Air temperatures that hover just above freezing for a prolonged period of time in the spring can kill most tender plants if they’re set out too early. So if you set out your houseplants outdoors too early, even if it’s actually above freezing in terms of the air temperature, they simply can’t endure it after being accustomed to the warmth in the house.

All air temperatures affect plants, as it affects most of their physiological processes such as their absorption of water and nutrients and time of flowering, fruiting, and also seed production. Each plant species has its own critical air temperature, which includes the minimum and maximum temperatures between which a specific plant can continue to exist, grow, and/or thrive. There is the perfect temperature for it to grow optimally and the lethal temperatures, both high and low, when a specific plant will expire.

All these levels are also, of course, influenced by the individual plant’s stage of growth and development. Seed germination requires more warmth than vegetative growth as well, and optimum temperatures are different for both day and night.

The important point here is, of course, do not take risks by setting out your annual plants too soon. Always be cautious for it is better to be sure than sorry when considering when to plant outdoors in the spring.