Pocket BotanistEvery autumn there is a week when the nights turn and the pots on the patio need a decision. Some plants can take a frost and some cannot, and the ones that cannot need to be indoors before it arrives, not the morning after. Here is which plants to move, when, and how to keep them alive in a house that is warmer and darker than they would like.
The plants that must come in
Houseplants that summered outside: monstera, ficus, palms, orchids, ferns and the rest. They should be back inside before nights fall below about 10 °C (50 °F), earlier than the frost.
Tender perennials in pots: pelargoniums (the plants most people call geraniums), fuchsias, salvias from Mexico and South America, osteospermums, marguerites, heliotrope, bougainvillea, brugmansia, and lemon verbena. These die at the first frost outside and live for years inside.
Fruit and Mediterranean plants: lemons, limes and other citrus, which take a light frost but not a hard one; olives and bay in cold regions; and dwarf banana plants. Succulents and cacti, including echeveria, aeonium and aloe: a night at freezing turns them to mush.
Tuberous and bulbous plants that can either come in whole or be lifted and stored dry: cannas, dahlias, begonias, gladioli, and caladiums.
When to move them
The trigger is the forecast, not the calendar. Start once night temperatures drop to 8 to 10 °C (46 to 50 °F), usually a few weeks before the first frost, because the shock of moving from cold nights into a warm house drops leaves. Houseplants go first. Pelargoniums, fuchsias and citrus can wait until a frost is actually forecast, since they prefer the cool. Whatever happens, everything tender is indoors the evening a frost is forecast.
Check for pests before they cross the threshold
A summer outside collects aphids, whitefly, scale, mealybugs and the odd slug in the pot, and a warm windowsill is where they multiply. Before each plant comes in: look under the leaves and along the stems, hose the foliage down, spray with insecticidal soap if anything is found, and slide the plant out of its pot to check for slugs, snails and ants in the root ball. Quarantine anything suspect on its own for two weeks. Now is also the time to cut back leggy growth by a third, which makes the plant fit on a windowsill and reduces what it has to keep alive.
Where to keep them
Most tender plants want to rest through winter, and rest means cool and bright: an unheated conservatory, a porch, a bright spare room, a frost-free greenhouse or a garage with a window, ideally between 5 and 12 °C (41 and 54 °F). A warm living room keeps them growing weakly in poor light, which is how they get leggy and pest-ridden. Houseplants are the exception; they go back to their usual warm spot. Citrus is happiest cool and bright; a heated room with dry air is how it loses its leaves.
Care through winter
Water very little. A resting plant in a cool room may need water once a month; the soil should be nearly dry before it gets any. Do not feed until March. Remove yellow leaves and any mold promptly, and give the plants space so air moves between them. Pelargoniums and fuchsias can be kept almost dry and leafless in a dark, frost-free shed and will shoot again in spring. In March, move everything into brighter light, start watering, and harden them off outside over two weeks after the last frost.
What can stay out
Hardy plants: roses, lavender, box, most shrubs and trees, hardy perennials and hardy herbs like rosemary, thyme and sage in most of the UK and central Europe. Pots of hardy plants still benefit from being moved against a house wall and lifted off the ground on feet, because roots in a pot freeze faster than roots in the ground. Wrap terracotta pots to stop them cracking. If in doubt about a plant, its hardiness rating is on its label or in the app, and anything rated for a zone colder than yours can stay.
Know which of your plants are tender
Pocket Botanist knows the hardiness of every plant in your garden and your zone, so the plants that need to come inside are listed before the first frost, and Premium sends the warning the evening before. Free on iPhone and Android.
Sources
- Royal Horticultural Society, "Frost damage." rhs.org.uk
- Royal Horticultural Society, "Prevention and protection." rhs.org.uk