University of Illinois Extension

Acclimating Plants Outdoors

Taking plants outdoors in the spring time is a breath of fresh air for your plants that have been cooped up in the house all winter, said Matt Kostelnick, U of I Extension horticulturist. "When houseplants get a break outside, they get a chance to grow more actively and expose any indoor pests to the elements outside," said Kostelnick. "People also enjoy bringing their houseplants outdoors in the warmer months because they can add an exotic touch to the outdoors.

"For example, bringing palms outside in the summer can bring a taste of Florida to the outdoor landscape, deck or patio. But before you rush to venture your plants to the outdoors, there are a number of things to keep in mind."

First and foremost, plants don't appreciate shock of any kind, just like humans or animals. Taking a plant from its indoor "bubble" environment to the elements outside can easily cause shock to the plant. "One of the biggest shocks to a plant going outdoors is light intensity," he said. "Light intensity (brightness of light) is far greater outdoors than indoors. The light intensity from direct sunlight outside can easily be one hundred times brighter than indoor lighting conditions. Plants need light for life, but they don't adjust well from going from one extreme to another."

For example, if you took a tropical plant like Dracaena (Corn Plant) from the indoors and placed it in bright sunlight outdoors, you would find the leaves badly sunburned within a few hours, he added.

"Part of the reason for this is the shock to the plant," he said. "The other reason is the Dracaena does not prefer full sun conditions. When taking a plant outdoors, don't place it in direct sunlight. Instead, place it in an area that is well shaded, like a porch or under a tree."

Wind is another factor to consider when taking plants outdoors. Indoors, plants are subject to very little if any wind. Outdoors can be much different.

"Too much wind outside causes the plants to transpire (lose water through the leaves) very quickly," he said. "Wind also causes the soil to dry up quicker. Additionally, wind can toss plants around or knock them over easily. Avoid wind exposure by placing plants in a well sheltered area that is protected from wind. Choose a first day outdoors that is not windy. During times of exceptional wind or thunderstorms, bring the plants inside."

Temperature outside varies much more than indoors. Keep in mind that most house plants are native to tropical or sub tropical regions of the world. Temperatures for most houseplants outside should not dip lower than 55 to 60 degrees during the night.

"Freezing temperatures will kill houseplants," Kostelnick said. "Acclimating plants to outdoor conditions is a good way to start the adjustment to outdoors. Acclimating can be done by introducing the plant to a few hours outdoors the first day and then gradually increasing the time outside. After a week or two, the plants should be fully acclimated to the outdoors."

During the summer, plants are more actively growing, therefore using more water and nutrients. Plan on watering your plants outdoors more frequently than you would when they are indoors. However, don't waterlog or over-fertilize the plants either. Use suggested fertilizer rates on the fertilizer label.

The principle of acclimating or "hardening off" seedlings from the indoors to the outdoors is very similar to the steps of acclimating houseplants. Cold frames are commonly used with seedlings in the hardening off process.

"As seedlings are very delicate, they are sensitive to shock," he said. "When seedlings go from indoors to outdoors, they are leaving their 'protective bubble.' Although the idea of acclimating plants outdoors is to 'toughen them up' the key is to gradually introduce the seedlings to their new environment, not force them into drastic conditions they aren't used to.

"Shock to seedlings (and all other plants) stresses them. Too much stress will lead to plant injury or death and make the plant more susceptible to pest problems. Mild, gradual doses of stress will be enough for the plant to handle, and at the same time, acclimate the plant to its new environment. Also keep in mind that tender annuals need to go out later in spring than hardy annuals. Following these steps will help ensure the survival of your plants journey from indoors to the outdoors."