How to care for dormant plants
Some houseplants go completely dormant in winter. The most extreme cases are bulbs such as Jacobean lily (Sprekelia formosissima) and blood lily (Scadoxus multiflorus syn. Haemanthus multiflorus), which lose their leaves and may look dead. Just pull off any yellowed leaves and place plants in an out-of-the-way spot like a closet or basement until they show signs of life again, usually by mid- February. Amaryllis (Hippeastrum spp.) prefers a short dormant period; it usually awakens by late January.
Other plants don't visibly change but are still semi-dormant—Clivia, for example, prefers to be ignored until February or March. Stop watering in late November until you see flower buds, then start a regular watering schedule again.
Desert cacti (both the stubby, short ones, such as Mammillaria, Lobivia and Rebutia and the tall, candelabra ones, such as Cereus and Cleistocactus) are a special group: they like their winters cold and dry, with temperatures between 4 and 7°C. If you can maintain this temperature in a room with no light, don't water the cacti until late March. They'll shrink considerably but will plump right back up when you start watering them again. If ambient temperatures are warmer, such as 9 to 15°C, continue watering but only very lightly—perhaps once a month—since the plants are partially dormant. Cool, dry winter conditions mean heavy flowering come summer, so if your cacti have never bloomed for you, now you know why.
Plants that clean the air
All plants absorb harmful toxins given off by substances such as household paints, plastics and cleaning products, and they also help suppress harmful bacteria. The following 10 houseplants have been shown in ongoing NASA studies to be particularly effective in cleaning the air:
• Reed palm (Chamaedorea seifrizii)
• Areca palm (Chrysalidocarpus lutescens syn. Areca lutescens)
• 'Janet Craig' dracaena (Dracaena deremensis 'Janet Craig')
• Rubber plant (Ficus elastica)
• 'Alli' ficus (Ficus maclellandii 'Alii')
• English ivy (Hedera helix)
• Boston fern (Nephrolepis exaltata 'Bostoniensis')
• Miniature date palm (Phoenix roebelenii)
• Lady palm (Rhapis excelsa)
• Peace lily (Spathiphyllum spp.)
Image: Peace lily. Photography by Marilynn McAra.