Hardening Off/Acclimating 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.

Low Temperature Tolerance of Annuals
62 °F
AlyssumAgeratumAsparagus FernAgapanthusAngeloniaBasil
DianthusBrussels SproutsCeleryBacopaCannasBridal Veil
Dusty MillerCabbageChinese CabbageCalibrachoaChenilleCaladiums
Flowering CabbageGaillardiaCleomeCornCucumberCelosia
Flowering KaleKohlrabiCosmosGomphrenaDahliaColeus
SnapdragonsLobeliaEndiveJasmineHeliotropeFrench Tarragon
NicotianaFibrous BegoniaMarigoldHibiscusGerman Ivy
ParsleyGazaniaMoss RoseMelonImpatiens, New Guinea
Sweet PeaGeranium, IvyPumpkinMorning Glory
Geranium, ScentedPurple Fountain GrassPepper
HeatherScaeveolaSwedish Ivy
Monkey FlowerVinca
NierembergiaWandering Jew
Spider Plant