Tolerance v Preference

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.  

Rosa mutabilis. You almost couldn’t kill it with drought, but it will seriously underperform in 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.

William Waldorf Astor’s garden, designed by Harold Peto, at Villa Tritone, Sorrento, Italy. If I gardened in a frost-free spot like this, I’d happily resign myself to a quiet summer and concentrate my floral efforts in the cooler months. Where I live, that’s not an option

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.

Aster x frikartii ‘Monch’ (the mauve in the middle) survives in my garden with no water, but only flowers well in the occasional stress-free year. Sedum ‘Matrona’ (the pink thing) is only compromised in the very driest years, and then hardly at all. Calamagrostis ‘Karl Foerster’ (the bolt upright grass) never fails to deliver its optimum performance, no matter how challenging the summer

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.  

Clematis maximowicziana, draping itself over Buddleja

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!

Discussion

  1. If the end of the millenium drought in 2009 taught us nothing else it showed a) what stood out as unshakeable (Sedum, Artemisia, Agapanthus if not absolutely exposed to the western sun), and what recovered with phenomenal speed (almost all my mediterranean bulbs and shrubs, Syringa, grasses like Miscanthus, Aster,etc). Unfortunately most of that 2nd group were devastated for weeks before the rain arrived, so your distiction between surviving and thriving is totally on the money. Most summers we can take the edges off this blighting with a bit of supplementary water, but why not focus on what can do it on its own?

    1. Or simply allocate more space to those plants that perform acceptably in the dry, and less to those that don’t? As for your bit of supplementary water, I’d love to have given space to a further category in this piece – those things for which a single deep watering, or maybe two, can make a vast difference to performance. My Clematis maximowicziana is a great example. I’m convinced that in a very dry year, a single deep watering in late Jan/early Feb would be enough to get it over the hump, and result in good flowering in March. A single good watering in Feb can also trigger the Zephyranthes to bloom, without which it may not even appear in leaf

  2. I have a large garden near Paris and the last three summers have been terribly dry. Three months without any rain worth talking about. I have decided to try some of the easier Mediterranean plants OlivierFilippi has written two very good books on gardening in a dry climate. I do not know if they have been translated. He has a beautiful nursery near Montpellier in the south west. I can give you the address if you are passing through !!!!
    Thank you for your web site. It is a pleasure to read By the way, William Waterfield ( Le clos du Peyronnet in Menton) died recently Quid the garden?
    Judith Fumex ( guide potager St Jean de Beauregard)

    1. Yep – they’ve been translated. I have both ‘Bringing the Mediterranean into your Garden’, and ‘Planting Design for Dry Gardens’ – both amongst my top picks of books from the last ten years. I love that he not only makes you aware of plants appropriate to dry climates, but ways/modes/models of how to use them, which has been very hard to come by.
      I was aware of, and saddened by, the death of William Waterfield. My best gardening buddy used to garden and stay with him back in the early 90’s, and I’ve forever wished to be able to download his huge database of dry-gardening wisdom direct to my brain. What a loss.
      Great to hear from you, Judith. Was it you that showed me and my group around the house and potager at St Jean de Beauregard in 2015, I wonder?

  3. I did my hort studies toward the end of the millennium drought here in temperate Australia and despite our teachers always reminding us how lucky we were to be studying horticulture during such drouthy times, because it would make us better gardeners, it certainly didn’t feel like that at the time. It felt like the worst time to be studying plant husbandry, I certainly remember it being like that for me anyway. It wasn’t until years later that I ‘got it’.

    Now though, I’m constantly looking for plants that will not only merely survive but, as you say, perform in the dry. If drought tolerance means survivability, then that’s an appallingly low bar to set when choosing plants. I love the idea of a scale of rating plants for ‘drought performance,’ it would convey much more information than the ubiquitous line on plant labels ‘drought tolerant once established,’ which conveys bugger all information that I find, quite frankly, redundant. The phrase essentially means nothing.

    The search for performers in the dry has challenged many of my plant prejudices though, big time. I use some dwarf agapanthus cultivars a lot these days, but it took me a long time to accept them as useful plants, despite my continued aversion to them. They are solid performers, without a doubt. But in the right context they reliably work, whatever summer throws at them.

    That aspect of doing artistry with plants here in Australia is a constant source of enjoyment for me – what plants do I hate that actually perform in our challenging conditions and how might I use them well? I guess it’s an advantage of a climate like ours; yones prejudices get sapped and eroded by the dry at the same time your gardens do. It forces you to rethink everything continually.

    1. ‘Drought tolerant once established’. Means nothing – you’re right. Like nearly all plant label info.

      I love the idea of fatiguing prejudices. And like yours, mine against dwarf agapanthus has been challenged. Twice in the last few weeks I’ve been driving, and spotted a great mass of bloom up ahead, and wondered what it could possibly be, in midsummer. Only right up close have a realised that I was looking at a mass planting of dwarf agapanthus, blooming in consistent height and consistent density. Neither made up even faintly imaginative planting, but I found myself conjuring up contexts for them. But I couldn’t help but wonder if this has been an exceptionally good year for them. In both locations, I must have driven past them before, at flowering time, without being at all impacted.

      We need to get James Hitchmough on to this. He’d immediately have the terminology, and the methodology, for a serious study into the decline in performance in response to the dry. I’m ready to launch into a Ph.D

  4. Absolutely spot on with this post – I want to know what will look good – not just survive. I’ve seen some pretty dodgy “drought tolerant” labels too!
    I’ve had a lot of success with Sedum Autumn Joy, Calmagrostis and Cotinus in the Adelaide Hills. Banksia rose, dwarf spirea, callistemon, artemisa, and agastache have all worked well.
    The standout however is Ceanothus Yankee Point – I created a hedge at the front of the house, planted in builders rubble. Might have watered it a couple of times in its first summer, and only rainfall since and it always produces fantastic blue flowers in October. Been going for nearly 10 years now. Everyone always asks what it is when it’s in flower and out of season it has glossy dark green leaves which are larger than normal for a ceanothus.

    1. Funny you should mention Ceanothus ‘Yankee Point’ Tracey. I got sick of mine last winter – they got too big for the spot I’d planted them (I’d planned to keep them clipped tight, but you know how it goes), and I’m not that keen on giving important space to stuff that flowers way earlier than all of the neighbouring plants – and I cut them right back to stumps. I mean seriously to stumps – stumps too low to then dig around and lever out. All three of them shot back from old wood in a way that I’d never expect a Ceanothus ‘Blue Pacific’ to do. They now low, and contained, and totally refreshed, and I ran the hedge trimmers over them just yesterday, to keep ’em tight.
      You’re right. They’re a brilliant plant for a rubbish spot. Probably less so for a really top spot like I’ve given mine

  5. Thanks Michael and all for this discussion. As Gary notes, 2009 felt like the year when we finally had to face reality about our plant selections. I’ve just done a bit of research on the BOM. In December, 2019, our area had 9 days of daytime temperatures over 35 and 3 ml of rain. And that was just the start of the 2019-20 summer. “Drought tolerant once established” is meant to indicate a plant has low-ish water needs. It doesn’t necessarily factor in the impact that 35 degree-plus days with a dessicating hot northerly wind over more than a few days can have on our summer gardens and their occupants. The star performers/thrivers in those conditions have been Calamagrostis KF, Salvia Nemerosa, Perovskia, Miscanthus Zebrinus and Sedum AJ and Matrona. In the meantime, this beautifully benign summer has brought unmitigated joy. So much lushness, so much intense colour all at once.

    1. Yeah, I was thinking of a follow up piece on the different effects of spring, summer and autumn dry. That November/December of 2019 was appalling. Apocalyptic. The relief when the rain started in early Jan! And it’s been relatively regular ever since. We swapped climate stress fo Covid. What a blessing there was no overlap. Not yet at least

    2. Thank you, Michael for your blog and this intelligent discussion. As a new gardener, I appreciate all the comments and have taken copious notes to aid in choosing plants that will perform in the dry.

    3. Fabulous! And don’t forget to add here what you learn along the way!

  6. What a great parallel discussion for the year just past, where lockdown was the deprivation and ‘drought’ conditions and most of us ‘survived’ but certainly couldn’t ‘thrive’. I feel like I was seriously underperforming last year!
    This wet, humid summer has messed with my assumptions about what to expect from plants entirely. Living in Melbourne’s sand belt, C. Karl Foerster has finally delivered better flowering this year (rain? division? sheer luck?) and its partner, Sedum ‘Autumn Joy’ looks horrendous for the first time ever – blotchy fungal looking leaves and sparse flowering and you can see its knees – not plump domes of foliage. So disappointing not to get both singing at the same time.
    And I know, soils aint soils – I’m sure many things are not giving their full floral potential because they’re stuck in my poor, hydrophobic sandy soil versus something more water retentive. Not helped by my lack of soil improvement and mulching – all the things I used to preach to others! So it’s never a level playing field is it? So many variables deliver such different results!

    1. I certainly underperformed last year. I’ve decided I’m like a Covid-immobilised plane that’s been sitting too long on the tarmac – not sure what problems might emerge when I attempt to go back into service.
      Yeah, I’ve never been able to explain those odd bad years for the taller sedums. But they happen. When everything else is looking good

  7. Had thought the small agapanthus needed water to look good. They would have been useful in the reinstated landscape (1.9k St Georges Rd median), but as you say perhaps this past year has been kind. There is a PhD in the performance of plants in St Georges Rd median, with Celtis australis surviving the Millenium drought, ( selected by James Hitchmough for thé site constraints), but not so many of the understorey plants from reinstatement in one section in 1997 have survived. Watching the new section (1.9k) and particularly the understorey planting – prostrate rosemary is pretty good.

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