How do you overwinter tender plants?
By recognising that different plants fail for different reasons — some are killed by frost, some by winter wet, and some simply by being too cold for too long — and the right protection depends on which.
Understand what actually kills them:
Frost damage, where ice forms in tissue and ruptures cells.
Winter wet, which is the underestimated killer. Many Mediterranean and alpine plants tolerate considerable cold and are killed by cold, wet soil rotting their roots and crowns. For these, improving drainage does more than any covering.
Cold wind, which desiccates evergreens when the ground is frozen and roots cannot replace lost water.
Waterlogged pots, which freeze solid and kill roots.
The methods, by plant type:
Bring indoors. Truly tender plants — pelargoniums, fuchsias, citrus, succulents — to a frost-free place. Cool and light is better than warm and dark: a warm room causes weak, drawn growth and encourages pests. A conservatory, porch or cool spare room works; a heated living room does not.
Lift and store. Dahlias, cannas and begonias can be lifted after the first frost blackens the foliage, dried, and stored in barely moist compost somewhere frost-free. In milder areas with free-draining soil, dahlias are frequently left in the ground under a thick mulch, which is increasingly common.
Mulch the crown. For borderline perennials, a thick layer of bark, straw or leaves over the crown, applied after the ground has cooled. This protects the growing point even if top growth dies.
Wrap in situ. Tree ferns, bananas and palms — pack the crown with straw and wrap with fleece or hessian. Use breathable material, since plastic traps moisture and causes rot.
Protect pots by moving them against a wall, grouping them together, and wrapping the pot — roots are far more vulnerable than top growth because they have no soil mass insulating them.
Raise pots on feet so they drain.
Reduce watering substantially for dormant plants.
Timing matters: too early encourages soft growth and rot; wait until temperatures genuinely fall, and remove protection promptly in spring.