It’s an emergency. One by one, the toughest garden plants of the past decades are succumbing to newly-discovered, debilitating and often deadly pests and disease. Elm, ash, buxus, roses, horse chestnut, agapanthus, impatiens, clivea, lilly pilly, loropetalum…the list goes on. If you haven’t heard of problems with these plants in your part of the world already, you soon will, as globalisation eventually defeats even the most vigilant biosecurity measures.
Every time we plant a hedge, a single-species border or a sweep of identical perennials, we spread and encourage either existing or future pestilence and, in doing so, eventually remove that plant forever from our gardens, our streets, our natural landscapes and eventually our entire country.
“I think we do mass planting because we’re not clever enough to do anything else”
Well it’s high noon, and high time we got much, much cleverer.
A history lesson
During my landscape design training years in the early 1990s, there was a strong reaction to the 1980s Decade of the Cottage Garden. Mass planting was now the key to good design, with grouped swathes or hedges of a single plant to provide ‘unity’ and ‘harmony’ in peaceful and beautiful gardens. And it’s still being promoted and preferred as the perfect, quick and easy solution for any garden, by garden design magazines and websites, by nurseries, by gardeners thinking it’s the key to that elusive ‘low maintenance’ garden, and by unimaginative garden designers.
Since starting GardenDrum back in late 2011, I’ve become increasingly aware of this creeping, insidious but largely unrecognised problem with our design and planting choices. Although I garden in south-eastern Australia, I read widely about what’s happening in the USA, UK, Europe, New Zealand, South Africa and South-east Asia.
And the news is really bad.
In all of those countries, plants that have become highly popular for mass planting over the past few decades for their adaptability, cheap and simple propagation, easy-to-grow habit and strong pest and disease resistance are now succumbing to new problems that are, in some cases, making them completely ungrowable in several countries.
Why is this happening? Why are these previously reliable plants now attracting so many persistent pest and disease predators?
First, the pathogens that are attacking these mass-planting favourites aren’t usually unknown but when we start to supply them with unlimited food all conveniently co-located, it’s akin to erecting an ‘ALL YOU CAN EAT’ neon sign over your garden. Any pest or disease that can attack that plant, if it finds it, will have so much food that it will multiply and spread rapidly. The plant species affected doesn’t have time (ie enough seed-grown generations) to develop resistance. This mass-planting encouragement of pests and diseases is of course, well-known in agricultural food crops, with wheat rusts and potato blight responsible for many famines through history, as a new and more virulent strains of fungal diseases, bacteria or viruses defeat their host plants.
Second, the vast majority of ornamental plants being grown today are clones of a very limited number of parent plants. Vast production nurseries grow tens of thousands of one plant, and often they are all clones of one original parent plant that had the chance mutation which gave us that sought-after new and unique flower colour or form. Today it’s the only economical way to run a plant production nursery, but whether it’s propagation by tissue culture, division or cuttings, the genes of the parent plant are repeated in thousands and thousands of offspring, greatly restricting the genetic diversity of our ornamental garden plants.
I suspect this is an especially critical factor in the rise of deadly fungal diseases, which usually develop in an ‘arm’s race’ style with a single species. The fungus attacks the plant; some plants of the species, because of their genetic diversity, have or develop resistance and survive, the fungus adapts and again debilitates the plant…and so on. But when so many host plants are co-located, and they are likely to be clones without the genetic variation needed for adaptations to arise, they are unable to develop resistance and the fungus increases to the point where it wipes them out.
Third, we transport plants great distances both within our own countries and internationally, carrying pathogens and pests in cut flowers, live plants, seeds, soil and mulch. Although most countries have vigilant biosecurity measures, it is inevitable that things slip through, as happened with myrtle rust entering Australia five or so years ago. I heard of suspicions that this fungal disease, which already threatens to send many Australian native plants extinct by the end of this century, came in on imported cut flowers (but that’s not a proven fact).
What plants are affected?
Potentially anything we mass-plant, whether it’s a tree, shrub or perennial and particularly plants that are grown from cloned vegetative material, which means cultivars rather than species plants. The first well-known ornamental plant mass extinction through pest or disease of which I’m aware was the elm in the mid-20th century. Elm trees had been mass planted as avenue trees all over Europe and the UK and when Dutch elm beetle spread Dutch Elm Disease, the result was fatal for the elms of Europe. There is now only a handful of isolated survivors in the UK, some elms in southern Australia and also New Zealand (although Dutch elm disease arrived there in 2008).
Many of the elms were replaced with horse chestnuts and ash trees, which are now starting to suffer the same fate.
Note that in my descriptions below I am not going to recommend how to control these pests and diseases because that’s not the point of this story.
Chalara dieback in Fraxinus – Ash tree
Chalara dieback of ash is now widespread in Europe and spreading rapidly through the UK. It is terribly sad to think that one of England’s most well-known folk songs ‘The Ash Grove‘ may soon be about an extinct tree as the UK’s 80 million ash trees gradually succumb. Chalara dieback is a fungal disease that probably spread to the UK from Europe via the importation of infected ash nursery stock. New growth wilts and blackens, cankers develop and the canopy gradually dies. There is no cure.
Bleeding canker and leaf miner of Aesculus hippocastanum – horse chestnut
Widely planted to replace elm trees, horse chestnut now struggles against several problems, including a disfiguring and debilitating leaf-mining moth and a number of bacterial pathogens. The first symptom of bleeding canker is yellowing foliage and leaf drop and then a sticky liquid starts oozing from the trunk. The canopy dies and eventually the tree also. Over half of the UK’s trees are now infected.
Box blight or boxwood blight and box tree moth caterpillar on Buxus – English box, boxwood
The renewed popularity of formal gardens in the 1990s saw the popularity of box soar. Soon every garden in Europe, the UK, USA, New Zealand and Australia had a box hedge, box topiary, or both, giving instant structure to a town garden throughout the year as, in a cold climate, box is ‘sempervirens‘ ie evergreen.
Box blight is a newly discovered species of fungus called Cylindrocladium pseudonaviculatum. It was first recorded in Europe in the early 1990s and appeared in New Zealand in 2002 and in the USA in October 2011 and has now spread throughout its eastern states. It primarily affects Buxus sempervirens ‘Suffruticosa’, the small edging box, causing leaf spotting, leaf drop and then whole branches to progressively die back, ruining the look of the hedge. Although it doesn’t (yet) appear to cause significant dieback in other box species like Buxus microphylla, research by the NCSU Extension found that other cultivars can harbour and spread the pathogen. Sarcococca and Pachysandra species are also susceptible.
Box blight rates No2 on the RHS list of disease reports in the UK and gardeners are no longer using box due to the likelihood of the disfiguring disease. It is not yet in Australia but I suspect it will arrive within a few more years. And if this can happen to Buxus sempervirens, then it can (and I predict will) happen Buxus microphyllla soon enough.
The newest woe for the mass-planted box throughout Europe is the box tree moth caterpillar, which comes from Asia and was first recorded in Germany in 2006 and the UK in 2008. It has no natural predators and birds seem to find it unpalatable. It can reproduce 3 times each year and survives very cold winters, so it is now doing serious damage to buxus plantings, as profiled by Helen Yemm in the UK Telegraph in September 2015. The caterpillars are voracious feeders and quickly cause large patches of the hedge to die back.
Paropsides calypso leaf-eating beetle on Syzygium australe and cultivars – Lilly Pilly
Lilly pilly, native to Australia, is now the go-to hedging plant all along the eastern coast. Countless cultivars have been developed and most of them are now resistant to the disfiguring pimple psyllid. But a few years ago, something started happening to lilly pillies, particularly Syzygium australe cultivars, and the leaves on many are now so badly eaten, the plants are permanently tattered and torn and become increasingly threadbare.
The culprit is a tiny leaf-eating beetle called Paropsides calypso, identified by Dr Chris Reid, entomologist and leaf beetle expert at the Australian Museum in Sydney – see more below**. (Note that this leaf beetle been incorrectly identified elsewhere as Calomela pallida, the green strip leaf beetle).
Paropsides calypso now infests lilly pilly hedges from north Queensland to Melbourne. And we made this happen – me included – by mass planting lilly pillies.
Mealy bug and gall midge in Agapanthus
Can you remember the good ol’ days when agapanthus was the bullet-proof choice for every warm temperate garden? There were nurseries in the 1980s devoted to pumping out mainly this one plant during its peak popularity. It’s still widely planted around the world but there are now new pestilences arising.
In Australia where once these plants survived our long heatwaves, those in full sun now get badly burnt. Many plants show deformed, pale, thin and wavy leaves. The culprit is a mealy bug that infests agapanthus plants and weakens the leaves so that they can’t translocate enough water during hot and dry periods. And the mealy bugs are impossible to eradicate.
In the UK, it’s the agapanthus gall midge that’s doing the damage, causing flower buds to abort.
Rose Rosette Disease (RRD) on Roses
Roses are a one of the world’s favourite plants. Although gardens throughout history have had both species or new hybrid roses on show, they were always specimen plants in limited number…then the mass-planting rose arrived on the scene and slowly but surely, the ground rules changed. Rose Rosette Disease (RRD) is sweeping through the USA, spread by a tiny mite, and no rose is immune. RRD causes ‘witch’s broom’ new growth, red leaves, oddly thick growth, mottled flowers, reduced winter tolerance and plant death after 2-3 years. RRD has already destroyed significant rose collections in eastern USA, including in the Brooklyn Botanic Garden, and has spread to Canada. Even the highly disease-resistant ‘Knockout’ roses succumb.
Downy mildew on Impatiens
These cheery little short-lived perennial plants brightened many gardens from temperate to tropical zones until a few years ago. Tried to buy one lately in the USA or Australia? Chances are you’ll only find New Guinea hybrid impatiens plants on offer. A virulent downy mildew, Plasmopara obducens, attacks Impatiens walleriana, causing stunting, leaf yellowing and leaf fall and eventual plant death. It was first recorded in Australia in 2006 and in the USA in 2011.
In the USA it is now spreading to other ornamentals that have been mass planted instead, like Gynura and also basil cultivars. The New Guinea impatiens, Impatiens hawkerii is resistant but, with it now being mass planted instead, how long before a new mildew arrives to defeat it?
Cypress bark weevil, cypress canker and cypress aphid on xCuprocyparis leylandii ‘Leighton Green’
For some, this pest and its often accompanying disease can’t spread fast enough to rid suburbia of inappropriately-planted ‘monster hedges’ of Leighton Green cypress. Symptoms of both the bark weevil and the Seiridium canker fungal disease (which can enters the tree through damaged bark) are browning and then dieback through the canopy. Trees that are stressed through drought are more susceptible, especially when it’s followed by warm and humid weather. Monterey cypress (Cupressus macrocarpa) is also infected. Cypress canker is becoming more widespread in the USA and Australia.
In the UK cypress aphid is also responsible for widespread browning and dieback on Leighton Green hedges, especially during the spring growth season. As climate change reduces the number of severe winters, entomologists expect that cypress aphid infestations will increase throughout southern England.
I’m thinking that you are now thoroughly depressed. So what’s the solution? How do we have beautiful and structured gardens without mass plantings? My thoughts to come next week…
** Dr Reid says: “Paropsides calypso has spread in the last 10 years from NE NSW where it is native and only seems to feed on Syzygium australe. It first turned up in Sydney about 5 years ago and probably poor quarantine between nurseries has helped its spread. Its eggs are laid on leaves or leaf buds, and the green larvae are relatively solitary (not clustering). Pupation is IN SOIL at the base of the plant. There are possibly 2 generations – a long winter one and short midsummer one, or maybe just one generation. It’s now known as far south as Victoria. The adults and larvae both feed on leaves and completely strip hedges/bushes.”