Though the Boise Valley is two months away from intensive gardening activity, a little time spent this month on preparation can make your home grounds a welcoming place for plants. Early spring is challenging for plants because of its freezing overnight temperatures, wind, and low soil temperatures. At a soil temperature below 40 degrees, plants are basically dormant and seeds of even hardy vegetables won’t sprout. But it is easy to create microclimates that mitigate cold and wind.
The first thing to do to warm the soil is to rake away any mulch that is shading the soil from sunlight. Do that as soon as possible wherever you plan to sow seed or set out plants so nature has time to reverse the winter’s chill. Even hardy bedding plants like pansies appreciate a warm place to rest their roots.
Next, consider putting down a heat-retaining layer of plastic film. Plastic, either clear or black, concentrates the sun’s warmth at ground level. Plastic for mulch is sold in rolls. For small areas, flattened bread bags or trash bags should work too. The important issue is to anchor the plastic so the wind won’t catch it and send it sailing through the neighborhood.
Another possibility is to use plant protectors that warm the soil and also protect young plants from frost and wind. As a protected nursery for starting seeds, nothing beats a cold frame. A cold frame is a bottomless box eight or so inches high with a glass top. Home centers now offer plastic ones at reasonable prices.
Farmers in France used to use glass bells called cloches to protect tender plants. Today we have less expensive and less fragile covers for plants. One that is used by commercial growers is the waxed paper dome called a hotkap. One hotkap will protect a tomato or pepper plant or a hill of melon seeds, but the hotkap lasts only one season and costs about a dollar. Cheaper or free alternatives include gallon milk jugs with the bottom cut out and two liter pop bottles cut in half. Ventilation is critical with all plant protectors: the spring sun can easily roast your plants.If the plant protector you are using has a vent in its top, like the upper half of a soda bottle with the cap off, just set the protector a bit above the ground on pebbles or chips of wood to allow air flow. Many plants that are hardier than warm weather vegetables appreciate protection from the wind most of all. Cut pop bottles into sleeves that are high enough to shelter small plants from wind. Whether using plant protectors or not, do not plan on setting out tomato plants and other tender plants until about May 20 at the earliest.
Other do-it-yourself plant covers that can be set up to warm the soil can be made with clear plastic film draped over a wire or wood frame. A small roll of chicken wire can be cut up to make many frames. Or build a tunnel over a garden row using plastic film with supports. Leave the ends open to allow ventilation and weigh the edges of the plastic down with soil. A new product that can take the place of plastic film is a woven material called “Frost blanket.” This material allows rain water to pass through to the soil below. If applied only one layer thick, it allows enough light to pass through so plants will grow with the blanket left in place. Multiple layers of frost blanket multiply its insulating properties but block light, requiring the gardener to remove the blanket from plants each morning.