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RHS winter plant damage survey – findings released

What died, what thrived and other surprises at the five RHS Gardens

The last year has tested many UK garden plants to their limits. A searing 40C summer, followed by a plunge to below -10C in early December, has provided a window into the effects that an increasingly volatile climate may have on our garden plants – and already, we can see both trends and surprises emerging. 

Navigating the challenges of gardening in a changing climate is a key research focus for the RHS, and our five Gardens, as well as observations sent in by gardeners at home such as for our Extreme Heat Survey, are proving valuable sources of information on how different plants respond to extreme weather events.

To help the RHS record plant performance in our changing climate, RHS Director of Horticulture, Education & Communities, Tim Upson, has gathered information from across all five RHS Gardens on how plants have coped with this winter's extreme cold. This enables us to identify patterns of what has suffered, what has pulled through, and what copes well with both extremes of temperature that we have seen over the last year.


A cold-damaged Phormium
Why was this winter so damaging?

Much of the damage we've seen after this brutal winter is likely to have resulted from the lack of acclimation. This is the process by which plants adjust to a change in their environment, such as winter cold. This year saw a prolonged wet and mild autumn and an early winter, with a sudden plunge to temperatures well below freezing.

This appears to be have been particularly extreme in the south and south east, with RHS Wisley, in Surrey, particularly badly affected. Here there were several consecutive days of sub-zero temperatures, with the garden reaching -10C and perhaps -12C in frost pockets.

The effect of microclimates has also been clear, with plants in known frost pockets most badly affected and often killed outright. Plants in milder microclimates have still suffered, but have seen better survival rates.

A particularly striking trend is the impact on plants grown in containers. It is known from previous studies that container-grown plants are more susceptible to extreme temperatures, and this has certainly played out at the RHS Gardens this winter, with many containerised plants lost while their counterparts planted in the ground have survived.

Much of the damage was due to rapid change from mild autumnal weather to immediately low temperatures for several days, with no chance for plants to acclimatise

- Tim Upson, Director of Horticulture Education & Communities


What was lost

Australian and New Zealand plants such as Hebe, Phormium, Corokia and Pittosporum (the latter two were being trialled as box alternatives) were badly hit across all five sites. Bulbs in unprotected pots also proved particularly vulnerable, with nerines, amarines, and newly planted hyacinths and daffodils lost, while those planted in the ground seem unaffected. These losses are thought to be due to cold damage to the bulb root plates.

  • Bulbs in pots – nerines, amarines, and newly planted hyacinths and daffodils died, while those planted in the ground appear unaffected
  • Shrubby hebe – whole trial devastated at Wisley and almost completely dead. Many badly scorched at Rosemoor (some green shoots noted in March). Those at Harlow Carr hardly affected – some just with light leaf scorch
  • Phormium – badly impacted with many plants killed
  • Pittosporum – many of the coloured foliage variants and cultivars such as ‘Golf Ball’ very badly affected at both Wisley at Rosemoor
  • Echium – E. pininana and E. candicans all seemed killed
  • Euphorbia – E. x pasteurii and E. stygiana dead or badly damaged
  • Cordyline australis – many rosettes killed. Yet to see if stems and trunks resprout
  • Daphne, especially D. bholua – those in open areas seem badly hit, some likely killed or at least badly defoliated
  • Rosemary – prostrate forms widely scorched or killed outright, while upright forms less affected and many undamaged
  • Lavenders – French and L. dentata widely killed. L. angustifolia and L. x intermedia fine in the ground, but some killed in pots
  • Acacia – A. melanoxylon, A. dealbata and variants killed at Wisley
  • Agave – A. americana killed outright at Wisley after several years outside. Some A. ovalifolium survived while others were killed, likely due to immediate microclimate
  • Corokia – most killed at Wisley
     

Several box alternatives being trialled at Wisley, such as New Zealand natives Pittosporum and Corokia, succumbed to the winter's extreme cold. Image: Olivia Drake

Unexpected survivors

In some instances, horticulturists were surprised by some tender plants that appear to have pulled through the extreme cold. Planting location has a big effect on a plant's vulnerability, with microclimates created by features such as walls and surrounding vegetation having a significant effect on the severity of conditions a plant experiences and therefore its chances of survival.

While large swathes of rosemary were killed, ‘Foxtail’ looks brand new

- Guy Barter, RHS Chief Horticulturist

  • Rosemary ‘Foxtail’ – while prostrate rosemaries were widely killed, this cultivar fared better
  • Agave parryi – unaffected and seem totally hardy
  • Veronica pimeleoides ‘Quicksilver’ and Veronica odora ‘New Zealand Gold’ – the only hebes to have survived in the trial at Wisley
  • Hebe parviflora – survived well
  • Acacia pravissima – tree planted in the ground at Wisley survived and was in full bloom in April, while a good-sized pot specimen in a garden 5 miles away was killed outright
  • Podocarpus – still looking great in Wisley walled garden (-10C) while many other box alternatives were killed. The best performers are currrently P. nivalis and P. acutifolius and their cultivars
  • Yuccas –​Y. rostrata, Y. torreyi, Y. gloriosa, Y. schotii, Y. filamentosa all okay at Wisley
  • Grevillea – mostly seem fine
  • Eucryphia – generally seem okay
  • Dasylirion acrotichum
  • Caesalpinia gilletii – surprisingly looks to have survived in a very sheltered, warm position at Wisley

Podocarpus is a New Zealand native that has triumphed over the extreme conditions, and is currently a front-runner in the Wisley box alternatives trial. Pictured is Podocarpus 'Young Rusty'

What's thrived through both extremes

Camellias coped incredibly well in the drought conditions, and have gone through and set flower beautifully because of that hot spell

- Peter Jones, Garden Manager, RHS Garden Wisley

Certain plants surprised the horticultural team by triumphing over last summer's extreme heat, and then going on to produce an impressive display even after bitter winter conditions. This has been particularly noticeable for hardy spring-flowering trees and shrubs, which have been unscathed by the winter cold, pulled through last summer's drought and made the most of the heat to promote wood ripening and flower bud set.

  • Magnolias – excellent flowering after hot summer with no cold damage to petals
  • Camellias – excellent flowering after hot summer despite extreme cold 
  • Cherries – excellent flowering after hot summer despite extreme cold 
  • Wisteria – covered in buds after hot summer
  • Rosemary ‘Foxtail’ – thrives in heat and drought and withstood extreme cold, unlike most prostrate cultivars
  • Agave parryi – thrives in heat and drought and withstood extreme cold, unlike most species

It's been a fantastic spring for flowering trees and shrubs such as magnolias, cherries, camellias and wisteria
Last year's hot summer is thought to have promoted good wood ripening and flower bud set

Wisterias are doing better and better in the heat. They’re a plant for the future

- Matt Pottage, Curator, RHS Garden Wisley


How to tell if a plant is really dead

Sometimes, a woody plant may not be killed all the way down to the ground, in which case there is hope of regrowth. To check this, gently scrape the

bark or outer layer lower down the plant with a fingernail or secateur blade. If this reveals green underneath the top layer, then this part of the plant is still alive and there is hope of regrowth. This may be encouraged by cutting back to the live parts once all risk of frost has passed.

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