Dunedin Botanic Garden’s Rhododendron Dell has gone through several phases, each with a different flavour. The most recent third of its life has been led by curator Doug Thomson, who is now retiring after 36 years.
The first phase, establishment and expansion, began in 1914.
David Tannock, the botanic garden’s superintendent at the time, had been well and truly making his mark since beginning his tenure in 1903. At that stage, the only developed part of the garden was on the flat.
During his early career, David Tannock worked at London’s Kew Gardens, which had a 60-year-old rhododendron dell. Perhaps that inspired another of his major projects.
Rhododendrons had been in New Zealand for decades, since about 1860, but David began to collect rhododendrons, a collection he later described as being the most extensive, complete and healthy in the Dominion.
It probably started with the planting of rhododendrons still visible in the lower botanic garden today.
In his annual report of 1913, he noted the planting of a border of rhododendrons, probably the stand of large old hybrids growing between the playground and the long herbaceous border.
By June the next year, he noted in his diary, "Rhododendron Dell. Started to clear, three men". Twelve days later, the clearing was finished and the first rhododendrons planted. Some are still alive now, on the track beneath the azalea garden and parallel to Lovelock Avenue — look for the interpretation panel headed "The Dell Started on this Spot".
By 1918, an avenue of Japanese cherries had been planted to create the Cherry Walk.
In his Otago Daily Times gardening column that year, he wrote, "Probably the ideal soil for rhododendrons is black bush soil overlying clay. Such a soil is never dry and hot, and when trenched, retains moisture for a long time."
Black bush soil is formed by forest leaves dropping to the ground and rotting down to form rich soil. The bush-clad hillside was thinned out, providing the partial shade that rhododendrons thrive on. This gentle slope with all-day sun was converted into an exotic woodland garden with pockets of bush, open glades, occasional tall trees and an understorey of exotic plants.
The Rhododendron Dell kept spreading. By 1923, a cottage garden aimed at beginner vegetable and flower gardeners had been transformed into today’s azalea garden.
By 1931, 31 different beds had been formed there and by 1939 the azalea garden had been extended four times.
The dell’s second phase of life started about 1940, when Mr Tannock was replaced by Maurice Skipworth. Wartime was a tricky start for him. Garden plots in the lower botanic garden were simply grassed over. By late 1944, the Rehabilitation Department was offering a 75% subsidy on the cost of wages and six men were employed for nine months to work on extending the dell. Mr Skipworth’s choice to use this subsidy to prioritise the dell over the rest of the neglected botanic garden reveals his enthusiasm for the plant collection.
His new assistant, Robert Balch, was just as keen on rhododendrons. They devoted many off-duty hours to breeding new rhododendron crosses, some of which still grow in the dell today. Balch’s Island, southeast of the Cherry Walk, displays hybrids born and bred in Dunedin.
Over these years, the dell was curated by one fulltime worker, Robert Kinnear, described as "one of the most reliable and capable men" on the staff. The woodland garden was becoming well established.
But in 1952 the woodland garden’s innately informal nature seems to have clashed with the fashion for a more manicured look.
Mr Skipworth replied, "It is fully the intention that these rhododendrons should encroach on the track and eventually arch overhead. No attempt is made to convert this area into a formal trimmed garden, as it is essential for the character of the various plants that the woodland atmosphere of the area should be further developed."
Other criticism was valid though, such as the wet and dirty conditions of the tracks, particularly around the Cherry Walk area. There were problems with drainage that would not be fixed for another 30 years.
In the early 1970s there was more criticism. While there was a new brochure for visitors, with special reference to the dell, the botanic garden was being managed more as a pleasure park than a botanically focused garden.
Retired staff had not been replaced and, in 1970, the dell had deteriorated to the degree that a community group lobbied for specialised staff to bring it back up to standard.
Around this time, the Dunedin Rhododendron Group decided to create its own woodland garden to plant new cultivars they had bred. They had little confidence the situation at the botanic garden was going to improve, saying in their 1974 bulletin that their own garden project, "may well be the only way in which a good woodland garden can be built and carried on long term".
They established Tannock Glen on a 1.6 hectare site up the road in Opoho which remains open for public visits today.
In 1973, the botanic garden had a false start with a new staff member who had proved unsuitable. By 1977, further new staff had rectified the situation to the degree a letter writer to the Otago Daily Times wrote about the "creative work" in the dell. The dell was back on track.
Its renaissance was well under way when Doug Thomson arrived from Scotland in 1986 to manage the dell. It had always been arranged horticulturally but, to give a clearer insight to the genus, needed more botanical themes so the rhododendrons’ similarities and differences could be appreciated.
Young Doug threw himself into transplanting more than 400 rhododendrons over 21 months so they could be displayed in their botanical groups before the National Rhododendron Conference in Dunedin in 1989. Many were mature or semi-mature, with root balls up to half a tonne in weight. It was a case of using sheer grunt to dig them up by hand, supplemented only by tractor and trailer to move the plants.
Around this time, it was also clear that, despite Mr Tannock’s original enthusiasm for the "black bush soil overlying clay", things had changed. The topsoil left by the original forest had been stripped or washed away over the years by weeding, watering and erosion.
Waterlogging in the heart of the dell had become a major problem. Some older plants were dying and paths were impassable in spring. The clay soil meant even sloping sites were a challenge to drain.
As a result, much of the focus of the last few decades has been installing drains in the clay soil and mixing in gypsum, a mined mineral that improves texture and allows for airflow and drainage. Copious amounts of compost have been forked in and, without the replenishment provided by the forest, this is a task that will probably have to continue indefinitely.
Paradoxically, dry summers in the late 1980s highlighted problems in the watering system. Only eight taps serviced the upper dell and the lower area was totally neglected due to corroded pipes. Some of the recently moved rhododendrons, still under stress from the move, were dying due to a lack of watering. Even mature, established plants died.
A new system was installed in 1990 and now there are 31 taps throughout the dell, allowing maintenance and new developments to continue.
Healthy soil is the basis of a healthy garden.
In a woodland garden, the layer closest to the ground is just as important as the top tiers of shrubs and trees. A rich understorey creates a supporting atmosphere of greenery and seasonal blooms. This has been built up over time so now there are colourful drifts of herbaceous plants such as Himalayan poppy, primula and iris.
New areas have been added such as the species border.
In the 1990s, China opened up its borders, allowing botanic garden staff to visit and get seed from wild rhododendron species.
Provenance is important to curating plants in a botanic garden and strict records are kept. But most other rhododendrons in the dell are hybrids, whereas species are particularly enlightening as they are the raw roots of these hybrid rhododendrons as they grow naturally in the wild.
A species border was established and in 1998 a new collection of New Zealand hybrids was established on the Cherry Walk.
An area nearby needed a makeover. The intersection of three tracks at the south of the Cherry Walk was a focal point offering an ideal, open viewing area. Horticulturally, though, it was minimalist with perennials providing a display for only a short time.
Dwarf rhododendrons and other acid-loving plants require moist, free-draining soil so Doug brought his Scottish familiarity with peat gardens and created one here.
Three years later though, in 1995, the difference between Scotland and the colonies had become apparent — Doug had used a recipe of peat, soil and grit suitable for Scotland but, with time, Dunedin’s clay soil compacted drastically. This forced renovation and replacement with a lighter mixture, this time also including compost and sand.
With time, environmental sensibilities kicked in — peat forms in nature at the rate of only 1mm a year and is as non-renewable as coal. But it’s a great, fibrous growing medium that holds water well. As a result, in 2019, Doug started using coco peat, made from the fibre of ground-up coconut husks. This rots more quickly than peat so needs more frequent replacement and, due to supply chain problems since the Covid 19 pandemic, is now impossible to buy in large quantities. An even more sustainable locally sourced alternative is ground bark, which will be the way of the future.
Rhododendron conservation is a focus for the future. In 2018 the Global Conservation Consortium of Rhododendrons was established by Botanic Gardens Conservation International. The project aims to ensure that no wild species of rhododendron becomes extinct.
The New Zealand contribution is being led by New Plymouth’s Pukeiti Rhododendron Trust. They have been surveying public and private gardens throughout the country, collecting data and seeds from known wild-collected species. Plants are grown then distributed to other participating gardens throughout the country.
Dunedin Botanic Garden’s propagation unit is one of three propagation sites, along with sites in Palmerston North and Pukeiti, growing seedlings on to a reliable size for being transported.
Some are grown on here in Dunedin to plant in the dell while the remainder are driven to Palmerston North, Pukeiti or Blue Mountain Nurseries in West Otago. The young plants stay there until they reach a plantable size before being planted in one or more of the 15 participating collection sites that most suit their cultivation needs, in effect becoming part of the seedbank of conserved species rhododendrons.
After November, Doug’s future still includes rhododendrons, just not those at the dell. Retirement is a time to choose whether you work at all, how much and how often.
Doug will continue his consultancy work for the Pukeiti Rhododendron Trust and turn his attention to Tannock Glen.
The dell has a special place in the heart of many Dunedinites. Doug sees his primary role as providing a service so visitors can relax, refresh and be uplifted.
The dell will continue to provide that and to develop and grow.
