Keep your tender plants safe
Tender perennials are becoming increasingly popular for adding an exotic touch to beds and borders. Young plants have also become a common sight in garden centres - sold alongside traditional annual summer bedding for use in hanging baskets and containers. They are easy to grow, flower profusely and most are drought tolerant - making them ideal choices for beginners and experienced gardeners alike. Unfortunately, being tender, or at least less than fully hardy, they are liable to die if left outside over the winter months. It’s often not so much the cold that does them in, but the winter wet. As a result, many people just accept them as one-year wonders and buy new each spring. This needn’t be the case, since most can be overwintered successfully with a little help….and in these days of austerity it’s an easy way to save a lot of money.
Tools for the job
Priority plants to overwinter
How to overwinter your plants
The best way to overwinter tender plants varies from plant to plant. Some, such as Cosmos, Fuchsia and Penstemon, can be overwintered where they’re growing in the border by giving them a little extra protection, while others, including begonia, canna, Pelargonium and marguerite do best if dug up and placed in an unheated room. If you grow a lot of tender perennials, it may not be possible to overwinter them all, so prioritise those that are expensive and difficult to replace, as well as those that are more likely to survive. It is also a good idea to take cuttings of your favourite plants as insurance against winter losses (see below).
In the border If your soil is reasonably well drained and the border not in a frost pocket, then you should be able to overwinter a range of the toughest ‘tender’ perennials outside in most years. Penstemon, for example, may die right back in autumn. If they do, they can have their crowns protected with an insulating layer to keep out the worst of the cold. A 15cm-deep pile of autumn leaves held in place with netting should do the trick. In milder winters and sheltered gardens they will keep all their foliage, and this can be cut back to within a few inches of the ground the following spring. A traditional technique for overwintering Pelargonium and fuchsias was to dig a trench in a dry border and bury the cut-back crowns just below the surface. I wouldn't guarantee its success!
Coldframe or unheated greenhouse
With the better protection afforded by a well-insulated, unheated greenhouse or coldframe, you can overwinter a wider range of plants including canna, helichrysum and verbena. You can double the protection by placing a coldframe inside the greenhouse. Box-up the cut-back plants as they start to go dormant, then pack them tightly together in deep trays, filling gaps between the rootballs with compost. Water only very occasionally to keep the roots and crowns alive, but not enough to make them sprout. The idea is to just keep them ticking over with good light, cool but frost-free temperatures and just enough water to keep them alive. The Victorians used to wrap their plants in newspaper and stick them under the bed. The plants may have looked dead when they unwrapped them in spring, but once potted up, there was enough life in the crowns for dormant buds to sprout. Sometimes!
Frost-free room With a well-lit spot indoors, such as a spare room, frost-free conservatory or porch, you can keep plants ticking over throughout the winter. This is a good way to overwinter larger specimens you don’t want to cut back as well as trained forms, such as standard Fuchsias. Keep a eye on the temperature to make sure it does not drop below freezing on the coldest winter nights.
Take cuttings as insurance If you are short of space, or your garden is particularly cold or wet during the winter months, taking cuttings can be an excellent backup if other methods fail. For me, this is the best solution of all. Cuttings take up relatively little space and most tender perennials strike easily from cuttings taken during late summer and early autumn. You can keep your insurance policy safe on the kitchen windowsill, or any other well-lit, frost-free spot. Once rooted, keep them cool and just moist, ready to be potted up in the spring.
