Mirage Landscaping of Calgary has three generations of experience dealing with everything this climate has to throw at us. There’s a lot you can do to help your garden grow successfully, including waiting until it’s warm enough to plant.
The Germination Temperatures of Common Garden Plants
According to the almighty almanac, in Calgary, May 23rd is the last “freeze date” of spring. From then on, our average growing season is 114 glorious days until the first frost, which happens around September 15 every year.
Just because the overnight lows are finally staying above zero Celsius doesn’t mean that it is time to plant the garden. The soil temperatures have to be consistently warm enough to allow the seeds to germinate.
Germination temperatures where various seeds will begin to sprout:
- 4.4 (5 to be sure)°C or warmer: “Cool weather” vegetables such as endive, lettuce varietals, kale, parsnip, peas, and spinach
- 10° C or warmer: Carrots, celery chard, onions, leeks, radish, and turnips
- 15° C or warmer: Asparagus, beets, broccoli, cabbage, cauliflower, green beans
- 21° C or warmer: Corn, cucumbers, eggplant, melons, peppers, pumpkins, squash, and tomatoes
How To Measure Soil Temperature
This is not tricky. Go to your favourite garden supply store or nursery and buy a soil thermometer. Once you have one of these devices, which are usually marked with both the Fahrenheit and Celsius scales on one side or the other, you can take measurements over a course of days to ensure consistent results.
Take a screwdriver long enough to penetrate about 13-15 cm into your selected planting site and then slip the soil thermometer into the holes you create so as to not bend or break the instrument. Make sure that you shade the thermometer from direct sunlight to prevent recording a false high temperature. Wait about three to five minutes for an accurate reading and take note of it. Remember that for proper seed germination you need the soil temperature to no longer be fluctuating below the minimums noted above.
Also note that the soil cannot be overly saturated with water before you plant. It needs to be somewhat loose and crumbly between the fingers first.
Consider Creating or Buying Starters
Calgary’s relatively short high growing season means that starting “warm weather” garden vegetables in late spring, ones that are harvested in the fall, can be a dicey proposition.
Raised vegetable and flower planting boxes are optimal for growing starter plants earlier than would otherwise be possible. A raised bed garden’s soil can be more readily tailored for prime growing conditions, in terms of moisture content, and the whole structure can be easily rigged to have a “green house cap” to provide a controlled temperature environment.
Remember that plants started in a warmer setting have to be gradually acclimated to the outside temperatures, a process called “hardening off.” It takes about seven to ten days of gradually lengthening exposure to the outside temperatures to achieve this effect before planting the starters.
What To Do In The Worst Case
None of us is infallible about the weather, especially the almanac. Somewhere it happens every year where vegetables have started growing, or fruit trees began budding, and out of seemingly nowhere, a last blast of winter threatens to put the freeze back on. What can the backyard gardener do to prevent frost damage in such a situation?
By far the most effective tactic against frost damage to your tender plants is to provide them overnight cover against the lower than expected temperatures. The most threatening time to young plants from spring frosts are the small hours of the morning until dawn. Condensation begins settling on the shoots and leaves and can easily become a frostbite situation at just an hour or two at zero or below.
One way to prevent this is by overlaying either plastic or cotton sheeting over the garden to prevent atmospheric water from settling upon it. Remember that these sheets have to be removed daily to allow the sunlight to reach your seedlings.
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