Pocket BotanistDahlias come from the mountains of Mexico and have no idea what to do with a frost. The first one turns the leaves black overnight, and it looks like the end. It is not: the plant lives in its tubers, which sit underground where a light frost cannot reach, and those can be kept for next year with an hour's work. The decision to make is whether to cover, lift, or leave them, and it depends on your climate and your soil.
A light frost: cover and carry on
The first frosts of autumn are usually light, a degree or two below freezing for a few hours before dawn, and they often come weeks before the next one. Dahlias covered on those nights keep flowering into November in a mild year. Drape horticultural fleece, an old bed sheet or a large cardboard box over each plant in the late afternoon, before the temperature drops, and take it off in the morning. Do not use plastic directly on the leaves; it conducts the cold straight to them. Plants against a south-facing wall or in a sheltered town garden often escape the first frost entirely.
When to lift
Wait until a frost has blackened the foliage: that is the plant's signal to send the last of its energy down into the tubers, and lifting before it means smaller, softer tubers that store badly. Then work within a week or so, before a hard frost reaches the ground. In a frost-free autumn, lift anyway once the leaves start to yellow in November; tubers left in cold, wet soil rot.
How to lift and store
Cut the stems down to about 15 cm (6 inches). Loosen the soil with a fork in a wide circle around the plant, well outside the stems, and lift the whole clump; the tubers spread further than you expect and a fork through one is the usual injury. Shake off the loose soil, rinse if it is heavy clay, and turn the clump upside down somewhere airy and frost-free for a few days so the hollow stems drain and the skins dry. Label them now, while you remember which is which.
Store in a shallow box or crate, in slightly damp compost, vermiculite or wood shavings, with the crowns just showing, somewhere dark, dry and cool but frost-free: a garage, a shed, a cellar, 4 to 10 °C (40 to 50 °F) is ideal. Check them monthly through winter. A tuber going soft or moldy is cut away or binned before it spreads; tubers shriveling badly get a light spray of water.
When to leave them in the ground
In gardens that rarely see a hard frost, USDA zone 8 and warmer, or the milder parts of the UK on light, free-draining soil, dahlias can overwinter where they grow. The enemy is not cold so much as cold and wet together: tubers in waterlogged clay rot regardless of temperature. Cut the stems back after the first frost, cover the crown with a mulch of compost, bark or straw at least 15 cm deep, and hold it in place with a pot or a board against the rain. Lift the mulch in April. Expect to lose a few in a wet winter, and take cuttings or lift your favorites as insurance.
Starting them again in spring
In March or April, pot the tubers up in compost in a frost-free, bright place, or plant them out directly once the last frost has passed, in May in most of the UK and central Europe, and after the last spring frost date for your zone in the US. Tubers started early in pots flower a month sooner. The shoots that come up can be taken as cuttings, which is how one tuber becomes five plants.
The evening-before warning
Pocket Botanist Premium sends a frost warning the evening before the forecast says freezing, naming the plants in your garden that are at risk, so the fleece goes on in time. The free app tells you your last and first frost dates from your hardiness zone. Free on iPhone and Android.
Sources
- Royal Horticultural Society, "Dahlias: growing guide." rhs.org.uk
- Royal Horticultural Society, "Frost damage." rhs.org.uk