Gardening Tips - how to over winter tender plants

How To Overwinter plants

There is information below about over wintering tender plants, whether in a greenhouse, lean to, conservatory or window sill. Wherever you place your plants for the winter there are two main pitfalls to watch out for. Over-watering is one keep plants dry . Plants need only a little water when dormant, water sparingly as too much wet will be fatal. Low light levels is the other problem, do not be tempted to overwinter in a shed or garage, the lack of light will mean the plant cannot thrive.

Overwintering plants on a trestle in a greenhouse

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

If overwintering plants under glass, the main problem is likely to be botrytis, grey mould particularly in a green house. Winter plants need good air circulation to prevent mould. During cold spells it is tempting to wrap up the plants and block off all the draughts, but this 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. 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) may need more protection than a greenhouse or lean to can offer, depending on the aspect of your garden.  If you are further north in the country, or have an exposed garden the more tender plants are best in an unheated conservatory 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 let the Pelargoniums carry on flowering in the conservatory, such a cheering sight in the winter gloom and I only cut back if they get leggy. In  the spring increase watering and start feeding.

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 depends on the minimum temperature in your greenhouse, ideally not below 5C. Another factor is the damp and mildew; to try 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.  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 may be 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 more so if the ground is well drained and a thick mulch is essential. Some borderline plants will survive in a sheltered garden provided they are not subject to too much winter wet. 

In cold areas, 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 invite 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 the 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.

Last updated 14.11.2020