BOULDER COUNTY – Has the unusually warm weather sparked the thought that spring has sprung in your mind? If you’ve been watching your spring bulbs shooting up and raring to go, you might be fooled into thinking we’re off to an early start.
NASA tends to agree: it released an analysis of data collected from their Global Precipitation Measurement satellite that shows spring-like rain and temperatures in the eastern United States spurring early bloom in plants. California awaits a massive “super bloom” of desert plants due to the deluge of rain they’ve been getting.
Here in the Rocky Mountains temperatures are bouncing from spring to winter to spring, often in the span of seven minutes. When someone counsels us to dress for the weather, it means tank tops under heavy parkas, hair bands that convert to face masks, and pants that convert to shorts with a quick pull of a zipper.
It’s no wonder our plants don’t know whether to break free and bloom or hunker down and wait a while. If you’re itching to plant, a little patience is a good thing. While our days are warm, the soil is not, at least not according to the temperatures I took in several areas in early March. The ground is hovering just below 40-degress, which is a bit chilly for germination of most seeds.
If you must plant, one of the simplest ways to warm your soil is covering the ground with plastic sheets. Use 6 mil or thicker, UV resistant clear or black plastic and lay it over the soil, weighing down all edges with rocks or soil to prevent winds from whipping it up, up, and away to Kansas. Alternately, you can anchor it down with wire U-shaped pins.
Check the soil after ten days to see if it’s warmer; for germination of cool season vegetables the minimum temperature needed is 40-degrees F. Typically, it takes two to three weeks for it to rise, depending on the soil type. Sandy or manufactured “planters mix” soils warm faster than wet, heavy clay.
If you’d like to speed the process, combine the plastic cover with an insulating layer. Using only clear plastic, lay a sheet on the ground, anchoring it as described above. Then drape a second layer of clear plastic slightly above the first, using bricks or other objects to make a small space between the two layers. Anchor the second layer securely by tucking its edges under the bricks or by weighing them down on the ground.
To plant, fold back the plastic drape and remove the plastic sheet covering the soil, cleaning, drying, and folding it away for use another time. Plant seeds of lettuce, radish, kale, collards, cabbage, broccoli, spinach, peas, onion and carrots and then replace the plastic drape over the spacers, creating an impromptu cold frame, anchoring the cover securely so it remains to keep the seeds snugly warm in the bed. There is no need for a plastic sheet on the soil once seeds are planted.
Patience is required when starting seeds this way, as seed take longer to germinate at minimum soil temperatures than they would later in the spring. Leaving the plastic on until the temperatures have risen higher than 40 degrees won’t take much more time and you’ll be rewarded with better germination if you wait. Monitor soil moisture and add water as needed.
Watch the weather and your plants closely; once the weather warms, the plastic tenting will trap heat and can reach temperatures hot enough to sizzle your plants. Open the cover on sunny days, partially folding back the cover and clipping the flap to prevent it from whipping in the breeze and tearing. Be sure to close the cover in the late afternoon to retain heat.
By Carol O’Meara, Colorado State University Extension Boulder County
Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, e-mail email@example.com or