Saxifraga is an incredibly large and diverse genus, encompassing around 480 species of alpine or woodland plants. They are generally small, made up of rosettes of green leaves, but can vary in habit from compact cushions to plants that become wide-spreading mats or that are tall and leafy-stemmed. They produce flowers in spring and early summer.
Because of the huge number of species, Saxifraga have been classified into many sections, but only three are widely employed in cultivation.
The section Porphyrion is highly prized for cultivation, especially among exhibitors at alpine gardening shows. These plants are the earliest to flower, between early and late spring, with S. burseriana flowering as early as February.
The Kabschia form bun-shaped cushions made up of rosettes with short, needle-like, lime-pitted leaves. Engleria generally have relatively large rosettes. Both these divisions have produced a range of attractive cultivars, and readily hybridise with each other.
Then there are the "encrusted" or "silver" Saxifraga that form cushions of stiff, flat rosettes. They flower in June, later than the Kabschias, producing sprays of pink, white or yellow flowers.
And there are the "mossy" Saxifraga, which form soft cushions of evergreen rosettes. They prefer semi-shade as they get scorched by the midday sun.
These are the main groups of garden interest, but there are a few worth mentioning from other sections. Saxifraga x urbium Award of Garden Merit (AGM) is commonly grown. It has 'cabbagey' rosettes of evergreen, leathery, spoon-shaped leaves and white flowers, spotted with red. Saxifraga fortunei is an interesting woodland plant that has lobed leaves more like a Heuchera than a Saxifraga and flowers in late autumn. It has yielded many cultivars.
Most species thrive in a well-drained, loamy soil, with additions of leaf mould, sharp sand and fine chippings. The high alpines like a very free-draining medium. Some prefer a calcium-rich medium, so ground limestone can be incorporated. The woodland and meadow species do best in soil with higher organic content.
Saxifraga prefer a moist, semi-shaded site as they need protection from the hot midday sun. The alpine types require drier conditions at their neck and so are best potted with a top dressing of grit and grown in cold frames or an alpine house to protect from the worst of the winter wet.
The most common pests and diseases include vine weevil, weevil grubs, mealy bug, root aphid and Botrytis cinerea, bacterial soft rot and rust, especially in encrusted Saxifraga.
WHAT THE SPECIALISTS SAY
Stella Rankin, owner, Kevock Garden Plants, Midlothian "We grow around 50 species and varieties of Saxifraga - it's a lovely genus. The encrusted group do particularly well - for example, S. 'Esther', which has very tight grey-green rosettes. It has an arching flower stem, which ends in clusters of tiny white flowers. Then there is S. 'Freckles', named because its white flowers are heavily speckled with dark-red spots. One of the best is S. 'Gorges du Verdon', which was found in the limestone gorge of that name. It reproduces well, divides well and takes easily from cuttings.
"Within the silver or encrusted group there are tiny little treasures, including S. x anglica 'Christine', S. cochlearis 'Minor' AGM and S. 'Doctor Clay'. These flower early and are ideal for troughs, crevice gardens or the corner of raised beds.
"The main reason people like Saxifraga is because the flowers are stunning. Something like S. Southside Seedling Group AGM, with its long arching stems and great plumes of white flowers, looks dramatic on a rockery. But there are also plants with good foliage - for example Saxifraga federici-augusti subsp. grisebachii 'Wisley' AGM. It has the most perfectly shaped rosettes - a very upper-class sax.
"There aren't really any drawbacks when growing Saxifraga. Most survive well in the garden but there are a few that need to be in an alpine house - for example S. 'Wisley' can be grown outside but performs best when plunged in sand under cover."
Terry Hunt, owner, Edrom Nurseries, Berwickshire "Alpines are not flavour of the month at the moment so we are selling more of the woodland type of Saxifraga. We sell about five or six hybrids of S. fortunei AGM - the woodland types are very attractive in leaf. For example there is S. 'Crystal Pink', which has pink leaves with splashes of white and green, and produces sprays of white flower in the late summer.
"Because there are types that flower in the early spring and others such as the S. fortunei hybrids, which can flower into later autumn, Saxifraga provide plenty of interest throughout the year. There are no drawbacks to growing Saxifraga - they are easy to grow, and most hybrids are hardy, though one or two might need a bit of shelter. The only problem they really have is that they can make the plants they are next to look dull."
Andrew Martinovs, Thompsons Garden Centre, Kent "We sell about half a dozen varieties. We used to have a vast range of alpines but they have decreased in popularity. Now we do get in plants in flower, so they attract customers.
"They are not difficult to look after in the shop. We try not to let them get too cold and damp. We promote them by displaying them in a wooden crate, using rocks and gravel to create an alpine garden - it makes it a good feature.
"We have found the mossy types are not so popular and Sedum seems to be edging Saxifraga in the popularity stakes these days, but when a Saxifraga is in flower they really sell."
SPECIES AND CULTIVARS
- Saxifraga x apiculata is a Kabschia hybrid that forms a wide, undulating mat of prickly green needles. It produces sprays of primrose yellow flowers. It is unusual in being able to cope with full sun all day without burning.
- S. x apiculata 'Gregor Mendel' AGM has small, shiny leaves and clusters of pale-lemon flowers on upright stalks in early spring.
- S. burseriana is considered one of the loveliest of the Kabschia. It forms a mat up to a foot across, with thick and spiny glaucous-blue leaves. It is one of the earliest flowering Saxifraga. Between February and March it produces large, single white flowers on red stems.
- S. 'Doctor Clay' is an encrusted type with intensely silver-grey rosettes of leaves. It has arching stems, carrying many white flowers.
- S. 'Elf' is a mossy type, forming a tight mound over which are produced the carmine saucer-shaped flowers between May and June. Ideal for the rockery or front of border.
- S. 'Esther' has rosettes of short, lime-encrusted leaves, which are dark red at the base. It has short stems bearing relatively large white flowers.
- S. federici-augusti subsp. grisebachii 'Wisley'AGM is larger than the species and produces pendent, carmine-red flowering stems above tight rosettes of silver encrusted leaves.
- S. fortunei AGM is a woodland species, producing a large clump of leathery, almost succulent leaves. It can be grown in shade and flowers in late autumn with a spray of tiny white stars.
- S. fortunei 'Black Ruby' a clump-forming plant, producing large, handsome purple-black leaves and carmine-pink flowers in October.
- S. fortunei 'Blackberry and Apple Pie' has apple-green leaves and red leaf stalks. It produces sprays of creamy-white flowers from October to December.
- S. Southside Seedling Group AGM has rosettes of pale-green, broad leaves. When rosettes are mature they send up a long stem bearing numerous white flowers, with red spots. After flowering the rosette dies, but new rosettes will already have been produced.
- S. umbrosa forms dense mats of rosettes with spoon-shaped, waxy-edged leaves and tall spikes of starry white flowers with red speckling in summer.
- S. umbrosa 'Clarence Elliott' AGM has dark green, spoon-shaped leaves with a prominent, greenish-white central vein. Rose-pink flowers are borne on red stalks in the spring. It can make an appealing ground cover.
- S. x urbium AGM is a mat-forming, evergreen perennial. It has rosettes of mid-green leaves with serrated edges. In summer it bears panicles of pale-pink flowers on tall, wiry stalks.
- S. x urbium 'Aureopunctata' is a cultivar of London pride, so a strong plant for part shade. It produces variegated rosettes and stems bearing sprays of white flowers.
- S. 'White Pixie' is a mossy type, bearing white flowers on short stems above a neat cushion of green foliage.