Hardiness - An Update

The concept of plant hardiness is not quite as straightforward as many believe. Bob Brown, from Cotswold Hardy Plants, guides you through the many factors that can affect plant survival during the winter months. - 24 August 2018

The public largely imagine hardiness is all or nothing.  Thus, in their imagination the thermometer drops to 0C (freezing) and the tender plants are reduced to mush.  In the world of tender bedding this may be true.  The nasturtiums that have been happy and even flowering during November with temperatures in single figures, dissolve to slime when, crash, there’s a real drop below 0C.  This kind of reaction is exceptional.

Lately Eastern Worcestershire has experienced some record low temperatures – officially about -20C at the met station on the side of the hill but as low as -26C at the bottom.  I can vouch that perennials and shrubs planted out on my nursery finally die at all kinds of temperatures; Puya spathacea at -10C, Pileostegia viburnoides  at -15C, Viburnum tinus ‘Gwenllian’ at -17C, Agapanthus caulescens at -20C, Agapanthus ‘Navy Blue’ and many others including the spectacular winter-flowering Moroccan Ranunculus calandrinoides presumably die somewhere out of sight below  -26C.  I conclude that hardiness is not an all or nothing quality.

Puya spathacea

It gets more complicated.  New modern Echinacea hybrids bred from species native to the Midwest of the USA are used to temperatures so low that freeze-drying operates and the activities of pathogenic bacteria and fungus are suspended.  Currently the British public watch them disappear overwinter and (rightly) complain.  During recent very cold winters I can report that they mostly lived.  My theory is that warmish wet British winters allow pathogens to rot the crowns and conclude that new Echinacea hybrids die when it isn’t cold enough.

Echinaceas survive cold, dry winters, but can struggle in mild, wet ones unless in well drained soil

Americans have known that their plants die at different low temperatures all their gardening lives and the US Department of Agriculture issue a zoning map based on average minimum temperatures.  Plants tend therefore, to come with zoning attached.  “Farfugium japonicum ‘Argenteum’ Z(zone)8a-10b” i.e. It’ll thrive planted out in places zoned between 8a and 10b where the average minima lie between -12.2C and +1.7C.  How sensible!  It beats my amateurish attempts to qualify the limits; “bone hardy”, “hardyish”, “needs the protection of a wall” or “tenderish” and so on.  The trouble is that American hardiness zoning doesn’t translate to Britain.  A plant like this Farfugium that’ll thrive to -12C in Virginia dies at -5C in Ely.  This is probably explained by the contrast between warm ripening summers and physiologically dry winters in USA and cold wet summers and cool wet winters in Britain.

Farfugium japonicum 'Argenteum'

In 2012 the RHS introduced a new British hardiness zoning system H1 to H7.  H1 is hot-house conditions, H7 is very hardy (below -20C).  0C is included in both H3 and H4.  Hooray!  A solution to the problem is in sight.  I’ve offered my four penn’orth and recorded H zones for plants I’ve killed and others are doing the same.  Eventually a consensus opinion about a plant’s hardiness will be agreed.  H numbers have started to appear in print.  They’re even on labels.

I grow, have grown, kill and have killed an enormous number of plants in a part of Britain currently experiencing record low temperatures.  I can say that Ranunculus calandrinoides even survives -26C.  Others who have only had -14C or -9C can only vouch for its hardiness down to their minima.  The places in Morocco where it grows do experience winter weather worse than ours. 

Ranunculus calandrinoides

I remember a trip in late February/early March when it snowed continuously between the Rif and sea level on the Mediterranean coast. At sea level no snow settled.  At higher altitudes it did settle.  I imagine that summer in the Rif is probably warmer and drier than our summer (I find this an easy concept to visualise).  Ranunculus calandrinoides ought to rot to death whilst it’s dormant in British summers.  However, it’s planted under a deciduous hawthorn tree which tends to dry the soil whilst it’s growing during the summer so I can’t do any more than suggest that maybe it’s summer hardy as well.  When I said the concept of hardiness is complicated I meant it!

To view the full table of RHS Hardiness ratings, please click here

Bob Brown owns Cotswold Garden Flowers and is a renowned plantsman, speaker and garden writer. The specialist nursery, based in Evesham, stocks a huge range of unusual perennials, with a focus on good old-fashioned plants, newly introduced plants bred for not only colour and form but for vigour as well, and plants newly introduced from the wild.  

Website: www.cgf.net