How climate change will affect the rhododendrons in your garden
The words climate change seem to be bandied around for just about any subject up for discussion these days and I make no apology for adding to the conversation. I feel like a bit of a dinosaur when I think of discussing these changes more than 30 years ago after 20 years at the helm of Pukeiti Garden. Then it was referencing the subtle changes like frost occurrence and swings in rainfall. Pukeiti had amassed rainfall records since it started in 1951 and the up to 4m of rain per year was oft-quoted to garner the unbelievable response. The rainfall total was not changing a great deal but the swings between wet spells and dry spells were more dramatic. The colder night temperatures that bring frosts were definitely reducing and periods of eight to 10 nights in a row were down to two to three nights, with corresponding damage to tree fern crowns almost non-existent. This effect on the native flora was subtle but much enhanced for the exotic introductions. READ MORE: * Heavenly hydrangeas: why New Zealand has the best garden plants in the world * Rhododendrons, azaleas and three generations of dedicated gardeners * Pukeiti 'globally significant' in rhododendron research One of the early joys of Pukeiti was being able to grow the likes of blue Meconopsis poppy species amongst a wide range of rhododendrons from the high alpines to sub-tropics. The Meconopsis gradually died out and primulas that spread and lined the small streams and damper areas gradually reduced to a few tough species. The effect on the rhododendrons was less obvious at first, but dwarf alpine and cool-loving high elevation species and hybrids began to show signs of stress, and no amount of replanting on new sites were any better. These are plants from high altitude, often spending months of the year under snow, safe from desiccating winds and bitter temperatures, so warm-temperate does not work. However, rhododendrons from the sub-tropics, the vireyas we know today, do appreciate these conditions and in the 1970s I started to experiment with a few that were available to see what their limits might be at Pukeiti and that might be a subject for another time. The other consideration was that warm-wet conditions bring a whole set of other problems compared with cold-wet, namely pests and diseases. Whilst always concerned that drainage was paramount in any part of Pukeiti and the friable volcanic soil was basically good for this, attention to saturation spots was a key part of the planting regime. This greatly increased with the climate changing and the need for raised bed planting or even epiphytic planting became more of a priority. Rhododendrons and azaleas have naturally fibrous and shallow root systems, emanating from their epiphytic nature, particularly in the subtropical range of species. A requirement for any plant is that there is air available at the root system otherwise you get poisonous anaerobic conditions and very few plants can adapt to this. Nothing has really changed in this respect but soil pathogens are increasing now that the soil itself is warming up and they spread faster in warm-wet situations. Phytopthora and other soil diseases are on the increase worldwide and these are only part of the problem. Plant diseases on rhododendron foliage or stems are spreading and climate change is directly involved with this. Mildews and rust diseases are increasing on some types of rhododendrons, both evergreen and deciduous, and controls are pretty difficult with the range of chemicals now available. Most pathogens are airborne through microscopic spores and there is not a lot you can do to prevent this spread other than achieving the best cultivation environment for the plant. This involves a free-draining acid (low PH) soil, shelter from strong cold winds, avoidance of very hot or cold positions in the garden, and enough regular moisture to prevent drying out of the fine root system. These are basics and in most gardens there will be a compromise between all facets to arrive at a suitable planting site. Existing plantings, particularly other trees and shrubs, will offer good solutions both to shelter and temperature but the density of canopy shade and root competition might be on the negative side. Deciduous trees with light shade canopies and deep root systems are the best. Evergreens are better as background shelter but often need regular pruning and canopy opening to provide better light if used within a planting. Having said that, Pukeiti is nearly 100 per cent evergreen forest and that constant, regular lifting of canopies, pushing back the margins or hard pruning is necessary. Tree ferns are a wonderful foil and overseas growers are so envious of our native species, so much so that a good trade has built up for their supply. Nearly 100 years ago a shipload of tree fern trunks as ship ballast was taken to Cornwall and became the originators of those still found in the most sheltered gardens in southwest England. Particular pests that have increased over time include thrips, white fly, leaf hoppers and some borer insects that tunnel into the branches and can cause dieback or whole stems to collapse. The spread of thrips is particularly concerning as it causes the foliage to turn a silvery-grey colour on the top and brownish below. That is caused by those minute little black sap-sucking insects, which also eventually can cause leaf drop and weaken the growth. A silver-grey rhododendron is not a good look and it stays with the plant until the leaf drops, by which time the next set of leaves is already affected because many generations occur per season. The warmer the weather, the longer the infection occurs. Many treatments have been tried for these pests but in this no-poisons age, we are left with few options. Getting the environment right is the first step and balancing good air movement with just enough shelter, plus not letting things get too dry, are all positives. Good old-fashioned summer oil or neem oil are probably the only options for safer spraying, but these need to be timed regularly through the main growing season to be effective. It is vital that the application covers all leaf surfaces but particularly the undersides, where many of the minute pests hide. A clean water-jet hosing of the leaves will also remove some pests plus assist with moisture around the plant. The choice of new species and varieties is a problem as it takes time to turn around a set propagation range that have been considered top performers for years. Nurseries constantly look to see what is selling well and new hybrids always come with some razzmatazz to help the sales, and that is good. Also to consider is that New Zealand is a long, narrow country with a complex weather system coupled with various soil and topography situations. One size does not fit all but eventually those plants that continue to do well in most situations will survive and those that once were top performers but do not match the new criteria will fall off the lists. Quite simply, the warmer climate species and hybrids will prevail because that is the way we are tracking and it is not going to turn around in the next 80 years. We will always have the winterless north, which will become tropical and the deep south will be warm-temperate, both with the proviso that elevation and exposure will modify every situation. Dont expect pineapples along the waterfront in Bluff for a while yet! What are the likely ones to look for? The biggest group that we expect to become more popular are the Maddenias. These are fine foliage plants with shiny leaves and what are called scales on the underside. These look like small dots, sometimes almost touching across the whole surface and sometimes well separated. They can be dark brown, tan, yellow or greenish. The scales help protect the leaf from drying out in adverse conditions. Sometimes, the new shoots and flower stems are also covered with the scales. The flowers are usually in pastel colours, white, yellow, through all shades of pink and often with coloured blotches in the centre of each. They can be very fragrant, and the more than 100-year-old hybrid Rhododendron 'Fragrantissimum is one of the most well-known; it was originally bred for greenhouse cultivation in England. Maddenias are usually classed as subtropical but they are found in the mountains of northeast Asia right through to China, Myanmar, Vietnam, Taiwan and southwest Asia, so are incredibly variable in hardiness. Invariably epiphytic in nature, they are totally adaptable in the garden. They can flower from early spring right through to early summer and the new growth on some species and hybrids can be coloured in silvery scales or be deep purple-red until they fully develop. Flowers are normally trumpet-shaped and though not in full round trusses like the hardy varieties, they are incredibly beautiful. Hybrids to look for in your garden centre could include pale pink Charisma (a Pukeiti selection); John Bull, white with rich scent; Chrysomanicum, early bright yellow; Yellow Dawn, huge, creamy, scented trumpets; Mi Amor, tall, huge, white, scented flowers; Bernice, rich rose, scented; Barbara Jury, soft yellow, scented; Floral Dance, large white trumpets, flushed rose and fragrant; Floral Magic, full trusses of white, flushed cream and pink, and scented; Felicity Fair, apricot yellow, late flowering; Platinum Ice, wide, palest lilac trumpets; and the new Maddy Bern with rich reddish-pink and edged white, scented trumpets flowering late in the season, released from Pukeiti this year. Species that are worth seeking out include R. nuttallii with spectacular foliage and bark; R. lindleyi , with fragrant lily-like flowers usually white but pink-tinged forms are around; R. dalhousie with three to four large greenish yellow trumpets on a rangy shrub; R. edgeworthii , the parent of R. Fragrantissimum, one of the most scented of all with distinctly woolly foliage and white, often flushed pink trumpets; R. maddenii , a dense foliage plant with variable white to pink flowers, long-tubed or widely flaring, always with a good scent. There are many more but unfortunately they are not so frequently available in garden centres at present. Another group that have been around for a long time are the R. arboreum varieties and some of their hybrids. R. arboreum itself is a tall tree in the Himalayas and flowers very early in the season, covering mountainsides with red, pink or white displays. Early hybrids from these were also big plants and some, such as R. elegans and R. Sir Robert Peel, are found throughout New Zealand even as street trees but are not suitable for small town gardens. However there are smaller forms that hold hope for new, small hybrids in the future and the long-named R. arboreum ssp. albotomentosum , from one mountain in Myanmar, is promising. An outstanding foliage plant, it is a miniature tree in form with narrow, glossy dark green leaves, white below and with small intense red flower trusses in late winter. Only introduced in the 1950s, it has adapted to New Zealand conditions very well and Pukeiti is starting to cross this with other species to breed a better race of rhododendrons suitable to the changing climate not a quick process, but I am excited at the potential that we have in front of us.