Ask a Master Gardener: Toughen up your greenhouse plants
"Hardening off" means acclimating plants to the outdoors before leaving them outside overnight.
Q: I bought some annual flowers at a local garden center. Is it too soon to plant them outside or in baskets?
A: The nights we have been getting in the 20s are too cold for any annuals or new perennials. And everything needs to be hardened off before it can be left out or you will wind up with miserable, or even dead, plants.
Hardening off means acclimating plants to the outdoors by first moving them out into a shaded area for a few hours every day, eventually extending the time and the amount of sun they get. It’s tedious, but if you don’t do it, the plants will be damaged.
Compare it to human behavior, and you'll see acclimation works for us, too. Some people think shorts are a good idea when it is 45 in the spring, but they wear a winter coat at 45 in the fall. The plants are coming from a constantly warm greenhouse with bright light but no direct sun — they can get sunburned like someone on spring break.
Although some annuals are pretty tough once hardened off, most that are considered cold tolerant are happiest above 40 degrees. The exceptions are pansies and violas, which can go down to about 30.
Other cold-tolerant annuals are calendula, cyclamen, dianthus, dusty miller, flowering kale, forget-me-not, love-in-a-mist, lobelia, osteospermum, poppies, snapdragons, stock, sweet alyssum and sweet peas. Vegetables include broccoli, Brussels sprouts, cabbage, cauliflower, kale, kohlrabi, lettuce, peas and Swiss chard. Both sweet peas and garden pea seeds can be planted outside before the last frost, but be aware that sweet peas take about two months before they start to bloom.
Wait until nights are above 55 before leaving these out: amaranth (love lies bleeding), begonia, coleus, impatiens, heliotrope, nicotiana, sweet potato vines, verbena and zinnias.
If you are in doubt, wait.
Written by U of M Extension Master Gardeners in St. Louis County. Send questions to email@example.com.