How to delay the effects of climate change on our gardens

We shall not see a seismic shift in the types of plant we can grow in our gardens – and this is true wherever in the British Isles you live – unless and until there are no frosts.

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It is several years since you could last open a newspaper and not see it mentioned – on most if not all the pages.  However, for once, I do not mean the B-word but the two C-words. Yes, Climate Change now vies with Brexit for the amount of printer’s ink taken up on a single topic. But while both clearly impinge on most aspects of our lives, climate change has the edge in the garden.

In truth, folk have been asking me for a long time about what was once more usually called global warming, and the effects it will have on our choice of plants and gardening activities in general. It is 17 years since the Royal Horticultural Society first published a report on the topic called “Gardening in a Global Greenhouse”. It was updated in 2017 and makes for a thought-provoking read.

Moreover, the subject is especially relevant at this time of the year, in connection with something else that crops up repeatedly in my correspondence: namely, when I am asked what exactly is meant by terms such as hardy, half-hardy and tender.

They are all in truth moveable feasts, and the definition of relative hardiness has now become more refined with plants accorded grades from Tropical H1a (tolerate a minimum of 15°C) to Very Hardy H7 (tolerate a minimum of below -20°C).

But the most critical factor in all these designations and definitions is the likelihood of frost. I have said many times that we shall not see a seismic shift in the types of plant we can grow in our gardens – and this is true wherever in the British Isles you live – unless and until there are no frosts.

The impact of a minimum temperature above freezing for 364 days of the year will be undone instantly if on the 365th day there is a frost. This was brought home to me dramatically and frustratingly many years ago when I was undertaking research on a disease of larch trees. I wished to test a notion that suggested the disease only arose after the trees suffered frost damage. After a prolonged search I found a plantation on a sheltered coastal South Devon estate where no frost had occurred for 20 years. I set up my experiment to test the theory and yes, of course, that year a frost occurred – rendering my carefully planned trial meaningless.

It is not always realised that many of the plants we grow in our summer gardens as annuals are in reality perennial in their native lands. Pelargoniums, among the most popular summer bedding plants in Britain, are good examples. They originate in South Africa but if I leave mine in my Warwickshire garden all year round, they will be wiped out by the first significant autumn frost. By contrast, I see them surviving every month of the year in sheltered window boxes in inner London, where frost is effectively non-existent.

My message then is always to err on the side of caution. Be prepared to restock every year if you know your plants are frost susceptible – albeit perennial. Alternatively, grow them in pots and move them under cover for the winter – to a greenhouse or conservatory, for example. 

Despite what politicians tell us, it will surely be impossible to stop climate change: but collectively we should be able to delay it or, on a personal gardening level, mitigate its effects. Which may all be a deal more than you can say for Brexit. 

Next week: Nina Caplan on drink

This article appears in the 29 November 2019 issue of the New Statesman, The English Question