Marin Master Gardeners
University of California
Marin Master Gardeners

Get a head start on spring with cold frames

cold frame
Can’t wait to dive into your spring vegetable garden? Consider cold frames. Like mini-greenhouses, these small covering structures provide a protected environment to start seeds and grow seedlings. Cold frames are easy to assemble and can be made from inexpensive materials. Because they are better acclimated from the outset, plants that start life in cold frames often do not experience the transplant shock that many plants face. Among the most transplant-vulnerable seedlings are basil, beans, cabbage-family plants, lettuce and peas.

Cold frames create a stable environment for frost-tender seedlings by protecting them from big temperature swings. Keeping delicate seedlings in cold frames helps them thrive until winter temperatures stabilize. Cold frames also work to protect seedlings from pests, including birds, cutworms, earwigs, slugs, and snails.

How do you make a cold frame? It’s easy – and it depends on your situation. In essence, you want to create a protected environment to give your plants a cushy place to get started. To make a standalone cold frame, place an old window pane or a rigid sheet of plastic on top of a four-sided box filled with soil. Be sure that the box receives plenty of sunlight. The box can be made out of any material that holds the soil in place, including wood, straw bales, metal, cinder block, or brick, to name a few. If you want to reuse your cold frame every winter, consider adding a hinge to one side so you can easily open and close it.

Since our winter days in Marin are rarely very cold, and occasionally may be quite warm, you will need to ventilate the glass or plastic-topped cold frame. Keep the soil moderately moist but not wet (soggy soil causes seedlings to rot). But be careful to check frequently that the soil stays moist, as soil inside a cold frame dries out more quickly than outside.

Cold frames create a stable environment for your tender seedlings. If you can’t wait for spring to start your vegetable garden, consider planting seedlings now using a cold frame to ensure that your veggie seedlings thrive.

By Julie McMillan
Photo courtesy of UCANR

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