The mesembs (sometimes called mesems) are a family of succulent plants properly named Aizoaceae. Although the family has roughly the same number of species as the cactus family, and although the majority of them are found in a single country (and almost all of them in two countries), it manages a huge diversity of growth habits and climatic conditions. There are alpine summer growers, dead-stick summer dormant species, large shrubs, and single leaf pairs no bigger than your thumbnail. So knowing how to grow them can be something of a challenge. This article will discuss the basics of cultivation for mesembs while individual growth cycles and habits will be explained in the description of each genus.
Mesembs share a few common traits that will be found throughout the family. One is that all are leaf succulents with the frequent habit of recycling resources from older leaves to new growth. They are mostly adapted to relatively predictable rainfall patterns rather than extreme drought and irregular rainfall. Total rainfall may be extremely low, but water is available at least seasonally or through fog and condensation. This leads to or allows plants which are not especially large and sometimes very small, and affects the way they need to be treated in cultivation.
There are a small number of annual or very short-lived species. Most are rare in cultivation, although Dorotheanthhus has become a staple bedding plant. Despite the limited number of species, one of them now has the largest range of any mesemb. Mesembryanthemum crystallinum has colonized virtually every Mediterranean climate zone in the world, probably with the help of seafaring man. The growth habit of these ephemerals tends to be somewhat sprawling with large moderately succulent leaves. The lifecycle is commonly one of boom and bust, a period of rapid growth followed by an extended decline usually coinciding with flowering. Cultivation is quite easy. Treat them like a normal plant, provided they get good sun and are not waterlogged, until they are mature and then just let them get on with it.
There is a much larger group of perennial shrubby mesembs, including well known plants such as Lampranthus and Delosperma. Cultivation of these is usually easy, provided only that they are given enough space. Mesembs haven't developed tree forms, or even true woody shrubs, but some of the shrubby species are large and sprawling, far more than most pots can handle. These large leafy plans, and also some smaller choice clumping species, are often divided into winter and summer growers, but this is something of an artificial distinction. Certainly in habitat some are from summer rainfall areas, some from winter rainfall areas, and some in between, but they readily adapt to different climates. Growth will be vigorous during cool to warm conditions when water is plentiful, but they will become dormant in excessive heat and need less water. In hot climates, species like Lampranthus will grow strongly in winter, followed by spring flowers, while in cooler climates they will grow strongly in spring and flower in summer. Most Delospermas, and some others like Malephora, will grow best in slightly warmer conditions. With just a little care given to good drainage and a sufficiently sunny spot, these types of mesemb can easily be grown as garden plants. Most will take some frost and some Delospermas are extremely cold hardy.
Although there are many hundreds of shrubby mesemb species, the main interest in cultivation is in the smaller mesembs with more reduced growth habits such as Conophytum and Lithops. The opposite leaf pairs produced along the stems of the shrubby species are reduced to tufts of a few leaf pairs on almost stemless plants, and ultimately to a single pair of leaves with the stem completely invisible. Perhaps the best known mesemb of this type is the Lithops, just a single pair of leaves that is replaced each year by a new pair. This reduced physical structure usually comes with a more or less strict annual growth cycle which can be key to successful cultivation. The mesemb habit of sacrificing older leaves to feed new ones becomes a strict rule where one new leaf pair is produced each year and the old pair dies off to maintain the same limited structure, hopefully a little larger than before. Deviating from this cycle is likely to produce an unhealthy plant and long term no plant at all. A common variation has two leaf pairs produced each year, but two very different pairs. Typically one leaf pair will be large and succulent, produced for growth and flowering in a cool wet season, while a smaller or even completely dried up leaf pair is produced for water conservation during a hot dry season. For obvious reasons getting this cycle out of sync can cause rapid death if the plant ends up with the wrong leaf for the season.
In common with most succulents, soil with good drainage is critical. A few of the annual or shrubby mesembs can tolerate periods in waterlogged conditions but most are likely to rot without excellent drainage. The most xeric mesembs inhabit grit pans and rocky hillsides as well as the iconic quartz fields. They avoid sand, and even the handful of species that do live in sandy places root in a dense or rocky soil underneath and just poke through the sand. The larger vigorous shrubs may inhabit richer soils, but for the most part it is best to avoid anything that a Petunia might survive in. Soil pH is usually not critical. Many mesembs grow in alkaline limestone based soils, but are happy enough in normal soil provided it is not too rich and drains well. Using a limestone soil can help to produce compact plants with strong colours and textures, or stunted plants in the opinion of another grower. A handful of mesembs do seem to react badly to alkaline soils, as well as to hard water, and certainly the more vigorous ones will do better with a soil containing some organic components. It is hard to use too much grit in mesemb soil and even the more vigorous species are usually happier with a large root run in a gritty soil than a cramped pot of peat loaded with fertilizer.
A major difference from many succulents is that mesembs, certainly the smaller and choicer cultivated species, thrive on frequent but very light watering. There are exceptions of course but a common technique for dealing with mesembs is to spray them frequently. Even in deep dormancy an occasional light spray can be helpful, and in hot climates it can be critical to the smallest species. Obviously some species are adapted for heavier more erratic rainfall, including the poster genus Lithops, but generally long periods of drought other than during dormancy are not helpful to mesembs. It can be difficult to match regular watering with the extremely low overall water needs of many mesembs. A small set of strict winter growing mesembs have a deep summer dormancy inside dried sheaths, followed by rapid lush growth when it is cooler, and these species can be extremely thirsty in leaf despite the cool humid season they choose to grow in. The difficulty here is simply believing that they should be watered every day in the depths of winter while they must remain dry in summer or run the risk of terminal rot.
Another requirement shared by nearly all mesembs is sunlight. Although a few may huddle in shaded rock cracks in a harsh rainless habitat, most will do best with as much light as possible in cultivated conditions. Frequently they can only be found on exposed rocky areas, including the painfully bright quartz fields. Only in the sunniest locations will mesembs need to be routinely grown in the shade. There are limitations to this exposure. Plants coming out of a long dark northern winter need to be acclimated again to the sun or they can scorch surprisingly easily, especially under glass. Sheathed dormant plants can be kept alive in summer by mimicking the frequent morning dew of habitat, but a simpler solution is just to shade them until it is cool enough for them to restart growth. It is extremely easy to boil a tiny mesemb in a greenhouse, so good ventilation is key. In very hot conditions it will be necessary to shade smaller species and those not adapted to high heat. Again Lithops are slightly unusual in the family and their unique structure makes them very sensitive to overheating in cultivation even while they crave strong direct sun. Sudden loss of colour is the warning sign, but a plant can be dead within hours.
While mesembs thrive in high light, none are adapted for extreme heat. Most hug the coasts of southern Africa where cold ocean currents produce fog and mist, or they compete to populate high ridges and mountains in the interior, while avoiding the hottest interior lowlands. Apart from the strict summer dormant species, many mesembs have adopted an annual growth cycle that is flexible enough to let them grow whenever it is moderately cool but there are still long hours of strong sun. This leads to almost the archetypal mesembs growth seasons of spring or autumn. These may be shifted earlier or later depending on the local conditions, but they stop growth in the hottest part of the year and also when the sun isn't strong enough. At its extremes this can lead to a single species being a winter grower in a very hot climate and a summer grower in a very cool one, so it is best not to be too strict about assigning a particular growth habit to a mesemb. As an example, the popular description of Lithops as summer growers and Conophytums as winter growers is clearly too simplistic when they grow side by side at many locations.
So the basics of mesemb care are very simple, with free-draining soil, plenty of sun and ventilation, and regular light watering in the right season. Yet the difficulties are endless, trying to adapt to the mesembs' own adaptability and to follow their growth habits in your particular conditions.
Author: Ian Nartowicz