Sowing success a matter of temperature

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.

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Fast tool for taking your soil’s temp

Ok, I’ll admit it:  I rush the season.  When the weather warms and eagerness overcomes my better sense, I push seedlings and seeds into the soil, tucking them in at the first hint of spring. 

 Because of this I wait as days turn to weeks, wondering if the seeds rotted in the soil.  I wait with impatience as seedlings sit and sulk, unimpressed by their new home.  They’re not dead; I check – those seeds just lay in the ground doing nothing as I put up trellises and string, ready sun shades and mulch.  Clearly one of us is slacking.

Turns out it’s the soil.  If your ground isn’t warm and cozy, the plants won’t thrive, and the jump start I gave the season will fizzle  until that soil heats up.  

Fortunately for us there’s a nifty website you can go to, to check the soil temperatures in your area.  Check out greencastonline.com, a site run by a major company who likes to make sure its seed succeeds.  I’m not endorsing anything here – I just like their soil temp site.   It’s under agronomic crops, should you decide you’d like to click over to weather, then back again. 

Boy, this is fast – currently our soil is 48F, good for planting kale, lettuce, pak choi, parsnips, peas, and spinach.  But days to maturity come into play too, so think ahead to conditions that will be in play once the plant starts producing.  Peas may fry and lettuce bolt in our balmy 90F summer.  But a few more degrees and I can plant the chard or leeks.

Beans typically need soils to be warmer than 60F, ideally 68 – 70F; all of the truly tender crops – tomatoes, eggplants, peppers, cucumbers, squash, pumpkins, corn and melons need the soil to be 70F.  Otherwise they sit there, brooding over the fate that left them in the hands of a rude, thoughtless gardener who plops their bottoms into chilly soil.  Fedco has a great chart for crops and the soil temps they need for planting.

With these two tools you can click on your area to get information on your soil, precipitation, weather, etc.  And this year I intend to use them, so that when I pop my plants in, they’ll leap from the ground and I can get down to the business of gardening, not waiting.

 **Yes, you can warm the soil with plastic, walls o water or cold frames.  Take your soil temp after five days to see how far your temps have increased.

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