Terry Smale

Xerophytes & Geophytes

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CULTIVATION OF LITHOPS

Introduction

Lithops is just one genus in a largely southern African family called the Mesembryanthemaceae, often just abbreviated to mesemb. There has been debate as to whether this should be recognised as a family and it is often incorporated into the larger plant family Aizoaceae. There is great diversity of form among the mesembs, ranging from highly evolved miniatures such as lithops through to relatively normal looking plants such as the annual Livingstone daisy Dorotheanthus bellidiformis.  However, if one compares the flowers of lithops and Livingstone daisies, the relationship becomes apparent. Moreover, they share the characteristic of their flowers only opening during the day time when the sun is shining, a feature that has given rise to the common name “mid-day flowers” for the family. In between the two extremes, many mesembs have adopted the growth form of low shrubs with thickened leaves and in some parts of South Africa these form the dominant part of a vegetation type known as vygieveld; vygie or little fig being the Afrikaans name for members of the family. Some of these shrubby mesembs, for example lampranthus and carpobrotus, have become naturalised in milder parts of the British Isles such as coastal south Devon and Cornwall.

Lithops occur sporadically in Namibia and much of South Africa, apart from the wetter southern and south-eastern parts. This distribution area mainly experiences summer rains and dry winters, although a few species grow in the winter-rainfall area of the Western and Northern Cape. In practice, all lithops can be treated as summer-growing for cultivation purposes. As with plants in most parts of the world, lithops are under threat from agriculture, mining, urban development and commercial collecting; so far they are managing to survive apart from a variety of L. aucampiae that has disappeared under asphalt.  Certain species, for example L. lesliei which occurs in Gauteng and neighbouring Provinces, have a very wide distribution with many individual colonies; others such as L. viridis are known from only one small wild population and ideally need a greater degree of protection. Lithops are small, slow-growing and long-lived; they could not compete against more vigorous plants and therefore tend to inhabit marginal areas where strong-growing plants with their greater resource requirements could not survive. Consequently, they are usually found on stabilised stony flats or on rocky hillsides, where they grow with a range of other dwarf succulents. The colours and patterns on lithops enable them to blend in with the surrounding stones and render them cryptic. They are found on a wide variety of rock types and the different selection pressures associated with each type have resulted in the plethora of different leaf designs. A simple example is L. marmorata which has white to pale grey leaves as a result of growing among white quartzite pebbles.

The first lithops was discovered in 1811 by William Burchell, but it was only in the twentieth century than many species started to enter cultivation. A number of personalities have studied lithops over the years, but the outstanding contributor to the field is Professor Desmond Cole. By profession he is a student of African languages, but in the late 1950’s he became interested in lithops and his spare-time researches on them culminated in the now-scarce monograph published in 1988. With his wife Naureen he tracked down nearly every recorded wild population and discovered very many new populations in the process, all of which were carefully recorded. With full approval from the appropriate authorities he took plant specimens from nearly four hundred different localities together with samples of the soil and stones in which they grow. The plants were then maintained at his nursery in their habitat soil and used as a basis for his extensive published studies. When plants were in flower, they were covered with a fine gauze “lithops contraceptive” and hand-pollinated for seed production. The majority of plants now in cultivation around the world have originated from this seed and a “C” number on their labels is testimony to their source. Through this work, all Lithops taxa are readily available as cheap seed-raised plants and selective collecting pressure on wild populations has eased. Unfortunately, much C - number seed now in circulation is second or third generation produced by nurseries that take less care in its production than the Coles did.

 

Morphology

The term succulent is used to describe any plant that has tissue which is modified for storage of water; this is a useful survival strategy in areas where the rainfall is seasonal or unreliable. In a cactus the water is stored in its fat modified stems; the leaves are just reduced to protective spines. By contrast, the mesembs have normal stems but the leaves are fattened to perform the water-storage function. The shape of a lithops approximates to an inverted cone with a slit across the top. This is actually composed of a single pair of leaves which is almost completely fused, apart from that gap at the top. If one was cruel enough to break the plant apart, the growth buds that produce the flower and new leaves can be found deep down inside that fissure. The curious appearance of lithops has given rise to various vernacular names. In  the anglophone countries, “living stones” or “flowering stones” are the most common, but in their native lands there are more colourful expressions such as “beeskloutjies” (little cow hoofs) and “oogies” (eyes) or even less politically-correct names such as “Hottentottenpopos” (Hottentot's buttocks).

Currently,  there are 37 recognised species of Lithops and a further 53 subspecies and varieties. Furthermore, in the wild there is appreciable variation between populations and even within single populations, so there is considerable scope for forming a collection of these plants. There is some variation in leaf shape between species, but it is the incredible range of colours and patterns on the leaves that define the individual species and also make them so interesting for the enthusiast. Written descriptions refer to windows, channels, warts, islands, dusky dots and more, in an attempt to put in words those features that are more easily recorded in a photograph or painting. Very occasionally in the wild or in cultivation an interesting mutation can occur. For example an individual plant of a normally yellow-flowered species might produce white flowers because there has been a disruption of one of the genes responsible for the yellow pigment. Such variants are often stabilised as a seed strain and given cultivar status. The most sought-after cultivars are those in which the leaves have lost their brown pigment resulting in various shades of green, for example L. bromfieldii ‘Sulphurea’, and those in which leaves have acquired purple colours as in L. optica ‘Rubra’.

 

Growth

The annual cycle of a lithops begins in late spring, when in response to moisture the existing leaf pair (often known as a head or body) starts to increase in size. Depending on species, at some time between July and December in the northern hemisphere, a single flower bud will force its way through the fissure and open to a daisy-like white or yellow flower that can be as large as the plant body and lasts for about a fortnight. In one species, L. verruculosa, the colour is very variable and can even be red. Some time after flowering, a new pair of leaves starts to develop and as it grows through the winter and early spring, the old leaves are assimilated and dry up. This occurs during a period of drought for the plant and indicates the way in which they are economical in their use of water and nutrient resources. Sometimes, two new heads will develop inside the old one and by this method the plant will eventually form a clump; the record was a plant of L. salicola comprising over 350 individual leaf pairs. Most lithops are not self-fertile, but if two individuals are interpollinated when in flower, seed capsules will form. These take almost a year to develop and when fully ripe, they are quite woody. Seeds are well-protected in these cocoons until the capsules becomes wet, whereupon they open like stars to reveal the seeds inside; closing once more when dry. In the wild, opening would occur during a rainy period, when the raindrops would splash the seeds out of the capsule onto moist soil suitable for germination; yet another survival strategy for an unpredictable climate.

 

Light  & Temperature

Most lithops are easy to grow as long as their specific requirements are recognised, however there are two species, L. comptonii and L. viridis, which are notoriously intractable. Lithops require a very bright and sunny position at all times, but during particularly warm summer periods this must be accompanied by good ventilation to prevent scorching. In common with most succulent plants, the stomata only open at night to allow absorption of carbon dioxide. This normally occurs below about 18oC, therefore also maintain plenty of ventilation at night during warm weather. In the wild, some species experience frost for short periods. However, in cultivation it is safer to keep them frost-free and most growers try to maintain a minimum temperature of 4 - 5oC in their greenhouses.

 

Watering

The summer watering is started when the new leaves are well developed and the old leaves more or less dried up; in United Kingdom greenhouses this tends be sometime during May. Pots are allowed almost to dry out before the next watering and the process is continued until October. During high summer, watering will usually be required at least once per week. Pots are kept completely dry through the winter until the next May, but growers in warmer climates such as California sometimes need to give a very small amount of water during this period. Occasional feeding with a low-nitrogen liquid fertiliser during the growing season is advantageous.

 

Potting

Potting is best carried out just before the first watering is due and a well-drained loam-based compost is to be preferred; I use a 1 : 1 mixture of John Innes No. 2 and 4 mm grit. Peat-based composts are often used by commercial nurseries for propagation, but are not very satisfactory for long-term use. Remove all the old compost and do not worry that the fine roots are dead after the long resting period, they will soon regrow from the thicker permanent ones. Most enthusiasts use plastic pots, although a clay pot might be safer if you succeed in growing a large specimen. Very small pots are always difficult to manage and some growers prefer to put several plants into a larger pot. Sometimes seen at shows, an attractive extension of this is a large pan containing a number of specimens with the spaces filled by matching grit and pebbles. Plants will only need to be repotted about once every two years.

 

Pests

Potential pests are mealy bugs and western flower thrips but good hygiene helps to avoid these. Careful removal of old dried leaves eliminates hiding places for mealies and twisting off dead flowers removes possible food sources for the thrips. Thrips damage is sometimes seen as scarring on newly emerged leaves. If necessary, the usual range of garden insecticides can be used on the plants; imidaclopride (e.g. Provado) is particularly useful against mealy bugs and sciara fly. Poorly-drained soils or overwatering can lead to rots, but these occasionally occur in even the best-managed collections.

 

Propagation

It is possible to take cuttings of lithops, but there is not usually enough suitable material to make this worthwhile. Sometimes, very old plants become untidy and it might be best to chop them up and start again. Heads are cut off immediately beneath the leaf pair during early summer and potted in the soil mix mentioned above. Grown and watered with the mature plants, they will root quite quickly.

The usual method of propagation is by seed, which is available as individual species and forms for as little as 20p to 40p per packet from specialist society (such as the Mesemb Study Group) seed distributions or cactus and succulent seed suppliers. A well-drained, fine-textured compost is required, for example a 1 : 1 mixture of John Innes Seed and 1 mm grit. Sowing is carried out at any time in the spring when you can maintain a temperature of about 20oC and the seed is scattered onto the surface of the compost. Keep the pots moist and in a closed environment until germination takes place, this will only require about one or two weeks, then allow good air circulation to avoid damping off. Provide shade for the young seedlings during their first spring and summer and prick off into trays about a year after sowing. The 5 mm long, transparent, thread-like larvae of sciara flies can make a meal of young seedlings, but a sticky trap just above the plants will provide some control of the adults. If all goes well, flowers can be expected in two to three years from sowing. Suggested species for the first-time raiser are L. aucampiae, bromfieldii, hallii, hookeri, karasmontana, lesliei, olivacea, pseudotruncatella, salicola and schwantesii in all their subspecies, varieties and cultivars.

 

Current Literature

Cole, D.T & Cole, N.A. (2005) Lithops Flowering Stones. Cactus & Co, Italy.

Hammer, S.A. (1999) Lithops Treasures of the Veld. British Cactus & Succulent Society, UK.

 

Useful web site

Full list of Cole Lithops field numbers is on the Nick Rowlette site (under LLD)

All established Lithops cultivars are listed by Keith Green

 

Modified version of article originally published in the RHS Journal "The Garden".

© Terry Smale