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CULTIVATION OF SOUTH AFRICAN BULBS

Introduction

The term “bulb” is being used on this website to describe true bulbs, corms and tubers; a more accurate but less familiar collective name for these plants is “geophyte”. Such plants have adapted to life in a climate with seasonal rainfall by disappearing below ground during the dry season. The subterranean organ is a gene bank and food store that enables flowering and reproduction when conditions are favourable during the wet season. Thus they have developed a different but complementary survival strategy to succulent plants for coping with life in areas that have prolonged dry spells. Bulbs can be found growing wild over much of South Africa and thus it is not easy to generalise about cultivation. The most fundamental division into two groups for cultivation is based on whether the bulbs are from the winter-rainfall areas of the Western and Northern Cape Provinces or from the summer-rainfall areas of most other parts of the country. The greatest diversity of geophytes has developed in the winter-rainfall areas and these are by far the commonest in cultivation. My sales lists indicate whether plants are winter or summer-growing, otherwise consult some of the reference books. Frequently encountered summer-growers include Clivia, Cyrtanthus, Crinum, Eucomis, Zantedeschia, Gloriosa, Nerine, Ledebouria and Rhodohypoxis, but even in certain of these genera there are some winter-growers.

Watering

Summer-growing bulbs are treated like many more familiar greenhouse plants. Most are kept dry over winter, although those with evergreen foliage appreciate a little water. Watering is commenced when greenhouse temperatures start to rise in March and is continued until about October; yellowing of foliage often indicates the end of the growing season. Allow the compost to partially dry out between waterings and feed regularly with low-nitrogen, high-potassium liquid fertiliser. In many cases the pots of bulbs can be stood outside on the patio for the summer and stored under the greenhouse staging during the winter.

Winter-growing bulbs require a warm dry summer rest; if they do not get this they will sometimes fail to produce growth in the autumn. Watering is started at the beginning of September and continued through the winter until foliage starts to die in April/May. Watering in mid-winter should be done with caution and it is advisable to wait until a slight limpness in the leaves indicates that water is required. During the main spring flowering season, watering needs to be copious. Since the bulbs could originate anywhere from the relatively wet Cape Peninsula to the dry Richtersveld, a little research about your bulbs could help you to tailor watering to particular needs. I give very little feed to these bulbs once they are flowering-size because under poor UK winter light levels the foliage and flower stems tend to elongate too much if growing conditions are not sufficiently austere.

Light

During the winter, the growing bulbs will need all the light that you can provide for them in order to keep their typical characters. Some shading applied towards the end of March will help to keep them in growth for as long as possible and therefore produce large flowering bulbs for the next season. The summer-growing bulbs will also benefit from the shading if it left on the glass until late summer; Clivia and Scadoxus are forest plants and need to be particularly well shaded.

Temperature

In the main, the hardiness of South African bulbs in Britain has not been tested, although a few examples such as Nerine bowdenii, Amaryllis belladonna and Rhodohypoxis milloides have been shown to be happy in the open garden, at least in the south of England. Therefore it is advisable to grow your bulbs in a frost-free greenhouse until you have enough to experiment with. In the wild, many montane species from areas such as the Drakensberg, Cape mountains, Kamiesberg and Roggeveld plateau experience significant frost at times and should be temperature hardy. Therefore try growing spare bulbs in a cold frame or unheated greenhouse with either a free root run or pots buried in a plunge; even Eurasian bulbs can sometimes be killed if their roots are frozen. I have grown some Lachenalia, Tritonia and winter-growing Gladiolus for several years in an unheated Access frame in this way. Overhead glass enables control of watering and resting seasons can be respected. Ventilate the greenhouse or frame whenever possible during the winter. In summer, maximum ventilation is needed to stop the temperature becoming too high, certainly try to keep it below 35^oC.

Potting

In the wild, South African bulbs grow in a range of different soils, but in nearly all cases the soils are well-drained and relatively low in nutrients; this should be reflected in the potting medium. My current mix consists of 2 parts 4mm quartzite grit, 1 part John Innes Compost No.2 and 1 part ericaceous compost, but other free-draining mixes will be suitable. Repotting should be carried out during the dormant phase and I try to repot my bulbs every two years. Old roots and tunics are cleaned away and the bulbs in most cases can be put back in the pots quite close together, for example 25 Lachenalia in a 13cm pot. The only exceptions are bulbs with broad prostrate leaves such as Massonia, which need room for the foliage to develop naturally. Plant bulbs quite shallowly in the first instance: Amaryllids (members of the family Amaryllidaceae) with their nose at soil level and others with it about 3cm below the surface. When you subsequently repot certain species, you will notice that they have pulled themselves deeper in the soil by means of contractile roots; these bulbs should be put back at the depth that they have indicated they want to grow. The flowering of most amaryllids is inhibited by repotting and therefore when you think that bulbs of these are getting close to flowering size they should be left well alone. Furthermore, a few amaryllids such as Boophone disticha, Cyrtanthus obliquus and C. falcatus grow with their bulbs above the soil. I normally use plastic pots for bulbs in the frost-free greenhouse but clay pots or aquatic-plant baskets for material plunged in the cold frame.

Pests & Diseases

Viral diseases are common among commercial bulb stocks and usually manifest themselves as pale streaks on the leaves and perhaps distorted flowers. There is no easy cure and such material is best destroyed. Viruses are carried between plants by greenflies, therefore any infestation of these should be immediate eliminated using a suitable insecticide. Red spider mites are not normally active during the winter months, but the mites often attack the leaves of summer-growing bulbs. Their presence is revealed by a yellowing of the leaves and a very fine web, usually on the undersides of the leaves; the mites themselves need a magnifying glass to be seen. They easily become pesticide-resistant but currently, preparations containing bifenthrin are effective. Mealy bugs sometimes occur among old scales around the tops of bulbs; this is one reason for cleaning away dead scales and tunics when repotting. If mealies are present then treat with a suitable insecticide. Finally, flowers that appear during humid weather in autumn and early winter are often infected by botyritis as they die. If the dead flowers are not removed, the botrytis can transfer to the leaves and eventually to the bulb, with fatal consequences.

Vegetative Propagation

Many geophytes produce underground organs that divide or produce tiny offsets. These can be separated out at repotting time and thus stocks of a single clone built up. Where this does not happen, true bulbs can be forced to produce offsets by twin scaling or removal of the base plate. The grower should refer to a specialist manual if these techniques are to be attempted. An easier technique is leaf cuttings, which have been shown to work for some Lachenalia, Eucomis and Haemanthus. Cut off half a leaf as soon as it is well developed, but fairly early in the growing season, and insert the cut end into gritty compost. If all goes well, it will root and produce a number of bulbils by the end of the season. Any virus infection will be transferred during vegetative propagation, that is why seed is to be preferred.

Seed

I sow seed, just a few millimetres deep, in the same compost as used for adult plants. A packet of 25 seeds will go in a deep, 7cm square plastic pot. Summer-growing bulbs are germinated in gentle heat during early spring. Winter-growing bulbs will germinate when temperatures are falling, therefore these are sown in early September and placed in a shady cold frame. Kept moist, they will usually germinate by the end of October when they are brought into the greenhouse. Occasionally seeds will not germinate during the first year; any empty pots should be dried off and then watered again at the start of the next growing season. Space out bulblets during their first dormancy and many will flower at two or three years old; feeding during the early stages shortens the time to flowering. Seeds of all South African amaryllids except Cyrtanthus are viable for just a few weeks. Even if the seeds originated in South Africa (and consequently available at the wrong season for us), they must be sown as soon as available and conditions provided that mimic those that would prevail at their normal germination time.

© Terry Smale