Gardening Tips - how to over winter tender bedding plants

How To Overwinter plants

Overwintering plants on a trestle in a greenhouse

Overwintering plants is a good way of keeping tender, and slightly less than hardy plants, from year to year. Which plants you can overwinter depends really on available frost free space. 

If overwintering plants under glass the main problem is likely to be botrytis, grey mould particularly in a green house. In the winter plants need air and the greenhouse needs air to circulate to prevent mould. To protect against cold it is tempting to wrap up the plants and block off all the draughts which will create just the still damp conditions mould enjoys.

To reduce the risk of mould, space plants well apart, avoid their leaves touching and prune if necessary. Water sparingly, open vents and doors as much as possible and raise plants off the ground on a trestle, see image below. At the first signs of mould, which forms on leaves as grey fuzzy powder which collapse to the touch remove the plant and any leaves to stop it spreading.

Overwintering in a conservatory

pelargoniums overwintering in conservatory

Really tender plants, lifted from summer bedding such as Fuchsias, Pelargoniums (also known as Geraniums) are best in an unheated conservatory  which is ideal and where they may continue flowering. Conservatories, even when unheated, are warmer than a greenhouse, and less damp which keeps at bay mildew and mould which is the problem when over wintering in the greenhouse.

 Later in the season, say November, you can if you wish, cut back the plants by about a half or more. it is essential over winter to reduce watering so it is just sparing. Personally, I tend to let the Pelargoniums carry on in the conservatory and only cut back if they get leggy although reducing watering is essential.

Overwintering in a greenhouse

Overwintering in the greenhouse is colder, although tender plants can be nursed through a winter they need to be frost free  so it depend on the minimum temperature in your greenhouse, ideally not below 5C. Another factor is the damp and mildew; totry and reduce this it is a good idea to raise plants off the ground, see image above left. This reduces the impact of cold from the ground, and helps to increase ventilation which is vital to the plant's survival over the winter months. Damp and mildew can bea problem in the green house, ventilation helps. Use a few bricks and planks to raise the plants off the ground. If overwintering in the greenhouse it is better to cut the plants back to 10cms and very sparse watering.

Overwintering in a porch or windowsill

You can also overwinter plants in a bright porch, or on window sill. It is much more difficult in a garage because of the low light levels, although a garage maybe suitable for storing Dahlia tubers. Plants on an inside windowsill, which is warmer, will often carry on growing and so require more watering and regular turning because the light coming from only one side.  If the plant is not turned it will grow unevenly towards the light. In a porch it is likely to be cooler so water less and you may need a fleece in prolonged cold spells.

Some plants can be over wintered in the ground especially if you are in a sheltered area, such as Dahlia, Salvia, Agapanthus especially if the ground is well drained and a thick mulch is essential.

Dahlias  should be lifted later in the year after the first frost blackens the foliage is the ideal time. To do this dig up and lift, shake off all soil. Tip upside down and drain until they are very dry-any dampness will cause mould and they will not survive the winter.  Store in dry, frost free place in trays of old potting compost and hope. Check every month for signs of mould or rot and remove any affected.

Other tender bedding you can pot up into greenhouse or under glass and cut back to about half water sparingly and see what the spring brings. 

Semi tender such as Agapanthus, Canna Lillies may survive outside if covered with a heavy  mulch and provided they are in a reasonably dry spot they will survive in many areas, if in doubt pot into containers and overwinter in the greenhouse. Some Agapanthus are more tender than others, check growing Agapanthus

You can also seed in early autumn and over winter hardy annuals in the greenhouse to grow on in the spring.