Season Extension

Why bother

With roughly 110 frost-free days in much of Wisconsin ¹, every week matters. Covers and structures that trap the sun’s heat let you plant earlier in spring and harvest later in fall; together they can stretch the season by six to eight weeks ². They all work the same way: clear or translucent covers let sunlight in, the soil and plants warm, and the cover slows the heat from escaping at night.

Row covers

Floating row covers are lightweight fabrics of spun polyester or polypropylene laid right over the plants, letting light, air, and water through. They keep covered plants 5 to 10 °F warmer than uncovered ones and protect down to about 28 °F ². They’re also excellent insect barriers, and they shield young plants from wind and pounding rain ². Lay them loosely so plants can push them up, and weigh the edges down with soil or sandbags. For crops that need bees, such as squash and cucumbers, take them off when flowering starts.

Cloches and hot caps

A cloche is any clear cover over a single plant: a water-filled wall around a tomato, or a clear jug with the bottom cut off. They’re used mostly for tomatoes and peppers and add three to four weeks to the spring season ². Vent them on sunny days ².

Cold frames

A cold frame is a bottomless box with a clear, hinged lid, and it’s the most useful and versatile season extender of all ²:

• harden off transplants in spring,
• grow lettuce, radishes, and spinach weeks before and after their normal outdoor season,
• store carrots and parsnips or force bulbs in winter ².

Old storm windows make good lids; plexiglass or double-layer plastic is lighter ². The danger isn’t cold. A closed frame in spring sun overheats fast and wilts or cooks the plants, so hinge the lid and prop it open on sunny days ². An automatic vent opener that opens with heat solves the problem for the days you’re not home.

Add a thermostatically controlled heating cable in sand under a frame and it becomes a hot bed for germinating seeds in sunlight. The old method of heating with fresh manure isn’t recommended for food crops because of the food-safety risk ².

Hoop houses

A hoop house is a cold frame you can walk into, or at least reach into: metal or plastic pipe bent into hoops over a bed, covered with 4 to 6 mil polyethylene buried at the edges. It adds six to eight weeks to the season ². A low tunnel (hoops a couple of feet tall over a single bed) is the small, cheap version. Like a cold frame, a hoop house has to be vented on warm days by lifting the ends or sides ².

Warming the soil

Black plastic mulch laid on the bed a week or two before planting warms the soil 5 to 10 °F ¹, which helps tomatoes, peppers, melons, and squash, the crops that sulk in cold soil. Cut holes to plant through it.

What to take from this

Row covers give 5–10 °F and frost protection to about 28 °F, and they keep insects off. Cloches add three to four weeks for single plants. Cold frames are the most versatile: harden off, grow greens early and late, and vent them on sunny days. Hoop houses of 4–6 mil poly add six to eight weeks and need venting too. Black plastic warms the soil 5–10 °F for heat lovers. Skip fresh-manure hot beds for food crops.