This month, Sally Petitt scales the rock garden to bring us fascinating facts and inspirational ideas for introducing alpines to our outdoor spaces
The Rock Garden at Cambridge University Botanic Garden is one of our key features, sitting beside the lake and displaying a host of alpine plants from across the globe such as Primula marginata (silver-edged primrose), Tulipa montana (mountain tulip), Aethionema grandiflorum (Persian stonecress), Pulsatilla rubra (red pasque flower) and Sisyrinchium macrocarpum.
Of course, in our mountain-free region of Cambridgeshire, the Rock Garden is man-made. It was constructed in the 1950s using 500 tons of carboniferous limestone from Milnthorpe in Cumbria. Covering approximately one acre, it was developed on an open site under the watchful eye of the then superintendent, Bob Younger, who directed the construction to include planting pockets, crevices and screes to provide full sun and shade for a diversity of species.
Today, the Rock Garden is an established historical feature where bulbous, herbaceous, perennial and woody plants grow, all of which are beautiful and jewel-like. For me, it’s hard to equate the delicate appearance of these alpine gems with the tough, inhospitable conditions from which many of them originate.
Top of the rocks
Across the country there are many well-known rock gardens to admire, each with its own distinct character. Perhaps the most iconic is the rock garden at RHS (Royal Horticultural Society) Wisley, where sandstone blocks are embedded into a steep, north-facing slope.
The sandstone rock garden in the Royal Botanic Gardens, Kew has a wide path meandering through large boulders, with alpines from six mountainous regions of the world, while Royal Botanic Garden Edinburgh’s rock garden has around 5,000 alpine species in the naturally undulating landscape. More recently, RHS Garden Harlow Carr installed a limestone rock garden adjacent to its alpine house.
Each garden has water features, which add to the charm, but more importantly all have effective drainage and an open position necessary for alpine plants to flourish. While the heyday of rock garden construction may be a thing of the past due to financial and environmental concerns, there is plenty of scope to include alpines in your own garden.
What are true alpine plants?
True alpine plants grow above the tree line in inhospitable mountainous regions at altitudes up to 4,000m above sea level. Here, they are covered by an insulating layer of snow in winter, but they also endure low temperatures, limited
water, strong winds, intense light, low nutrient levels and poor, rocky soils.
In response to these conditions, they have compact habits, small leaves, extensive root systems and a short growing season in which to produce flower and seed. Today, many of these ‘high alpines’ are under threat from climate change, which is affecting snowfall and the timing of snow melt, and consequently the fragile ecosystems in which they grow.
In gardening terms, the word ‘alpine’ extends to small plants from other habitats that enjoy the same growing conditions of good drainage and ventilation.
Alpines for every garden
Whether a high alpine or a small plant from a lower altitude, all are suitable for cultivation in our climate, whether it
be individually in pots or a medley of plants in a window box, trough, raised bed or rock garden. And, of course, one of the great benefits of these charming plants is that you can squeeze many into a small space, making them ideal for even the
most compact garden.
There are a few guiding principles you should follow if you’re venturing into alpine cultivation. If you’re using a container for growing alpines, position it in an open site to encourage air flow, put a layer of crocks (broken pots) or grit in the bottom of your container, add plenty of grit to your compost to aid drainage, and apply a top dressing of grit on the surface of the compost to ensure the plants don’t rot from sitting on wet compost.
If, though, your alpine ambitions are grander and you wish to build a dedicated rock garden, you should:
- Ensure your chosen site is weed free before you start.
- Add a base layer of rubble, stone and shingle and top this with a permeable membrane to prevent soil loss.
- Add peat-free compost with at least 30% extra grit.
- Use a mixture of rock sizes, with larger rocks as feature keystones.
- Embed the rocks so at least one-third is buried in the soil and try to make them look as natural as possible.
- Plant up with young plants (these will establish more quickly).
- Top dress with gravel or grit.
Spring alpine highlights
At this time of year, many garden centres have a range of alpines including Narcissus bulbocodium (hoop petticoat daffodil), Sempervivum tectorum (houseleek), Dianthus alpinus (alpine pink), Saxifraga ‘Peter Pan’ and even our native Pulsatilla vulgaris (pasque flower) to entice even the least experienced gardeners to give these delightful spring bloomers a go.
If you are looking for a wider range of alpines for your patch, there are several specialist alpine nurseries offering a selection of plants, from varieties and cultivars of Cyclamen coum to plants for the dedicated alpine enthusiast such as the South American Benthamiella patagonica, which requires alpine house cultivation.
If you do develop a penchant for alpines, you could join the Alpine Garden Society, which has numerous local groups and alpine shows throughout the country – and whose members hold a wealth of expert knowledge to share with like-minded gardeners.
Whether you’re merely dabbling, or developing an extensive rock garden and alpine collection, they will grow well in the East Anglian climate, fit into the smallest garden and are sure to add charm to your garden throughout the year.
Gardening jobs for April
1. Give your lawn a cut if you’ve not already done so, remembering to lift the blade height for the first few cuts of the season.
2. Weed, weed and weed – time spent weeding now will save you time later in the year.
3. Stake herbaceous perennials to prevent them from falling over neighbouring plants and paths later in the year.
4. Sow hardy annuals such as love-in-a-mist and sunflowers, and continue sowing vegetables such as beetroot and lettuce to give a succession of crops.
5. Plant your second-early and maincrop potatoes.
Sally Petitt is head of horticulture at Cambridge University Botanic Garden. From gardens for the senses to beds of roses, explore more of Sally’s gardening tips.