Nonhardy flowers and vegetables have been in stores for more than a month, but it’s only now safe to even think about planting them out in the garden without protection. A neighbor and I always hit the Pickens Tech plant sale that is held every year around Mother’s Day. This year, the sale was May 6. We bought tomatoes and geraniums, knowing full well that we were creating extra work for ourselves.
The average frost date for the metro area is around the same time as the Pickens sale, but it dropped down to freezing almost every night, and snowed several times to boot, for a week and a half after that not-so-prescient date. Anticipating this possibility, we stowed our purchases inside at night, bringing them outside to a sheltered location only when the daytime temperature rises above 40 F.
A neighbor of my friend who also attended the sale planted her flowers out on Mother’s Day weekend and lost every single one to frost. This explains our caution. Hard experience has taught gardeners not to risk planting anything frost-tender until two to three weeks after the average last frost date.
There are several categories of annuals, flowers that grow for a single season regardless of how long they might live in their natural habitats. (Geraniums grow into shrubs in Southern California, and I was amazed to see hedges of fuchsias in Southern England — both are considered annual flowers in Colorado.)
Tender annuals have tropical and subtropical origins. These flowers, foliage plants and vegetables are absolutely intolerant of frost. This includes tomatoes, peppers, eggplants and houseplants, as well as some of our most popular flowers: celosia, cosmos, marigolds, Madagascar periwinkle and zinnias. All blacken and die at 31 F. They really aren’t happy until temperatures stay over 50 F., even at night.
Cool-season annuals may or may not be frost hardy. They all require consistently cool temperatures to perform well. Frost-intolerant salpiglossis, schizanthus, dimorphotheca, nemesia and calceolaria are better choices for mountain gardens, planted well after the last frost date. Hardy cool-season annuals such as snapdragons, stock, pansies, calendula and Iceland poppies, also do best in mountain communities and are grown primarily in spring gardens on the Front Range.
A few frost-hardy annuals, including petunia, bacopa, begonia and verbena, prefer heat but can take quite a lot of cold (which explains why everyone loves petunias).
Protecting frost-sensitive flowers and vegetables from cold is only part of the equation for success. Moving plants in and outside and storing them in a protected spot for a couple of weeks gives them a chance to acclimate to low humidity and bright sunlight before subjecting them to the harsh world of Colorado’s climate.
After you put your plants outside, be prepared to cover them in case of occasionally really weird weather. I keep a pile of old bed sheets just for this purpose. You can also get dedicated frost blankets from garden centers. One year it snowed at my house in June.
Marcia Tatroe’s most recent book is “Cutting Edge Gardening in the Intermountain West,” ($29.95, Johnson Books). E-mail her at mtatroe@q.com.
