By Carl Wilson, horticulture agent, Colorado State University Cooperative Extension, Denver
While April is the typical date to plant cool season vegetables outdoors along the Front Range, a March start is possible with some February preparation. You will need to warm your soil in February before March planting and add a protective shelter.
To warm your soil, cover the ground with sheet plastic. Use a 4 mil or greater grade of clear or black plastic spread over the soil. Anchor with wire bent into U-shaped pins. While plastic on top the soil is effective by itself, combing this with a protective shelter works even better.
An easily made protective shelter uses full bags of bark mulch, soil amendment or potting soil that you already have on hand. (Don't use bags of fertilizer because moisture will ruin the product.) In rural areas, hay bales may be available and work quite well. Form an open circle with the bags or bales, leaving the sunny, southwest side open. Drape a second sheet of plastic over the open circle, tucking it under the bottom of the bags or bales. On the open side, anchor the plastic cover to the soil with more U-shaped pins or with soil placed on top the plastic edges.
Allow the soil to warm in its plastic wrap for two to three weeks. Light, sandy or imported "planters mix" soils will warm quicker than heavy clay soil. The minimum temperature required to germinate seeds is 40 degrees Fahrenheit.
To plant, roll back the plastic drape and remove the plastic sheet covering the soil. The soil warming plastic sheet has served its purpose and won't be needed again. Plant the seed and replace the plastic drape over the open circle, again anchoring the cover on the open side with wire U pins or by weighting with soil. Seed will take longer to germinate at minimum soil temperatures than they would later in the spring. Although moisture will not escape quickly under plastic protection, monitor and add water as needed.
Cool season crops that can be planted this way include lettuce, radish, kale, collards, cabbage, broccoli, spinach, peas, onion and carrots.
As the weather warms, the protective cover will retain heat and may reach temperatures high enough to damage your plants. Vent the cover on sunny days by partially pulling back the cover on the open side, clipping the flap with clothespins. Close the cover at night to retain heat. When the weather has warmed sufficiently, remove the cover completely, retaining the circle barrier for wind protection for another few weeks.
By using plastic to warm the soil and providing a protective cover, you will be rewarded with nutritious fresh vegetables much earlier than is possible by planting in the open.
Photograph courtesy of Debbie Burns, Beulah Land Homestead, Bells Tx.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010