them outside daily for increasing periods of time. With larger numbers of plants, a coldframe is indispensable as it eliminates moving the plants. A coldframe is also used to grow seedlings to a transplantable size and hold them until the garden is ready. Once transplants are set in the garden, cloches can be used to protect tender plants from spring frosts.


Illustration of slanted wood box

Use a coldframe to grow seedlings to a transplantable size, and to hold plants until your garden is ready.

Diagram with four steps showing how to assemble a coldframe

Figures 2a through 2d

Building a coldframe takes a few hours and requires only simple tools such as a hammer, drill, nails or wood screws, and a saw.

First, buy a 4x8 sheet of waferboard or plywood and three 8-foot lengths of 2x2. Cut the waferboard or plywood yourself or have the lumberyard cut it into four 2x4-foot pieces. Cut one 2x2 into four 2-foot long pieces. Cut the other two 8-foot 2x2s into four 4-foot pieces and save them for the cover.

Set the 2x4-foot pieces to form an open box with the edges of the side wall pieces flush with the edges of the front and back pieces so the box will be slightly wider than deep. Nail or screw the edges of the 2x4s into the 2x2s to reinforce and stabilize each corner. The finished product will be an open-ended box, four feet wide, four feet deep, and two feet high (figure 2a). Paint the structure inside and out with a white, oil-based paint to prevent deterioration. The white paint reflects light inside the coldframe.

Place the coldframe in a site which receives direct sun at least half the day. To let the sunlight in, lower the front by digging a trench six inches deep along the front and tapering back along the sides. Secure the coldframe by packing the soil back along the sides and front of the coldframe. (Gardeners with woodworking experience could cut the side panels at an angle and shorten the front wall, instead.)

Build a cover or use old storm windows. To build a cover, buy a clear sheet of wavy fiberglass and cut it in half so it forms two 4-foot-long pieces. (Use a power saw as a hand saw will splinter the fiberglass. If you have no power saw, the lumber yard will cut it for you.) Make the cover frame from 4-foot lengths of 2x2s. Set the side pieces to overlap the front and back pieces so the top will fit well (figure 2b). (You can also use 1x2 lumber, but the weight of the 2x2s helps keep the cover from blowing off in a storm.)

Buy two 8-foot pieces of fiberglass molding cut in half. Use one piece to fill in the gaps in the front and back of the lid. The other is an arched piece for each side (figure 2c). This frame may be painted to match the coldframe.

Set the molding pieces on the frame and fit the fiberglass pieces over the molding (figure 2d). There will be some overlap in the middle. Drill holes for the nails to keep the fiberglass from cracking when it is nailed or screwed into place. Hinges can be used to secure the lid to the frame, but they are not necessary.

Eight standard 21"x10.5" nursery trays of seedlings will fit in this coldframe. To keep the undersides of the trays clean, put a piece of landscape fabric in the bottom of the coldframe or make a slotted floor from lath strips spaced and nailed onto two other pieces of lath. Prop the lid open with a stick or leave the lid ajar a little more each day to harden off the plants inside.

Coldframes can also be used to hold purchased plants until the garden is ready and to start fall crops of broccoli and other cool-season plants.


Illustration of a cloche

Figure 3. A temporary cloche can double as a portable coldframe.

"Cloche" is French for a bell jar or dish cover. In a garden, a cloche is a temporary structure which serves as a mini-greenhouse set directly into the garden.

Use plastic milk jugs with the bottoms cut out to protect small, individual plants from cold. These will generally last a season or two, but need to be replaced since they become brittle in sunlight over time. To cover a row or rows of plants, use flexible fiberglass sheets held in an inverted U shape by stiff wire hoops or small wooden stakes.

Make temporary cloches by arching black, semi-rigid poly piping over the row or bed and sticking it into the ground on each side. Lay clear plastic over the arches. If the beds are enclosed with lumber, attach brackets to the inside edges of the boxes or sink short pieces of pipe with a larger inside diameter along the sides to hold the arches.

The arches can be used to support old sheets, shade cloth, Reemay or similar products to ward off frost if they are fastened more securely to the raised bed frames.

It's easy to make a free-standing cloche which can double as a portable coldframe. Purchase two 2x4s in the length you need. Drill corresponding holes 18" apart into the 2x4s. The holes must be large enough to snugly accommodate the poly pipe. Set the 2x4s the desired width apart and nail or screw each end to a piece of lumber. Cut black semi-rigid poly pipe long enough to span the 2x4s and provide the desired clearance (figure 3).

Cover the poly frame with clear plastic, tuck the edges under the frame or anchor it with weights to keep the plastic in place. On warm days, pull the plastic loose on the ends to provide air circulation and reduce heat build-up.

When this cloche is no longer needed, simply remove the plastic sheet and poly pipe ribs and store them until the following season.

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