The last frost of spring and the first frost of fall are often isolated events. A short cold snap in April or an early freeze in September can shorten a growing season by weeks. If you can protect your plants through these frosts, you can extend your growing season.
You can get a jump on spring by starting seeds well before the weather is warm enough for planting directly in the garden. You can plant seeds indoors, or outdoors in a cold frame, hotbed, or individual covers to get an early start. These simple structures provide warmth and protection for the developing seedlings. In the fall, cover tender plants or provide a heat source to evade that first fall frost, and harvest vegetables for an extra couple of weeks.
Seeds can be started on a sun porch or a windowsill 6 or 8 weeks before the weather is mild enough to plant them outside. In short-summer growing regions, this is the only way you can get some tender vegetables to bear at all. In all regions, this method allows earlier harvests and greater yield from vegetables and earlier blooms from annual flowers.
Plants can be started outside or set outside early with minimal protection. A cold frame is a box with a transparent top. It collects heat from the sun during the day and retains warmth at night. Typical early spring weather is ideal for cold-frame use; the days are warm and sunny, but the nights are still too cold for unprotected young seedlings.
A hotbed is a cold frame with heating. Traditionally, heating was supplied by a deep bed of decomposing manure under the hotbed. Today, electric heating cables usually provide heat.
You can create individual covers around plants in the ground to capture sun warmth and to protect vulnerable seedlings from wind. Glass cloches, paper hotcaps, or even cut-off milk jugs provide protection for individual plants. Row covers, either floating or on a frame, protect whole rows of plants.