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Kalmia - Growing Guide
Kalmias are evergreen, ericaceous shrubs native to North America. In the US their common name is ‘laurel’. They were first introduced to the UK in 1734 by the colonists but have only recently grown in popularity in woodland gardens since cultivars with new flower colours have been bred and introduced. Kalmias are still however (unfairly) perceived as being difficult and risky to grow in the UK. That may well be because they are still uncommon in gardens and because the situations in which they will thrive are not well understood. Kalmias are also toxic to animals (including rabbits and deer) if eaten.
Kalmia latifolia (and many of its cultivars) grow eventually to around 12ft tall with a similar or even greater spread. In Burncoose Garden there is an 80 year old clump which is easily this size. As robust evergreen shrubs they may be best viewed as similar to rhododendrons in their growing habit. K. latifolia is known as ‘Mountain Laurel’ or the ‘Calico Bush’.
Kalmia augustifolia or ‘Sheep Laurel’ is perhaps more widely grown in the UK because it grows only up to 3 or 4ft in height and is even hardier and more robust than K. latifolia.
What makes Kalmia special and worth a try?
Kalmias are undoubtedly the most beautiful of evergreen shrubs in North America where they are very widely grown in gardens and municipal locations. Certainly they rival rhododendrons in their displays which occur in late May or early June after most rhododendrons are over.
Individual kalmia flowers have five parts and are produced in showy clusters. Each flower has a peculiar way of achieving fertilisation. There are 10 stamens and the anthers are held back in cavities in the flower. When the pollen is ripe and the flower is touched by a passing insect the anther releases with a jerk sending pollen several inches into the air and onto the insect.
As ericaceous plants from hills and mountainsides their preferences are very similar to those of rhododendrons, azaleas and heathers. They like peaty acidic soil and cool or permanent moisture at their roots.
Kalmias will not tolerate alkaline soil, waterlogged soil or drought conditions in summer heat where soils are thin and prone to drying out.
They flower better if planted in full sun but they definitely need a sheltered site with protection from cold winds and a good mulching around the plants when first planted. It is also sensible to plant with plenty of organic feed in the planting pit itself.
Frost pockets and frozen ground should be avoided too although this is probably not a serious problem in southern England or in normal winters.
If your soil conditions are poor and alkaline then kalmias (like rhododendrons) can still be grown in upraised beds with improved soil and organic matter.
Excessive slow release or nitrogen based fertilisers should be avoided at all costs as their use will often cause the new growth to brown off and die. Rather like daphnes, more kalmias are killed by excessive ‘kindness’ than from any starvation in normal soils.
Do not attempt to move your kalmia either. Transplanting kalmias with delicate deep root systems can easily result in disaster.
Many kalmias sold today in the UK have been tissue culture grown. Others will have been grafted which accounts, in part, for the higher costs of purchasing these plants which can be difficult too for nurserymen to grow on to a saleable size.
Kalmia cuttings are generally very hard to propagate successfully both in the mist bench or cold frame.
Most kalmias do however produce such a plentiful crop of capsular seeds that, as with rhododendrons, it is advisable to remove most of them after flowering to avoid the wasted energy by the plant on seed production.
Kalmia seeds is best grown in exactly the same way as rhododendron seeds. A seed tray of soil and grit overlain with a covering of moss. The seeds germinate within the moss exactly as they would in the wild.
Pests and diseases
Although kalmias can be temperamental to grow if they do not like their selected location their natural insect enemies in the wild do not exist in the UK.
Avoiding frost damage, cold winds and drought are far more of a worry than any other particular pest of this particular plant.