Greenhouse Gardening – over-wintering part 2

Wintry conditions can chill container plants to the roots, causing ill health and injury that continues well into the subsequent spring, even if it doesn’t kill the plant outright. Greenhouse owners have a natural advantage with a sheltered location and can benefit from using their greenhouse space to over-winter container grown plants, because plants, and even tree, when growing in the ground have something called geothermic heat to keep their roots safe – this means the soil acts as an insulator. For container plants, the pot actually acts as a barrier against which the cold rests on one side and the roots rest on the other and all the geothermic heat is lost in the surrounding air. As a result, roots of container plants are subjected to ambient air temperature, which can kill even a mature plant from the roots outwards.

So it’s important, when preparing plants for the winter, ensure they have been given a chance to reach maximum root hardiness. Do this by waiting for plants to become dormant before covering them. Always avoid covering wet plants which rots foliage. It’s claimed that square containers provide better insulation, although there isn’t any research to substantiate this as yet.

Cannas and Dahlias

Cannas and dahlias are lifted after the foliage has been blackened by frost. Remove dead leaves and loose soil and store in trays of sand, coir or vermiculite, with the crown just showing. Keep just moist in a cold, frost-free greenhouse or conservatory. Container-grown plants can be stored in their pots.

Fuchsias and Bonsai

Mature fuchsias need to be kept frost-free during the winter. Remove extra foliage this reduces demands on the plant and doesn’t allow pests a chance to breed on them. It also means you can put the plants closer together, and you can see the compost surface better, helping you judge the need for watering. Prune plants back by about a third, this is usually a couple of leaf joints above the last stop. Spray or dip plants using a combined fungicide/insecticide mix. Place in cold greenhouse Bonsai need to be kept cool but dry over-winter – don’t prune until spring but make sure that roots in shallow pots or special bonsai containers are not in any way exposed to frost, or even freezing wind when the greenhouse door is open.

Greenhouse gardening canna photograph by twoblueday, used under a creative commons attribution licence