As summer fades, it's time to begin to ready plants for a winter indoors. Tropical houseplants must come home from "summer camp" before the chill of autumn breezes begins. Tender perennials that spend winters indoors can return to the outdoors ready to bloom the following spring. Even annuals taken indoors during winter's cold can provide new plants for spring. Proper preparation is the key to successful transitions for all three groups of plants.
List all the plants you plan to bring inside in order of tenderness. Tropical houseplants like peace lilies need to come inside first, then tender perennials like geraniums or annuals like coleus that will overwinter or provide stock for spring propagation and finally, hardy plants like citrosa that will simply spend the winter indoors. When finished, you'll have a list to follow rather than trying to do all the plants at one time.
Flush plants with water weekly beginning in August, using a soft spray or watering can to discourage whiteflies, aphids and thrips from setting up new colonies. Cut fertilizer back by half. Plants will slow or stop growing in the lower indoor light levels; most will need little or no fertilizer over the winter. Clean out accumulated leaves and debris in pots that can shelter insects and fungi that cause root rot.
Pot up plants or re-pot those that have outgrown their pots (spider plants and asparagus fern roots can actually break their pots). Scrub pots clean with mild dish soap, rinse well and fill with light potting mix. Soak any pots that will be coming directly inside by submerging the entire pot in tepid water to drown springtails and other soil dwelling insects; the University of Vermont Extension (UVE) recommends a 15 minute soak, while North Dakota State's houseplant guide recommends using an insecticide drench.
Prune plants that have reached their desired size. Prune overgrown roots of plants like sprengeri ferns while repotting. Pinch back perennials and prune back leggy growth back--it will just become more attenuated stretching toward the light all winter. Prune no more than one-third of a plant's root and top growth; you can always trim it again in the spring to "bush" it up. The exception is plants that will be kept inside in a dormant state; University of Illinois Extension educator Sandra Mason recommends trimming fast-growing tropicals back to 8 to 10 inches tall for storage in a cool, dark garage or basement.
Treat plants with insecticidal soap and water well the morning before bringing them inside. Bring a few plants in each day, beginning with the tropical plants and ending with the hardiest of the tender outdoor plants. Tropical houseplants should all be in by the time the nighttime temperatures reach 55 and outdoor plants should be in by the time it dips to 45. UVE suggests that tropical plants will suffer damage plants between 40 to 50 degrees.
Transition plants gradually; find a sunny place for the first week inside. UVE suggests preparing by washing all the windows and building some plant shelves with lined trays to prepare a first stage in their move to the indoors.