In an annual ritual of spring, gardeners emerge from their houses in mid-March, blinking in the thin sunlight as they peer around the landscape, assessing what needs pruning, cleaning, or mending. All it takes to get our sap rising is for the month to turn to March, and we shrug off our winter lethargy in favor of planting the garden.
That harbinger of spring acts like a jolt of electricity on our minds, sending a surge of energy urging us to get our hands in the soil and sow seeds. But if you’re too quick to cave in to eagerness, you might find that your good intentions are not appreciated by the seeds you lovingly pop into the ground. They’re not impressed by the calendar; they could care less about leprechaun and daylight savings.
What the seeds want is warmth. To ensure seeds sprout instead of sulk, gardeners need to plan on two things: frost date and soil temperatures. Average date of last frost is what most rely upon to plan their plantings; it’s calculated from 30 to 40 years of data. In Boulder the probability of frost is 50-50 on May 3, in Loveland May 4, and Longmont May 5.
Because it’s still possible to get frost after these dates, many gardeners prefer to use the 10-percent probability of frost date as their benchmark, which in Boulder is May 13, Loveland May 16, and Longmont May 17. You still need to plan for protection from the odd cold snap, but the likelihood of frost is much less when waiting until mid-month.
Yet there is a second temperature factor that Colorado gardeners shouldn’t ignore: soil. Chilly soils will limit germination of seeds, and if the ground is both wet and cold, seeds will rot instead of sprouting.
Early spring vegetables vary in required soil temperatures. Spinach and lettuce are the most tolerant, needing soil to be 35-degrees to germinate and 45-degrees or warmer to grow well. Peas, chard, onions, mache, and parsley can grow in soils at 40-degrees, but do best in soils of 50-degrees or warmer. Soil temperatures of 60 degrees or warmer are perfect for fava beans, beets, carrots, and leeks, while scallions, Asian greens, kale and kohlrabi like is slightly warmer, at 65-degrees.
Can these plants germinate in slightly cooler soils? Yes, but for them to grow rapidly and thrive – key to sweet flavor and tenderness – gardeners should pay close attention to bringing soil temperatures up.
To do this, plastic mulches or water-filled walls are helpful. If you’re choosing to go with warming by plastic, clear plastic works better than black at warming soil, bringing the temperature up 10 to 15 degrees in a few days. Make sure you weight down all edges of the plastic to keep it from blowing in the wind, and lay it out so that it touches the soil a week ahead of planting to give your seeds a warm start.
Punch holes into the plastic to sow seed if you plan on leaving the plastic in place over the season. If you are removing the plastic once temperatures are stable, laying the sheets down in strips that can be folded back for planting space is a good way to leave the plastic between rows for a few weeks. If using water-filled walls, place them out one week before planting to help warm the soil.