Dr. Leonard Perry, Extension Professor
University of Vermont
While perennials are hardy in their native climates, if they aren’t in yours but die with cold in fall, they can be considered tender. Many of what we consider annuals in our climate are in reality perennials in their native habitats, such as annual geraniums (Pelargonium), salvia, snapdragon, begonia, and impatiens. Whether annuals, summer bulbs, woody plants such as hibiscus, or exotic plants such as angel’s trumpet, knowing which of the four main climates they prefer will help them to live (or to remain dormant in the case of summer bulbs such as canna and dahlia) successfully indoors all winter.
Sunny and warm conditions can be provided by windows facing east, south, or west in warm living areas. Temperatures should remain between 60 and 70 degrees F. A heated greenhouse or solarium also can provide such conditions. The cooler the temperature, the drier you should keep the soil. These plants require as much light as possible. If plants become spindly with thin leaves, consider adding supplemental light or moving plants to a southern exposure if possible and not there already. Sunny and warm mimics the winter conditions for many tropical plants. Plants keep their leaves, and growth slows but remains active.
Some common “annuals” that prefer sunny and warm conditions include coleus, begonia, impatiens, lantana, thunbergia, and Egyptian stars (Pentas). Hibiscus prefers these conditions too. Flowering maple (Abutilon), fuchsia, begonia, and succulents often survive fine near windows with east or west exposures. Succulents include such plants as aloe, jade plant and its relatives (Crassula), and kalanchoe.
Sunny and cool conditions can be provided in an unheated sunporch or guest room, minimally heated entry halls, or a cool greenhouse. Temperatures should remain between 45 and 55 degrees F ideally, or at least above freezing. These cool temperatures help prevent insect problems as an added benefit. The goal with sunny and cool is to keep plants from growing, or growing very slowly, most the winter. They are provided a rest period, and so are low maintenance.
Most tender perennials prefer sunny and cool winters indoors. Mediterranean plants such as pomegranate and rosemary, citrus such as lemons and miniature oranges, and silvery plants such as lavender prefer sunny and cool winters. Some South African natives such as lily-of-the-Nile (Agapanthus) and annual geranium are in this group, as are the Australian hebes. Tropical “annuals” in this group include cigar flowers (Cuphea), summer snapdragon (Angelonia), and sages (Salvia). Many subtropical and tropical woody plants prefer sunny and cool winters, such as oleander, gardenia, osmanthus, and angel’s trumpet (Brugmansia). Some such as fuchsia and hibiscus do well under these conditions as well as with sunny and warm, just with less growth.
Dark and damp conditions are useful for a handful of plants, especially tubers (swollen roots) of sweet-potato vine, dahlia, and canna. Store tubers in a plastic bin or bag in a barely moist, not wet, material such as sawdust or peat moss. Fountain grasses (Pennisetum) in pots can be cut back to 6 to 12 inches, and stored here as well. Some even have luck in dark and damp winters with pots of salvias and geraniums that normally need more light. Provide these conditions in an unheated cellar, garage, or crawl space. Lacking those, a cool closet may work. Temperatures for cool and damp mean between 35 and 50 degrees F.
Dark and dry conditions are needed for a handful of tender perennials, gladiolus corms being the most common. Some salvia in this wide-ranging genus can be stored here, unless they have formed tubers that should be stored damp. Similar to dark and damp, cool temperatures should range between 35 and 50 degrees F.
Plants dropping their leaves may be a sign of too cool, too much water (rotting roots), adjusting from high light outdoors to lower light indoors (in which case you should see new leaves soon), or simply may be normal. Some woody plants such as angel’s trumpet, butterfly bush, fig, and lantana normally lose their leaves in winter. They can be shaped, left in their pots without repotting (so not to stimulate new growth), and kept barely moist but not wet until growth resumes with the longer days of spring.
Much more on the correct conditions for many individual plants, as well as tips on such topics as problems and propagation, can be found in the excellent reference from Storey Publishing by Alice and Brian McGowan, Bulbs in the Basement Geraniums on the Windowsill.