Glasshouse interior with shade netting and potted plants.
A stocktake to record the roughly 1,000 Vireya Rhododendron cuttings growing happily in House 21 (Summer 2022)

On the south corner of RBGE’s Research Glasshouses sits a zone known as ‘House 19’, which holds a large part of the Ericaceous Research Collection. This research collection is home to plants of Vaccinium, Dimorphanthera, Agapetes and the Vireya Rhododendron collection to name a few. While the Edinburgh Biomes project involves the more noticeable decanting of both Victorian Palm Houses and the Front Range Glasshouses, there are several research collections going through an equally significant change behind the scenes. This collection holds nearly 4,000 plants, including over 250 different species of Vireya Rhododendron, and as part of the Edinburgh Biomes project the entire potted collection has been on the slow move since 2021 to another Glasshouse, just next door.

Winter morning exterior view of two glasshouses.
An October morning: House 19 (left) and House 21 (right) Research Glasshouses.

House 19

Interior overview of a large potted collection of plants.
From above, the Ericaceous collection in House 19 (early 2022)

With a high number of species in the collection and limited space, some of those species are represented by just one solitary plant, making them extremely vulnerable especially during any relocation. In early 2021, work started on mass repotting of the collection specifically to promote new root growth in some of the older, more fragile plants. In addition to this the team at the time – which included the manager of the collection Horticulturist Steve Willis and then-Biomes Team member (now Head of the International Conifer Conservation Project) Hannah Wilson – began with the construction of heated propagation cases with LED lighting. This allowed them to keep to schedule by promoting the growth of cuttings over the winter months. New plants began to grow, and older plants improved. Overall, this valuable collection became stronger and ready for its move.

New leaf growth, pink and yellow on the left. Rich pink flowers on the right.
Left: New growth of Vaccinium viscifolium   
Right: Rhododendon rugosum var. rugosum
Horticulturist potting a sprawling plant on a tray of sand
Horticulturist Steve Willis places Rhododendron saxifragoides onto a potting tray of sand, having previously grown out of its original pot and along the bench’s surface.
Vibrant, red new growth leaves
The vibrant red new growth of Dimorphanthera amoena

A Variety of Vireyas

RBGE is thought to hold the world’s largest potted collection of Vireya Rhododendron species, native to the mountains of Southeast Asia, with roughly 70% of all known species. Many of these species are threatened in the wild, and RBGE’s plants act as an important ex-situ conservation collection. We have found that successful cultivation of Vireyas requires a shaded, relatively cool glasshouse averaging a minimum night temperate of 13°C, and 15 to 18°C during the day. Since a majority of Vireyas grow as epiphytes in the wild (that is, grow on other plants, usually trees), care of them as a potted collection involves finding a balance in a potting medium and careful routines in watering. While the collection includes various hybrids and historic cultivars from the Veitch nursery such as ‘Princess Alexandra’ it represents a long legacy of invaluable work done by RBGE’s Dr. George Argent throughout the years.

Rhododendron himantodes Var. himantodes gold coloured star like scale arrangement
The golden, star-like scales of Rhododendron himantodes var. himantodes
White Rhododendron rousei flower on the left, Rhododendron maxwellii cream flowers on right.
Left: Rhododedron rousei collected in 1990    
Right: Rhododendron maxwellii collected in 1980

Decide and Divide

While their new Glasshouse is just next door, before they can move each plant goes through a process involving inspection, propagation and seed collection when possible. Starting with a stocktake to evaluate conditions of the collection, the process ultimately results in a focusing and strengthening of the collection by removing excess clones (plants that are genetically identical to one another) and giving the remaining plants the space and attention needed for their long-term survival.

It is a process of elimination based on multiple factors including history of collection, genetic diversity for conservation and research, and their current status in the wild. Additionally, the horticultural team also considers the quality of propagated material already taken, and if it is healthy enough this material can replace older, weaker plants as a method of reducing the footprint of pots within the house. Propagating is helpful in preventing overcrowding while also increasing the number of plants of any endangered species.

Interior glasshouse, two people discussing the collection of plants, surrounded by benches.
Glasshouse Supervisor Louise Galloway and Horticulturist Steve Willis work through each species of Vireya alphabetically, referring to the living collection database to help make decisions on propagation and reduction of numbers.
Interior glasshouse, small collection of plants on a bench.
A small holding collection of surplus plants, which were shared with Glasgow Botanic Gardens in November 2022, including Rhododendron multinervium from Indonesia and Dimorphanthera amplifolia var oblonga from Papua New Guinea.
Selection of yellow labels used to tag plants.
A small example of the tags each plant was given throughout the decision process.

House 21

Plants change the environments around them, and glasshouses are no exception. The moving of this collection triggered the building of propagation tunnels and closed cases lit with LED lighting, as well as some horticultural benching being dismantled and moved. Within a year a glasshouse can change multiple times, season to season becoming unrecognizable to the previous collection it once housed.

Glasshouse interior, two people work cleaning and unrolling weed protection fabric.
With the last of the previous collection moved to another Glasshouse, the gravel benches are weeded, raked and a sheet of a weed control fabric is laid out. Pictured: Horticulturists Roween Suess and Lucy Baines.

A common challenge with moving any collection is the change of area the plants take up; for example, when lifting larger plants from display glasshouses their footprint often increases, due to them being divided and spread out. When it comes to potted collections moving from one glasshouse to another, it is not a simple ‘cut and paste’ operation. Despite the two glasshouses mirroring each other in layout and design, re-potting and propagation initially cause an increase in space requirements – however once they are healthy enough, the smaller propagules can replace larger plants that would otherwise be dominating bench space.

Small cuttings of plants being potted on a bench.
Once the cuttings show signs of strong root growth, each individual cutting is re-potted and labelled. If cuttings happen to fail, they are taken again and the process repeats.
Leaves breaking from the stem of a plant.
New leaves emerge along the stem of Rhododendron javanicum ssp shadenbergii after being placed in a more open, light exposed environment in House 21 over the summer.
Interior of glasshouse. lighting in the process of being hung above the bench.
Preparations are made for the hanging of supplementary lighting previously fitted over benches in House 19.
Lighting above bench and collection of plants is switch on.
The lights are switched on, working off timers and giving an added boost of light as the days grow shorter.

As of November 2022, the move of the Vireya collection is complete. Horticultural benching within House 19 is freed up for part of the backup Temperate Collection to move in; a constant ripple effect that frees space in another area for plants to be lifted, potted and moved along.

Glasshouse interior, feeding plants with hose.
Horticulturist Steve Willis feeds the Vireya cuttings with an ericaceous, seaweed-based fertiliser.

From afar, moving a collection can seem as simple as moving pieces on a board, but on the ground, the team is now using this shift of collections to carefully curate the collections and make changes in their care.

Interior glasshouse, the sun shines through shade netting and glass on the roof.
The roof vents open fully to cool House 21 on a hot summer’s day.

The Edinburgh Biomes is a huge and complex project. Being on the ground and involved with the logistics of moving this one collection showed me how large projects often come down to much smaller pieces involving great effort and detail. This move included months of planning and attentive teamwork, all focused on an outcome that benefits the unique living collection that the horticultural team at the Royal Botanic Gardens Edinburgh care for every day.