Alpine and Rock Garden team blog

The Woodland Garden at Kew, around the Temple of Aeolus, is beginning to reach its spring best, with a range of bulbs and other woodland perennials flowering in the spring sunshine. One of the more unusual looking plants is a parasite known as purple toothwort, Lathraea clandestina. This species is totally dependent on the food produced by other plants to survive so it is called an 'obligate parasite'. It has parasitised the largest tree in the Woodland Garden, the black walnut, Juglans nigra, and the flowers sprout from the ground around the base of this mighty tree.

Purple toothwort (Lathraea clandestina) at the base of the black walnut

Lathraea clandestina is not native to the British Isles but was introduced in the second half of the nineteenth century and first came to Kew Gardens in 1888.  It grows wild in Belgium, France, Spain and Italy but has become naturalised in Britain and even New Zealand. There is a British native species, L. squamaria, the common toothwort, but it is not as colourful as this species.

The flowers of purple toothwort (Lathraea clandestina) can appear some distance from the host tree

The easiest way to introduce L. clandestina to your garden is from seed scattered around or mixed in the soil, near a suitable host plant. It normally parasitises poplars and willows but has also been found on a number of other trees and shrubs including maples, yew, rhododendrons and alder. In early spring the flowers appear out of the ground. They are pinkish purple hoods that form dense clumps, sometimes a fair distance from the base of the tree on which they are living.

Flowers of purple toothwort (Lathraea clandestina)

Being a parasite, purple toothwort doesn't have any green leaves for photosynthesis, and after setting seed it disappears from view. It connects to the root system of its host, often deep down in the soil. Like many parasitic plants, feeding off another plant doesn't seem to have a detrimental effect on the host, and this strange species makes a fascinating addition to the woodland garden flora.

- Richard & Katie -

The eight months preceding the opening of the Davies Alpine House in 2006 was a period of frenetic activity. Rockwork was constructed, the beds filled with soil, plants planted and the whole glasshouse cleaned and tidied for the official opening on 11 March. The first photograph below shows the Alpine House in August 2005, when the rockwork had been built been but before any soil or plants had been put in. Around the sides you can see the black pipes that come up from the underground chamber, where air is cooled before entering the glasshouse. These pipes were later cut down to fit in the landscape and fitted with stainless steel bends to direct the cool air over the plants.

The Davies Alpine House in August 2005

There have been plenty of changes over the last five years, with improvements to the plant displays, new rockwork built and the addition of extra features to show different ways of growing alpine plants.  More sandstone rocks have been added to provide additional nooks and crannies for alpines, and we have also built a wall of tufa. Tufa is a type of limestone, formed when deposits from water saturated with calcium carbonate build up to make a soft rock filled with tiny holes. Alpines can be planted directly into the rock where the roots work their way through the crumbly texture to find moisture, while the upper part of the plant is kept dry - just what many alpines like. The two photos below show the same area before and after the addition of the tufa, which was recycled from an old garden.

Top, the west side of the Alpine House in May 2006, and bottom, in March 2011

The next two photos show how the tufa and additional sandstone rocks have made a more interesting landscape and fully utilised the height of the surrounding walls:

Left, May 2006, and right, March 2011

Another new feature in the glasshouse is the construction of two dry stone walls, like cairns, to show another way of growing alpines. Plants are grown between the stone pieces, which have soil behind for the roots to reach into. One of these cairns is shown here, with the yellow Verbascum dumulosum in front, which flowers in May.

Stone cairn for growing alpines, in May 2010

The plants in the Alpine House have responded well to their new environment, especially the improved ventilation, the cooler summer temperatures and the higher light levels than in the previous alpine house, all essential for successful alpine cultivation. This is well illustrated by the next two photos showing the cushion plant Draba rigida. It was planted in 2006 between sandstone rocks on a vertical cliff. The first photo is from 2007 and the second from 2011, showing how well it has done. The high light levels inside the glasshouse have ensured the characteristic cushion shape has been maintained.

Left, Draba rigida in 2007, and right, flowering this week.

There are currently 443 accessions permanently planted in the Alpine House landscape and these are joined by the constantly changing displays of up to 100 additional plants brought in from the Alpine Nursery when in flower. Whatever the time of year, there is always something worth seeing

General view of the Davies Alpine House, March 2011

I hope this has given you an idea of how the Davies Alpine House has evolved over the five years since it opened. If you haven't visited recently then now is the time to come along to Kew Gardens and see how it has changed for yourself - over the next couple of months the display of plants will reach its peak.

- Richard -  

The native habitat of Tecophilaea cyanocrocus is the high Andes, where it grows on dry stony slopes from around 2000 to 3000 m altitude. In the wild it flowers in the Southern Hemisphere in spring, from October to November, and here in the UK from February into March. It immediately catches your eye in the Davies Alpine House with its amazing, deep blue flowers.

Tecophilaea cyanocrocus in the Davies Alpine House

With its crocus-like flowers it soon became known as the Chilean blue crocus although this plant is not actually a crocus but in a completely different family, Tecophilaeaceae. As well as the true species, two cultivars are also grown at Kew: Tecophilaea cyanocrocus 'Violacea' has flowers of a slightly more purple hue and T. cyanocrocus 'Leichtlinii' has more white in its petals.

Left, T. cyancocrocus 'Violacea', and right, T. cyanocrocus 'Leichtlinii'

Cultivation

We grow this eye-catching plant in pots in the Alpine Nursery but there are also some growing outside on the Rock Garden. Early each spring pots from the nursery are displayed in the Davies Alpine House.

Cultivation is quite straightforward. We use a free-draining compost and re-pot the corms each year in August. After re-potting, we plunge the pots in a sand bed, and water both the pots and the sand. The sand is kept damp until the corms come into growth, when we water them freely. Watering is eased off as the plants die back for their summer rest. During this resting period, the pots are sprayed over about once a fortnight, as they don't like to be dried out completely.

We keep the pots in a glasshouse to protect them from hard frosts but they will often survive outside in milder parts of the country, as long as they don't get too wet in the summer.

Pots of Tecophilaea cyanocrocus in the sand plunge in the Alpine Nursery

Propagation

Propagation is mostly by corm offsets, though occasionally we will sow seed in spring. Seedlings are left undisturbed in the seed pots for about three years. This means that when we come to separate them and put them into bigger pots, they have already formed a small corm, which is less likely to suffer damage during repotting. After about five years they should flower and dazzle you with their electrifying blooms.
 

- Sue -

One way that alpine plants can cope with their harsh mountain environment is to form a low mound or cushion, hugging the stony ground or clinging to cliffs. This shape exposes the minimum surface area to the high winds and freezing conditions. The genus Dionysia has some of the most striking examples of this type of growth habit and when they flower in early spring, these plants are covered with tiny blooms. You can see them now in the Davies Alpine House.

Dionysias on display in the Davies Alpine House

Dionysias come from Central Asia, especially Iran and Afghanistan. The cushions are made up of hundreds of tiny leaf rosettes and each one can produce a single flower. Individual flowers look like miniature primroses so it is not surprising that Dionysia is very closely related to the genus Primula.

Left, Dionysia mozaffarianii, and right, D. michauxii.

Growing Dionysias

Dionysias can be hard to keep alive in cultivation. They do not like too much water, especially in autumn and winter, and can easily rot away if there is not enough air movement around them. We grow them in a free-draining soil, incorporating plenty of 3 - 5mm grit (up to 50% by volume) to make sure excess water drains away quickly. The pots are kept in a cold glasshouse in the Alpine Nursery, to keep off the rain. They also like plenty of sunshine so our damp, dull winters are not ideal. To keep the collection going we have to regularly propagate our plants by taking cuttings. Each cutting is a cluster of leaf rosettes joined together by a thin stem. It is inserted into a pot of sandy soil in spring once flowering is over. After a few months they should have produced enough roots to pot them on.

Left, Dionysia cuttings ready to pot on, and right, young plants already flowering.

Young plants are potted on as they grow, into ever wider pots. A thick mulch of grit is used to keep the leaves away from the damp soil surface.  Eventually you may be lucky to have a specimen so covered in flowers you cannot see the leaves. The last photo is of a Dionysia hybrid, a cross between D. curviflora and D. tapetodes. Many other Dionysia hybrids have been produced by alpine gardeners around the world, who are all striving to produce that perfect plant.

A Dionysia hybrid covered in flowers.

- Richard -

The Rock Garden at Kew is built with Sussex sandstone and although in recent years there has been a lot of new construction, especially around the Davies Alpine House; some parts of the Rock Garden haven't been touched since the 1950s. Over time the rocks settle, the gaps between them widen letting the soil fall through and in the worst cases the rock can become unstable. The following picture shows some older rockwork, where you can clearly see how the rocks have moved and eroded, becoming uneven and creating wide gaps. 

Old rock work on the Rock Garden at Kew 

Before we begin working on fixing the rock we have to propagate or move any plants we want to save, so we have something to put back once the work is completed. The next picture shows the area we are working on at the moment. Most of the rocks have been rolled back or taken away, leaving a bank of soil and just the lowest layer of rock exposed. 

Rock removed to allow renovation work 

The top surface of the lowest rock layer is then levelled using an electric power chisel to break away any lumps and a spirit level to ensure the resulting surface is level. Of course, all the correct safety gear has to be worn - steel toe-capped boots, gloves, ear defenders and goggles! 

Chiselling the rock to create a level surface 

The rocks are then carefully put back in position so that they fit tightly together. Even then there is more chiselling to do, this time by hand, to make them fit together exactly. This locks them into place and makes them very stable.

    Left, a rock being moved back into place, and right, chiseling by hand to fit the rocks together

With a Rock Garden the size of the one at Kew, this work is ongoing and every winter we tackle a different area. It is a long process but worth doing well so the rocks stay in place for hopefully at least another 60 years.

 - Joanne -