It’s not surprising that one of the things we consistently want to understand about a plant is its drought tolerance.
But it’s not at all clear what we mean by that. The default position is to evaluate a plant on the extent or severity of drought that it will survive. And that’s important. But I want more. I want to know to what extent the drought will effect the plant’s ornamental performance – not just threaten its survival.
I guess that’s because I’m more or less likely to experience extended dry in any normal summer, and, with my current water supply, being tank only, I simply can’t water the garden. So a garden full of tough but resentful drought survivors isn’t going to do it for me. What I need are plants that will perform to somewhere acceptably near their peak despite the dry.
Curiously the extent and duration of cold over winter has to be factored into the equation. If I lived on the Cote d’Azur, or the Italian Riviera, I might be perfectly happy to have a summer garden full of semi-indignant drought survivors, as winter is so benign that it can be flower-loaded, and you can just swap the traditional downtime of an English winter for summer instead, enjoying a verdant and floriferous autumn/winter/spring cycle instead of a spring/summer/autumn cycle.
But where I live the winter is long, with frosts from April to November (and sometimes the summer months as well), so the winter downtime is forced on me. I refuse to accept a summer downtime as well.
So let’s just build into the particularities of my climate that a ‘quiet’ summer is not an option. It may well be the best option for you.
Eenough of the general talk. Let’s look at specific plant examples.
I have one bed in which I’ve repeatedly used Aster x frikartii ‘Monch’ and Penstemon ‘Blackbird’ (see first pic). Both are reasonably drought tolerant. But there comes a point, and my garden crosses it most summers, when they get so dry that they stop flowering, or don’t even start. They’re almost useless to me, and it beats me why they’re still here. It makes no sense to rely heavily on plants that might only perform properly once every five or ten years. Both are very drought tolerant. They survive. But they don’t perform, and that’s what they’re there to do. It might make sense to give a little bit of space to the odd favourite that follows this pattern, but only a very little bit indeed.
Then there’s those plants that rarely fail to perform due to dry, or whose performance is only slightly tainted by it. Sedum ‘Autumn Joy’, ‘Matrona’, and most of the telephium types perform well to very well in anything my climate can dish up to them. Admittedly the foliage can, at times, look a bit pinched. The purple blush on ‘Matrona’ can take on a look of cadaverous bruising during the worst of summers, and the flowering may be thinner than optimal, but it never fails to put on a worthwhile show. My Clematis maximowicziana has only once failed to produce a huge explosion of almond-scented bloom in late summer due to severe and extended drought and heat.
And then there’s those plants that have never failed to perform to the full extent of their capabilities despite the dry, such as Calamagrostis ‘Karl Foerster’, or Cotinus ‘Grace’, or (now that I’ve swung over into shrubs), English box. The latter clearly isn’t based on a floral evaluation, as all the others above have been, but the point is that it never fails to do less than what I expect of it, no matter what the summer serves up.
What I’d love, in a perfect world, is a quotient that indicated what proportion of full performance to expect from a plant during extended summer dry. Calamagrostis – 100%. Penstemon ‘Blackbird’ – 30%. My wife’s David Austin roses near the clothesline – about 5%
How useful would that be!