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Time to take care of those precious plants

The first 40-degree nights remind us: It's time to prepare the houseplants and tropicals to come inside for the winter. There's no rush – some of us delay well into October – but since some plants die from frost or even 40 degrees, keep a sheet ready to cover the vulnerable plants when a suddenly cold night is predicted.

For most plants that are going inside, the normal steps are basic cleanup, a decision about repotting and adapting them to new conditions.

Clean up: It is not necessary to spray all container plants with pesticides. Very few insects that happen to travel into the house will survive there, and those few usually aren't harmful. Brush off the bottoms of the pots, where pill bugs and centipedes congregate. Remove dead flowers or stems, and debris around the base of the plant. Clean up the pot itself. Brush off spiders and minor insects, and hose the plant with a strong stream of water.

If you see or have experienced more serious pests, use an insecticidal soap or dunk the whole plant in a tub of mild soapy water. In extreme cases, expose the plant's entire root system, wash the whole plant and then put it into fresh potting mix.

Repotting or not?: Fall and spring are both fine for repotting houseplants. If your plant is obviously cramped (so you'll be watering constantly), or if it's in an outdoors-only container, then do move the plant into a larger pot now. Choose a pot just one size up (no more than an inch larger).

Timing and adjusting: Many plants are shocked by changes – from cool to warm, warm to cold, humid to dry. Ficus plants are notorious for dropping their leaves after the least change of venue. A Christmas cactus can even drop its buds because you turned it around in place. So make the outside/inside move as gentle as possible. Do it on a mild day, when the heat is off in the house and the conditions are relatively similar.

Two conditions in most homes are radically different from outdoors – light and humidity. The best indoor light is about as bright as outdoor shade, so plan to supplement the lighting. An outdoor "shade" plant really won't love your darkest corner. And do plan to increase the humidity around your plants, since heated homes without humidifiers can be drier than a desert.

>Which plants, where?

Unless you have a greenhouse or huge, bright windows, it's triage time: Some plants must stay behind – call them compost – and some will get the best deal you can offer.

Group 1: Tropicals to the basement: With a cool basement or attached garage (40 to 55 degrees) you can provide a long winter's nap for these:

*Angels' trumpets, bananas, Tibouchina, Lantanas: Before the first hard frost, cut off their leaves and drag them to the basement, where they will sprout again in late winter.

*Bulbous or tuberous plants – Cannas, dahlias, caladiums, calla lilies, Agapanthus: After a hard frost, cut off the dead leaves and dig them up carefully – not to wound. Shake off the soil and air dry them; moisture promotes rot. Unless they fall apart, wait until spring to divide them. Some experts advise storage in peat moss or sawdust, but many gardeners just keep them in shallow crates or boxes.

For both groups, provide a few drops of water once in a while, before the soil gets bone dry.

Group 2: Keep as houseplants: Traditional houseplants such as begonias and schefflera became popular because they survive in warm indoor conditions that are not nearly moist or bright enough for most plants. As new annuals and tender perennials become popular, we're just learning which ones make good houseplants. Try any you love, and if they die – hey, you tried.

Good candidates include Strelitzia (birds of paradise), Colocasia, Alocasia (elephant's ears), coleus, hibiscus, Plectranthus, Helichrysum, mezoo, salvias and Alternanthera. I'm trying Acalypha for the first time. It's way too beautiful to just let go.

Some mandevillas and passionflowers have just begun to flower exuberantly, so bring them in before a 40-degree night and enjoy them as long as they are beautiful. When they stop flowering and get leggy, cut them back and move them to a cool, dark place.

Group 3: A cool, bright location: Few people have a cold cellar or unheated room with good windows, perfect for plants to think they live in Florida. If you have a cold space, above freezing, you could add plant lighting and provide perfect conditions for these: Agaves, Clivia, Cordyline, Crinum, Phormium and flowering maple (Abutilon)

>Two special cases

*Christmas cactus (shown on C1): People start to worry about how to force Christmas or Thanksgiving cacti into bloom, but it couldn't be easier. Just don't rush it inside. When the plant senses decreasing day length and night temperatures below 55 degrees for a few weeks, it is stimulated to develop flower buds. You will not get flower buds in a 70-degree house. Once the buds start forming, move it carefully inside and place it at the same angle to the light as it had been.

*Amaryllis: Ideally you have let your amaryllis leaves grow, watering and fertilizing them all summer. To produce a bloom, you must now force dormancy. You must cut all the leaves off (a step that feels very, very wrong!) and then lay the pot sideways so it doesn't get drowned in the rain.

Before a frost, take the plant into a cool room for two months of rest. After at least two months, spoon out a little of the old potting mix from the top of the pot and add some new material. (Repot only if it's absolutely cramped, because disturbing the roots can interrupt bud formation.) Then begin to water and fertilize gently and consistently. Sometimes amaryllis take two years to rebloom, but they are certainly worth the effort.

Sally Cunningham is a garden writer, lecturer and consultant.