Cold frames help enable early start to growing season

(EDITOR’S NOTE: This column is written by members of the Lycoming County Master Gardeners.)


Lycoming County Master Gardener

Q: What’s a cold frame and what does it do?

A: Cold-frame gardening works like a greenhouse but is little more than an open-bottomed box with a clear cover used to extend the growing season. Sunlight enters the cold frame and warms the soil while the frame protects tender plants from frost and winds — a bit like a sweater for your plants.

There are several benefits to cold-frame gardening, including giving gardeners up to a month’s jump start on sowing as well as overwintering cold-hardy varieties. This means harvesting kale in January and germinating spinach seeds outdoors in mid- to early February instead of March.

Most enticingly, gardeners can transplant six-week tomatoes and peppers in early May instead of early June. Just imagine! Getting long-season, but high-flavor, varieties such as pound-heavy Brandywines a whole month earlier. Did anybody else’s eyes glaze and mouth water just thinking about the bounty? “Big Boy,” oh, boy.

Other benefits of cold-frame gardening include bringing in more light to shady spots when the interior of the frame is painted a bright white or lined with metal foil as well as being somewhat deer- and pest-proof.

Gardeners can buy cold frames or make their own. One of the simplest designs is straw bales, bricks or wood in a square around some soil or plants and then topped with glass, perhaps an old window, a shower door or 6-mil UV-resistant plastic.

Whichever clear material you use, it should be easy to lift so plants can be weeded, watered and harvested, which you may be doing often as cold frames require more watering since the soil is warmer and so dries out faster.

Another consideration when making your own cold frame is including a vent, which will allow heat to escape and prevent plants from getting fried on very sunny days or when the temperature is above 50 degrees.

Some ready-made frames have an automatic vent, which saves having to add one more thing to what could already be a busy day.

Do-it-yourself frames lacking in the automatic vent will need to be propped open or vented in some other way, perhaps pulling back the plastic or using a brick to hold up the glass. This opening may make the plants vulnerable to pests.

Lastly, choosing the right site for your cold frame is essential. Soil should be well draining, which is true for most garden sites, and the cover should face south or southeast to capture as much sun as possible.

There is no reliable chart for when to sow plants grown in a cold frame as there are so many variables. Generally, though, cold frames will raise soil temperatures about 2 to 5 degrees higher than soil outside the frame, which is roughy a month earlier.

Once soil temperatures and risk of frost are no longer a factor, gardeners can remove the clear cover.

Cold frames also can harden off tender transplants started under indoor lights as well as overwinter plants that are a hardiness zone or two — or sometimes three, if our winter is mild — above ours such as potted dahlias, which do best in zones 8 through 10.

Marcello, of Montoursville, often can be found in the garden with her Buff Orpington chickens. She loves growing orange watermelon, black lettuce and other unexpected eats.