Kalmia produce exquisite cup-shaped flowers but unfortunately remain little-known. This is partly because they are acid lovers, which reduces suitable locations. They are also difficult plants to grow commercially, not doing well when container-grown and not usually producing that uniform look wanted by garden centre customers. But for gardeners with the right conditions, and the will to try to something a little unusual, Kalmia is definitely worth a try.
There are seven species, most originating in eastern North America, with one from Cuba. They are nearly all evergreen shrubs of medium size, flowering in spring and early summer. The flowers are stunning when open but even in bud they are striking. Many have likened the buds to childhood favourites "iced gems" because they have the same ruffled shape as the hard piped candy atop those little biscuits. Once unfurled they reveal parasol-like flowers, often with delicate markings in contrasting colours.
K. latifolia, also known as the mountain laurel, is by far the most popular species. The state flower of Connecticut, it provides spectacular flowering displays in spring in the New England countryside. It is an evergreen shrub or sometimes small tree with glossy green leaves, which are dull yellow underneath.
Its flowers, produced in clusters, can be white, pale-blue or deep-rose. There are more than 40 cultivars of this species, widening the colour range with shades of pink, red and purple. Some also have interesting darker markings of cinnamon or deep red. There are some shrubs too, selected because of their dwarf form with a dense, compact habit.
K. angustifolia Award of Garden Merit (AGM) and K. polifolia are both bog plants from North America that can spread to form thickets. K. angustifolia AGM is the more compact of the two.
K. angustifolia f. rubra AGM produces rosy red flowers from May into July. K. polifolia is much shorter — under knee height — and has clear pink flowers from late spring into early summer.
Suitable for planting in open woodland, the woodland edge or in the shrub border, Kalmia can be used to create hedges or barriers. The smaller K. polifolia is a good choice for a bog garden. If grown on slightly drier soil than it is used to, it will be a neater plant, dense and compact.
They grow well in acid-to-neutral, moist, but well-drained soils — even the boggy species. The RHS says
they can be planted on sandy soils if they contain plenty of organic matter and are lime-free. They grow in full sun or dappled shade and are all hardy down to -30°C. Mulch annually with acidic organic matter such as leaf mould or pine needles.
Pruning is not always necessary but older K. angustifolia AGM specimens may become bare at the centre. Should this be the case, prune hard, mulch and feed, and new growth will break from the base. K. latifolia needs a lighter prune. To rejuvenate, cut back just two-to-three branches to within the canopy and be aware that replacement regrowth will be slow.
What the specialists say
Peter Chapman, managing director, Perryhill Nurseries, East Sussex
"Kalmia grow well in our locality as they need an acid soil that is moist but well-drained. K. polifolia grows naturally in swamps and boggy places but I have only ever grown them in a moist but well-drained soil. They flower best in a sunny site, though will tolerate part shade, and the flowers are reminiscent of iced gems (the confectionery biscuit with icing on the top).
"K. latifolia and its cultivars are excellent medium-sized evergreens and generally flower in June. There are some superb specimens at Sheffield Park. There are quite a number of selected forms but for some reason they are not widely available in the UK, except by a few specialists — more are listed by Dutch growers.
"K. angustifolia f. rubra is more compact, to about 1m, and forms a thicket with rosy red flowers in May, June and sometimes into July. K. polifolia is a low-growing twiggy shrub with quite bright, light-purple flowers in late spring early summer. I think that they are often overlooked. We only grow the above and really don’t have any cultivation problems with them."
Kenneth Cox, managing director, Glendoick Gardens, Perthshire
"Kalmia are mainly grown for their exquisite flowers — the usually deep-coloured buds gradually open out like miniature parasols in June to July over several weeks. Kalmia like an acid or peaty soil and in Scotland they need full sun to bloom freely. They associate well with rhododendrons and other ericaceous and woodland plants.
"K. latifolia is by far the best-known species, with thick evergreen leaves that are inclined to yellowness. There are some excellent cultivars, many of which were bred by Karl Hubbers in Germany and Richard Jaynes in the USA. I think the best are ‘Peppermint’, ‘Mitternacht’, ‘Eskimo’, ‘Olympic Fire’, ‘Kaleidoscope’, ‘Bandeau’ and ‘Nani’.
"The flowers are some of the most attractive of any shrub, but these are tricky plants to produce commercially. They have brittle branches, an untidy habit and often chlorotic foliage, and they dislike container growing. In addition, they tend to take a few years to produce flower buds. So none of the major UK wholesale growers offer them for sale, not even ericaceous specialists.
"We grow them in the open ground in pure peat. They grow almost too well in this mixture and can be hard to lift and pack. They can suffer from petal blight in warm wet weather."
John Winterson, deputy plant buyer, RHS Plant Centres
"We love Kalmia and always struggle to get them. They are one of my favourites because the flowers remind me of iced gems — the small round biscuits you used to get with the iced tops.
"Our top seller by far is the straight species, K. latifolia — a fantastic feature plant for any garden with room for it to grow. Although there are many variations, with named cultivars, I think the problem is that most gardening books only ever mention the straight species, so this is the one our customers ask for — and nothing else will do."
"We can sell K. latifolia without flower, as it is a demand plant, from our shrubs A-Z area year round. But the named cultivars we sell as larger plants, usually four-litre or seven-litre, with flower to show off the fantastic colours, from both the A-Z area and from display tables.
"They usually have good colour labels as well. Our range varies but usually includes ‘Galaxy’, ‘Pinwheel’ and ‘Peppermint’. The other Kalmia that sells well is K. angustifolia f. rubra, but it is not as showy as the latifolia types.
"If we can get consistency of supply then K. latifolia sells all year round. The rest are generally sold in flower in May to June. RHS Garden Wisley has a stunning old plant of K. latifolia in the "Wild Garden" that doesn’t fail to amaze visitors each year when it is covered from head to toe in flower."
Species and varieties
K. angustifolia f. rubra AGM (H6) is a suckering evergreen shrub, forming a mound of erect stems
with narrow leaves and clusters of rosy red, bowl-shaped flowers in early summer. More compact than K. latifolia. Height: 1m.
K. latifolia, also known as the mountain laurel, is a medium-sized evergreen shrub. It has dark-green glossy leaves but its best feature is the flowers, which are produced in midsummer. In bud they resemble iced gems and once open they are possibly even better, like pretty oriental umbrellas. Many cultivars have been bred from this species. Height: 2.5m. Spread: 2m.
K. latifolia ‘Bandeau’ produces pink buds that open to reveal striking large, just off-white flowers, marked with a dark-maroon band sporadically decorated with tiny maroon dots. Its twigs are coppery orange to red. Height: 1-1.3m.
K. latifolia ‘Eskimo’ has pure white flowers, considered by some to be the best white on offer. Height: 1.35-1.8m after 10 years.
K. latifolia ‘Kaleidoscope’ is a compact shrub that has red juvenile growth and later deep-green foliage. Its flower buds are dark-purple to red. Once the flowers open they appear slightly lighter in colour,
with contrasting white margins. Height: 1.35-1.8m after 10 years.
K. latifolia ‘Mitternacht’ is a compact shrub with deep-green foliage and the darkest-red to purple flowers of all. Height: approximately 1.35-1.8m after 10 years.
K. latifolia ‘Nani’ is a lovely variety with white flowers and attractive maroon-purple markings. Height: 1.35-1.8m after 10 years.
K. latifolia ‘Olympic Fire’ AGM (H6) has dense foliage and produces large clusters of ornate, vibrant pink flowers from late spring into summer. They contrast brilliantly with the highly glossy, leathery leaves. Height: 2.5m. Spread: 2m.
K. latifolia ‘Peppermint’ has a compact, somewhat upright habit and produces lots of pale-pink flowers with red markings, like the spokes of a wheel.
K. polifolia is a low-growing, twiggy shrub with clear pink flowers from late spring into early summer. It grows in boggy conditions in its natural habitat. Height: 60cm. Spread: 90cm.
Thank you to Floramedia, which supplied the images for this article from its photo library www.floramedia-picture-library.com