Baby it’s cold outside

Our winter survival guide for your plants

Sat, Nov 30, 2013, 01:00

Sheesh, it’s been cold . . . or at least, in my garden it’s been cold. Cold enough for frost, ice and even snow, if only a barely there dusting of white.

Cold enough that the soft foliage of tender and half-hardy plants which were in flower only weeks ago has now been blackened, while I’ve taken to wearing a double layer of gloves while I finish planting the last of the spring bulbs. But then my garden – a frost pocket at the bottom of a damp hillside and at an altitude of 230m, is colder than most – while I, to my vague embarrassment (the firmly held view is gardeners should be a hardy lot), feel the cold more than some.

It is consoling to know that the same can said of many plants. Take the exceptionally cold winters of 2009 and 2010, when traditional garden favourites such as phlox, oriental poppies and hardy geraniums proved their ability to withstand temperatures as low as minus 17 degrees. Others didn’t, including many specimens of ceanothus, tree ferns, large-leaved hebes, bay laurel, fatsia, rosemary, phormium, cordyline, cistus, olearia and pittosporum.

Why so? The answer is that a plant’s hardiness depends on a vast and sometimes baffling range of factors, chief among which is its natural habitat.

Along with native plants, our gardens are filled with a rich variety of flora that originated in faraway places – in Europe, Asia, Australia, North and South America – and which evolved under a great diversity of growing conditions that influence their coping mechanisms when it comes to withstanding the stresses of extreme cold, heat, drought or flooding.

One example is the aforementioned choisya ternata, or Mexican orange blossom, that evergreen, flowering shrub so popular with Irish gardeners but one which was badly hit by those two recent harsh winters. It is not native to Ireland but to Mexico, where it grows on chalky soils in coastal areas.

As for the lovely, large-leafed fatsia Japonica that’s a feature of many smart town gardens? As suggested by its species name, it is native to Japan.

And the scarlet-pink fuchsia hedges of west Cork and Kerry? That is fuchsia magellanica, a species native to temperate regions of Chile and Argentina,.

Given such a great diversity of garden- worthy plants, horticulturists have come up with various systems that help us to gauge their individual hardiness, including the American USDA system and the freshly revised RHS system, the latter of which now rates individual plants on a scale of one to seven

At the most tender end of the RHS scale are tropical greenhouse plants (H1a) that require heat all year round, while at the other end (H7) are the many ultra-hardy plants that should survive the coldest of Irish gardens and the harshest of winters, to temperatures as low as at least minus 20 degrees.

That Mexican orange blossom bush I mentioned earlier comes with an RHS rating of H4, meaning it is hardy down to between minus 5 to minus 10 degrees but will suffer foliage die- back or stem damage in a cold garden in a harsh winter.

So, if you’ve failed to grow it successfully, then concentrate on plants that come with a H5, H6 or H7 rating, of which there are many.

But plant hardiness is far more than just numbers on a thermometer. It is also influenced by many other variables, from your garden’s soil conditions, exposure, altitude, average annual rainfall amounts, orientation, its slope/incline and the position of surrounding trees, hedges, buildings and hard surfaces to – just as importantly – the cultivation techniques you employ.

True success comes with the ability to cleverly manipulate that micro-climate to your benefit, by choosing or creating the right spot for the right plant., whether that’s a warm, sunny wall for the sun-loving trachelospermum jasminoides or a cool, shady one for the leafy hydrangea petiolaris.

The same goes for that Mexican orange blossom bush. If planted in a protected spot and given fertile, free-draining well-prepared soil, it has a much better chance of surviving an exceptionally cold spell than one grown in poorly prepared, cold, wet ground where it takes the full brunt of icy gales.

If you don’t have the right spot for a particular plant, there are ways to create it. In cold, exposed gardens, use windbreak netting or plant shelterbelts in the form of hedges and trees.

If your soil is heavy or poorly drained (a significant factor in plant deaths), add plenty of grit. If it’s poor, dry and compacted, add manure/garden compost, while keeping in mind that soils that are of good heart (those that have been rarely dug but regularly mulched with organic matter so that they’ve retained a good structure) always foster plant hardiness.

In very cold spells, protect plants with horticultural fleece, especially those grown in containers whose vulnerable roots are exposed to colder temperatures than those the same plant would experience if grown in the ground. Don’t be afraid though to experiment and take the odd risk, because without that occasional frisson, gardening might become a dull thing.
For details of individual plant hardiness, see rhs.org.uk

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