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Growing Cypripedium outdoors in Britain
Peter Corkhill gives advice on cultivating hybrids of these spectacular slipper orchids that have proved hardy in his south Cumbrian garden
Images: Peter Corkhill
The genus Cypripedium consists of some 47 species scattered round the northern hemisphere, most of which are totally hardy in the British Isles. Although they are well equipped to withstand the cold temperatures, many do not appreciate high winter moisture levels or the stop/go nature of British winters with mild intervals that can start them into growth too early. However, recent developments in micropropagation methods and the creation of many primary hybrids mean that garden cultivation of these superb slipper orchids is now within the reach of everyone.
Although Cypripedium cultivation is not easy, with careful preparation and attention to detail, some amazing results are achievable.
Choosing plants
Beginners with this genus and those attempting garden culture should only start with hybrids. In general they are more vigorous than the species, less prone to disease and unlikely to disappoint. To succeed with the species, a more specialist knowledge of their habitats is required and it is advisable to use pot culture and provide extra protection from an alpine house or plunge frame.
The first flowering seedling produced by micropropagation from seed was C. reginae, reported by Werner Frosch in Germany in 1985, and the first primary hybrid, C. Genesis (C. reginae x C. parviflorum var. pubescens), was registered with the RHS by Whitlow in 1987. For registration purposes, C. parviflorum var. pubescens and C. parviflorum var. parviflorum are regarded as separate species. Progress in producing new hybrids to flowering size has since been rapid and almost 70 (including naturally occurring hybrids) are now registered with the RHS at the time of writing.
Personal experiences
The first hybrid I tried in the open garden was C. Ulla Silkens (Malmgren 1996), produced in Sweden by Svante Malmgren and named after his wife. This cross between North American C. reginae and Chinese C. flavum is a must for the beginner and produces strong, multi-stemmed clumps which often flower five years ex-flask.
The flowers come in two main colour forms - pale lilac with multiple spots and a darker form reminiscent of the C. reginae parent but always displaying the delightful curled-back lateral sepals of C. flavum. After about 18 months I dug up my plant of Ulla Silkens to move it to a new garden and was amazed to find long healthy roots up to 45cm long, much bigger than plants from the same batch that I had continued to grow in pots.
Encouraged by this initial success I next tried C. Emil, a cross between C. parviflorum var. parviflorum, a small yellow North American variety, and its European counterpart C. calceolus, registered by Werner Frosch in 1993. I had repeated this cross myself in 1995 and so had a few seedlings with which to experiment. Amazingly, all survived garden culture with no extra winter protection and progressed to form clumps like the North American mother plant but with slightly larger yellow slippers often flushed with red when grown where they received a bit more sunshine. Some seedlings I had distributed to orchid friends did even better.
Promising new hybrids
In the past few years, C. Gisela (C. parviflorum var. parviflorum x C. macranthos, Frosch 1992) has been produced on a large scale by Michael Weinert at the Frosch Nursery in Germany and a number of British nurseries have imported and offered plants up to flowering size. Cypripedium Gisela produces flowers in several colour forms depending on which variety of C. macranthos has been used as one of the parents. I have not experienced any difficulty in establishing C. Gisela in my garden in south Cumbria and some spectacular displays are featured on the Frosch Nursery website in Germany.
Other promising hybrids for garden cultivation are C. Aki (C. macranthos x C. parviflorum var. pubescens, Frosch 1996), C. Sebastian (C. parviflorum var. parviflorum x C. montanum, Frosch 1998) and C. Andrewsii, which is a remake of the naturally occurring North American hybrid of C. parviflorum and C. candidum. My experiments with these are still at an early stage although all have survived their first winter without loss.
I am impatient to try newly registered C. Sunny (Frosch 2004), a cross between European C. calceolus and Chinese C. fasciolatum with huge pale lemon flowers. My seedlings of this hybrid have shown great promise in the early stages, and in Germany, where this cross has flowered, reports suggest the result will resemble a very large-flowered C. calceolus. This should be a great attraction in gardens and fulfil the fantasies of those wishing to grow a plant like the threatened British native lady’s slipper orchid without posing any threat to remaining wild populations.
Choosing a site
An ideal situation is a steep slope set in dappled shade but alternatively you can use a site that is shaded on the south side by a tall structure such as a building, fence or shrub. Cypripediums need to receive several hours of sunshine each day in midsummer in early morning or late afternoon but no direct sun in the middle of the day (say from 10am-5pm). Extra shade can be provided by strategically fixing shade cloth but a naturalistic setting is more attractive.
Ground preparation
Ideally plants will remain undisturbed for many years once established, so time spent on initial site preparation is well invested. To provide stable conditions choose an area that is at least 1 sq m (preferably bigger). Unless your soil is free draining or you live in a low rainfall area, it is best to create a raised bed about 22cm deep, but even lifting the level by 15cm has a dramatic effect on soil moisture.
Your aim is to produce conditions that provide perfect drainage and even moisture levels, so the planting mixture should have plenty of air spaces to avoid waterlogging. The final pH value should be around neutral or slightly alkaline.
Heavy and dense silty soils need substantial improvement with extra drainage material such as pumice, perlite or coarse river sand; up to three-quarters of the final mixture can consist of these materials.
Free-draining sandy soils also need to be improved to aid moisture retention, with Seramis Clay Granules or coarse fibrous organic materials such as pine duff (decayed pine needles), coir, composted wood chip or even pine bark with a light dressing of dolomite lime to counteract the high acidity. In general, avoid peat as most on the market is too finely milled and holds too much moisture, besides which its extraction threatens wild areas where other orchids grow.
A certain amount of experimentation may be needed to discover a mixture that suits your local climate. Here, in the wet northwest, a raised bed with a mix of equal parts of coarse grade perlite:pumice (3–6mm):fibrous organic material (probably a mixture of pine duff and composted hardwood chip) with a little added dolomite lime is required. However, in Norfolk this may well provide conditions that dry out too easily so the volume of fibrous material will need to be increased.
In a woodland garden, tree roots invading the Cypripedium bed may pose a threat. If they become too dominant, nutrients and moisture will be removed from the compost at an unacceptable rate and it will become impossible to lift your plants in the future without extensive damage to their root systems. A pond liner can be used to provide a tree root barrier, but take care that the drainage still functions efficiently.
Planting techniques
The best time to buy and plant new stock is in early autumn when the plants first go dormant, normally in October and early November. At this time of year bare-root plants can be bought by mail order and soil conditions are still warm enough to help them establish before the worst winter weather arrives. Winter rain helps to consolidate the compost and ensure good contact with the sparse root system and ensure early uptake of moisture and nutrients when the plants start into growth in spring.
When planting, excavate a hole in the compost mixture about 15cm deep and spread out the roots evenly, burying the plant so the final surface level just covers the tip of the bud. Some growers take the extra precaution of surrounding the crown/ rhizome area with pure pumice to ensure perfect drainage to ward off rots that can affect this area of the plant. I normally bury the tip of the bud, even though this is usually green, because the compost will compact naturally over the winter (do not be tempted to firm it by hand). Cover any buds that become exposed by rain and frost, as the crown area can become vulnerable to drying out.
If you plan to lift your plants in the future either to move them or divide the crowns, planting into large aquatic plant baskets will make the process easier and should help reduce root damage during the operation.
Finally, to camouflage the surface and prevent the perlite blowing about the garden, the bed can be topped with a mix of local loam and pine needles or even gravel.
Cypripediums look good in garden settings when grown in association with other garden plants but do not like to compete with anything tall or vigorous. Companion planting may well benefit the plants directly by helping regulate soil moisture levels and it is also possible that mycorrhizal links may form allowing nutrient exchange. Small ferns are suitable companions as are other low, carpeting, ground-cover plants.
Health care
Plants will require occasional fertiliser applications during the growing season using either slow-release granular fertiliser scattered thinly on the compost surface or a balanced mineral fertiliser watered on with a fine rose - which has a foliar feeding effect as well as providing nutrients to the roots. In general it is best to avoid high nitrogen levels and I use a 7:7:7 NPK formulation for seedlings and a higher phosphate/potassium 3:12:6 formula containing trace elements for flowering-sized plants and large seedlings, but anything similar is fine. Fertiliser levels are generally a quarter of that recommended for houseplants.
Cypripediums have many enemies, particularly when they are still small and certainly need protection against slugs and snails in most gardens. The small leaves in spring seem to be particularly attractive to molluscs. Mice and voles sometimes eat the shoots and roots, and blackbirds and pheasants can easily uproot new plants. To keep these and cats at bay, low cages of wire mesh will allow the shoots to grow through unimpeded.
A little time spent in planning and caring for these Cypripedium hybrids, will reward UK gardeners with glorious displays of colourful and attractive, hardy slipper orchids.
Peter Corkhill is a writer and photographer and was English Nature’s Manager of the Lady’s slipper orchid Recovery Programme. p.corkhill@daelnet.co.uk
Further information
The excellent monograph The Genus Cypripedium by Dr Phillip Cribb (Timber Press, 1997, £29.99, ISBN 0881924032) is undoubtedly the best introduction to hardy lady’s slipper orchids.
Several websites are worth a visit:
www.cypripedium.de provides a forum where questions can be answered and also information on reliable sources of micropropagated plants
Werner Frosch runs www.w-frosch.onlinehome.de with full details of species and hybrids including photographs and cultural information
In the USA, Spangle Creek Labs www.uslink.net/~scl and the Vermont Lady’s Slipper Co www.vtladyslipper.com show mouthwatering views of large-scale seedling production. A search using the keyword Cypripedium produces many others
The UK Hardy Orchid Society has www.hardyorchidsociety.org.uk and it is often possible to buy plants at the society’s meetings and shows.