Cold-Tolerant Annuals

An annual is any plant that doesn’t overwinter in our climate, so it only grows for one season and is planted annually.

Cold-tolerant annuals, also known as hardy annuals, can survive in lower temperatures when other annuals don’t survive. They can withstand cooler temperatures in the spring and often withstand temperatures slightly below freezing in the fall.

Cool weather annuals are also cold-tolerant, but differ in that they only thrive when the weather is cool. Blooming slows once the temperature rises in the summer.

Cold-Tolerant Annuals
Cool Weather Annuals
CalendulaBroccoli
DianthusBrussels Sprouts
Ornamental CabbageCabbage
Ornamental KaleCauliflower
PansyKale
SnapdragonKohlrabi
StockLobelia
Sweet AlyssumPeas
Sweet PeasRadish
ViolaSpinach

Hardening Off/Acclimating Greenhouse-grown Plants

Although cold-tolerant plants can handle low temperatures, they must still be given time to adjust to new environmental conditions, just like other greenhouse-grown plants. When plants are grown in the ideal conditions of a greenhouse, the outdoor environment is comparatively harsh. The different conditions, such as temperature, light, and wind, can shock them and cause plants to become stressed or die if introduced to the outdoors too quickly. This is especially so for annuals in early spring when temperatures are cool and can dip too low for many annuals to survive.

Acclimating or “hardening off” is the process of gradually adjusting plants to new growing conditions. At America’s Best Flowers, we bring cool weather crops, such as pansies, dianthus, petunias, dusty miller, broccoli, cabbage, lettuce, and leeks, outside as soon as possible. So when you purchase plants from us that are outside, they are already hardened off and can be planted right away. We are also the only local grower to use a special growing technique, known as DIF, which lowers the temperature in the greenhouse for a period each day helping harden off annuals by mimicking outdoor temperatures. That means our annuals are hardier and will acclimate to outdoor conditions more quickly than plants not grown with this technique.

To acclimate tender plants to cool weather, place them outdoors for 3-4 hours during the day when the temperature is 40 degrees or above in an area that is shady and sheltered from the wind. Then bring them in at night into a warmer, enclosed area such as a home, garage, or shed. Gradually increase the time outside and light intensity each day, and carefully monitor the weather forecast, especially the first few days. Keep plants indoors if strong storms or low temperatures are in the forecast.

The process can take a few days to 1-2 weeks depending on weather and plant variety. Once they are adjusted to the cooler temperatures, they can stay out overnight or be planted, but be prepared to move them indoors or cover if temperatures are below 40 degrees. Cold-tolerant plants should be able to withstand a light frost after several days of acclimation.

Frost Tolerance

Frost tolerance varies by the specific variety of annual. Most hardy annuals tolerate light frosts, but not freezing, though pansies and violas can tolerate several degrees below freezing. The average first frost date in Madison is usually October 10 and the average last frost date is around May 15.

Annuals planted in containers and ground beds should be covered with frost cloth if frost is forecasted. Remove frost cloth once the sun rises and temperature is above 32 degrees. Baskets should be brought inside.