Overwintering Tender Perennials

It may seem strange to think about overwintering annuals, but many plants that are called annuals are actually perennials that are not hardy in Iowa’s zone 5.  This list includes some very popular ‘annual’ plants such as petunias, coleus, sweet potato vines, zonal geraniums, and impatiens. There are two main ways to overwinter these tender perennials: taking stem cuttings or bringing the whole plant indoors.

Taking stem cuttings has several advantages. Several tender perennials get too large and unwieldy to bring into the house whereas cuttings are only single stems. For gardeners with a small house (or too many houseplants), the small size is necessary to overwinter these plants. Taking several cuttings also allows a single plant to grow into a multitude. Cuttings need to be taken before the first heavy frost of the year, but by taking cuttings in September there is a chance to take a second round of cuttings if some do not take.

To take a cutting, cut off a 3-5” section from a non-flowering shoot of a healthy plant. Check the cutting for insects or disease before bringing it indoors. Remove leaves from the bottom third of the shoot and place this section into damp potting soil. Cuttings should root within 3-5 weeks when placed in indirect light. It is important that the cuttings do not stay too wet or get too dry. Do not fertilize the cuttings during the winter. When winter ends and the danger of frost has passed, move the cuttings to a shady spot outdoors during the day to acclimate them to the outdoors, and then transplant them into the ground.

If a tender perennial is healthy and the right size, the entire plant can be potted and brought indoors. The plant will need to be acclimated to the indoors, and should be moved into a shaded location for several hours during the day, until it is ready to move inside. Start this process in the middle of September. Check the plant over for insects and disease and treat them as necessary before moving them inside.

Winter is a difficult time for plants, there is less light and lower humidity then they would prefer. If a plant does not make it through the winter, don’t take it as a sign of a black thumb. Try again the following year and eventually they will make it through to see the next growing season.

— prepared by Lindsey Smith, Plant Collections Curator

Photo caption Impatiens hawkeri ‘Pink Flame’