Tender perennials like coleus and pelargonium that are relatively easy to propagate are the best ones to overwinter as rooted cuttings under grow lights or on a sunny windowsill. Plants that are widely available, inexpensive and difficult to keep indoors, such as petunias, verbena and purple fountain grass, should be composted.
Plan ahead: Cuttings should be taken in late summer while plants are still in active growth. Don’t wait until a frost warning has you running around your garden gathering whatever you can save.
1. If you have room, the mother plants can also be saved by pruning them back to about 15 centimetres, and repotting them in a fresh, soilless mixture to keep plants in active growth throughout the winter.
2. Tropicals that have summered outdoors like hibiscus (H. rosa-sinensis) or calamondin oranges (×Citrofortunella microcarpa) should be checked for pests and treated if necessary. Use insecticidal soap for aphids and whitefly, and rubbing alcohol on a cotton swab for scale and mealy bug before bringing plants indoors; expect some leaf drop while they acclimatize.
3. Many tropicals and tender summer-flowering bulbs go dormant in winter and can have all their leaves removed (brugmansia and bananas, for example), or, in the case of dahlias and canna lilies, the entire top of the plant should be cut off at the base. The dry tubers, rhizomes, roots or stumps can be stored in a basement or cellar that doesn't freeze.
For a head start in late spring, bulbous plants may be potted indoors several weeks before your first frost-free date, or be taken directly back outside with other tender plants once all danger of frost has passed.