Growing Lewisias From Seed

Philip Baulk looks at this popular genus of showy plants.

Lewisias have been grown in cultivation for many years, but because they were generally considered to be difficult plants, they were usually to be found in the experienced hands of specialist nurserymen and dedicated alpine enthusiasts. They are now enjoying a much wider popularity, not only in Britain and the United States but also in many other countries around the world. The cultural requirements of the genus are now more fully understood and both plants and seed are becoming more readily available to the keen gardener.

There are some eighteen or so species within the genus, all of them originating from western parts of the United States and Canada, especially in the Rocky Mountains and surrounding districts. All have charm and interest, but there are two species which are particularly worthy of attention. These are L. rediviva, a deciduous plant often referred to as the 'Bitter Root', and L. tweedyi, which many consider to be the most attractive of all the Lewisias. Also available in nurseries and garden centres is a range of richly coloured hybrids which have been developed from selected plants of L. cotyledon. These have done much to increase interest in these charming plants, not only as excellent subjects for the alpine house but also as plants worth attempting in a well-chosen site on the rock garden.

Whilst I have grown L. tweedyi successfully out-of-doors, it is really a plant for the alpine house or cold greenhouse. Like most lewisias, it will withstand a moderate frost without harm, providing the plants are kept on the dry side during the winter months. Over-wet compost and poor air circulation are certain killers. L. tweedyi has large pale apricot flowers with a satin-like sheen, and flowers are produced primarily during April and May. A deep rose-pink form is also widely available but the pure white form is rather scarce. All can be grown from cuttings by removing side rosettes and inserting them in pure silver sand during March. I would favour producing plants from seed as they appear to live longer and develop into healthier plants.

September is an ideal month to sow seed of L. tweedyi. A proprietary seed-sowing compost can be used, to which can be added some extra flint grit, perlite or shredded leaf mould to create a very well-drained mixture. I use large clay pans in which I place some drainage material and then compost to within 1 inch (2.5cm) of the rim. This is then levelled, thoroughly watered and allowed to drain. A very thin layer of silver sand is then sprinkled on the surface so that it is easy to see the seed when sowing. The seeds can be space sown or scattered thinly on the surface, and then covered with about ½ inch (1.2cm) of fine flint grit. The pans are placed in a shaded cold frame until the end of December, by which time they should have been subjected to freezing temperatures. Early in January I move the pans into a well-ventilated greenhouse with frost protection. The compost is kept moist but not saturated as a few seedlings may already be emerging. Germination of all forms of L . tweedyi is very erratic. Sometimes they will emerge in large numbers in January and February. In other seasons they may germinate in small batches throughout the summer, and on one occasion, nothing came up until the second spring after sowing, at which time germination was superb. My advice then is to keep your pans for at least eighteen months and as seedlings become large enough to handle, carefully prick them out into individual 3 inch (7cm) pots using any good well-drained potting compost. Then water your seed pans and set them aside for more seeds to germinate. The young plants must be carefully watered at this point as over-watering can be fatal in the early stages. By August or September, the plants should be well established and ready to pot on again, this time into 4in (10cm) pots, using a loam-based compost with extra grit and leaf mould for improved drainage. L. tweedyi is best over-wintered in a cold, well-ventilated glasshouse and should flower well the following spring. The plants will increase in size annually and will flower well for several years.

L. rediviva is a very different plant and in many ways behaves rather like a bulb. It must be kept completely dry from the end of May until October or November, so in Britain, this means alpine house cultivation. Pot grown plants are top dressed with grit, and in September, a fresh green rosette of leaves appears through this top-dressing, and this before any water has been given. Watered sparingly from mid-autumn, the leaves grow slowly at first, quickening in pace as the New Year advances. By the end of April, flower buds appear and as the foliage begins to fade, large 2 inch (5cm) pink flowers like small radar dishes unfold. There is also a white form in cultivation but good plants can be hard to find. The 'Jolons Strain' of L .rediviva is a particularly good form with really large flowers. Seed ripens quickly after flowering and the plants become completely dormant again until autumn.

Seed is sown in September in the same manner as that of L. tweedyi. When the seedlings of L. rediviva emerge however, they are not pricked out but allowed to grow on for the first season in the seed pan.

By careful watering, they can be kept growing through the first summer without becoming dormant, and this seems to do them no harm. In the following October, they can be carefully removed and separated. At this stage, they have long orange tap-roots and look very like young carrots. Pot them into individual 3 inch (7cm) pots, using a very well-drained potting compost containing at least forty per cent grit. Finish the top third of each pot with pure flint grit. A few plants will flower in their second season, the rest waiting for another year before producing their striking flowers.

Undoubtedly the most popular lewisias are found in the cotyledon group, and it is here that the most brightly coloured hybrids have been developed. A good mixed strain will produce very many colours and shades including pinks, rose, apricot, orange, red, yellow and pure white. Some flowers are edged in white whilst others are delicately penciled in rose pink. In fact the only colour missing is blue. Plants are evergreen, usually multi-rosetted and flower predominantly in April and May, although young plants, especially yellows, will flower almost non-stop for their first two summers. They can be grown in the rock garden or scree, or planted vertically in a dry stone wall, but to achieve real perfection, they are best grown in pans in a cold, well-ventilated greenhouse. With annual re-potting and reasonably careful culture, the plants will easily live for ten years or more and flower profusely every spring.

I sow seed of L. cotyledon 'Ashwood Strain' any time between November and February. Plastic seed trays can be used and these are filled with a peat based seedling compost to which I add a little perlite for extra drainage. The trays are watered well and allowed to drain. Sow around two hundred seeds per standard seed tray, less for smaller containers. The seed is covered with a light dressing of fine grade flint grit and the containers placed in a shaded cold frame. A good freezing would appear to improve germination and this may take anything from fourteen days to several months, although a month to six weeks is more usual. When a good number of seedlings have appeared, the trays may be moved into a glasshouse with frost protection if this is available. This will speed the growth of the young plants, but they will grow on quite happily in the cold frame, albeit more slowly. When large enough to handle, the seedlings are pricked out, twenty-four per standard seed tray being about right. Any good potting compost is suitable at this stage but add a little extra grit or perlite to improve drainage. When the plants begin to touch, pot them on into 3½ inch (9cm) pots using a loam based potting compost with added grit. Water carefully at all stages and allow the plants to become just a little dry between waterings. If sown early, many plants should flower in the first summer after sowing, the rest waiting until the following spring. When well established in these pots, they may be planted in permanent positions in the rock garden or grown on under glass to produce really impressive plants which will be literally covered in flowers each spring.

There are several other species of lewisia which the alpine enthusiast may wish to attempt. Whilst some are quite easy, others are very tricky and also difficult to obtain. Two deciduous species which should be readily obtainable from alpine nurseries are L. longipetala which has pretty shell pink flowers and L. nevadensis which is white and ideal for growing in an alpine sink. Amongst the evergreen species, L. columbiana and L. canfelovii should not be difficult to find, and all can be grown from seed. When several different species of lewisia are grown in close proximity, honey bees and other pollinating insects will almost certainly affect pollination. If the resulting seed is collected and sown, some interesting hybrids will occur. These offspring will themselves be sterile and it may be difficult to reproduce them from cuttings, but they do generally flower profusely due to their hybrid vigor and lack of seed production. L. tweedyi, however, has so far refused to receive pollen from any other species and has remained pure. In some ways this is rather a pity because who can tell what beauties might emerge from a union with another species of these fascinating plants.