British Cactus and Succulent Society
by David Greenaway
Why do some of our succulent plants insist on growing in our winter, under glass, while the others are summer growers? You can water the former in the summer, along with everything else, and they either rot off or they just sit there; either way – no growth. Then, when they decide to, they start growing, and at more or less the same time every year.
It seems that succulent plants from any of the parts of the world that have a Mediterranean climate, with its hot, dry summers and milder, wetter winters, tend to a greater or lesser degree to be winter growers, both there and here in our collections. This climate regime occurs on the western side of the continents and at latitudes between 32 and 38 degrees north and south. SW Africa, California (and part of Baja California), central Chile, parts of Australia's southern coastline, and the Med. itself, cover the obvious areas.
Plants from places that have little rain at any time, but with what little there is falling in the winter, such as the Canary Islands, Djibouti, and the northern Somalian coast, fall into this category also. The coastal areas of north-east Brazil have an anomalous winter rainfall; could this be the reason for the summer dormancy of the Micranthocereus? (see later).
[Rainfall information from 'The World Weather Guide' by E A Pearce and C G Smith]
Many plants in all of these regions, not unreasonably, grow in their cooler, wetter season, but not in the hot, dry one. At those latitudes in winter, the sunshine is substantially stronger and the days longer, than here in the UK.
Even the ‘winter growers’ go into to torpor in mid-winter here; Autumn/spring growers would be a better name, or simply ‘summer dormant’ plants. The only reasonably comprehensive list I have seen is in the section ‘dormancy’ of the document at Highlands Succulents Culture Guide
Note that they say that there are a few exceptions for other species. We need to know those as well.
In addition to the genera that are listed in that guide, I have some other summer-dormant plants:
- Pachypodium namquensis;
- Euphorbia cylindrica, loricata, royleana, regisjubae, balsamifera;
- Crassula (especially: alstoni, namaquensis, deceptor, rupestris, ausensis, barbata, mesembryanthemopsis);
- Aloe variagata, brevifolia, pillansii, plicatilis, ramosissima, dichotoma;
- Hoodia gordonii, pillifera (up to now, though, I have treated these Hoodias as summer growers, successfully);
- Stapelia hirsuta;
- Ceropegia fusca, dichotoma;
- Umbicilis horizontalis.
From bi-modal rainfall areas (spring and autumn):
- Euphorbia ferox, enopla, meloformis, horrida;
- Aloe humilis, longistyla;
- Gasteria elliphieae, glomerata, bicolor;
- Faucaria tigrina;
- Haworthia limifolia;
- Euphorbia aeroruginosa.
In summer (May to August) I put all of these in a shady part of the greenhouse and water them sparingly; they are watered freely at other times of the year, save in mid-winter (mid-Nov. to mid- Jan.).
[The species information above uses rainfall and habitat information from 'Regions of Floristic Endemism in South Africa' by Abraham E van Wyk and Gideon F Smith]
A guide that I have found to very useful is at the Exotica site: Exotica
It looks as if Speks has included all the succulent plants that he grows, and all that he used to grow. For each species the watering for each month is given - normal, less or dormant (no water?). Some that I were calling 'winter growers' are dormant in the winter months of (at least) November, December, January and February. Some plants are dormant in May, June, July and August. The rest are watered throughout the year, but usually less from November to February inclusive. Minimum teperatures are also given, another factor which can be vital if cultivation is to be successful.
Some cacti from central Chile manage autumn and spring growth under glass here, so perhaps we should treat them as summer dormant and take more care with watering at that time? My Neoporteria wagenechtii regularly flowers in the autumn, until November, and N. microsperma buds up in the autumn, but waits until January and February to flower.
Keith Grantham and Paul Klaasen, in their book Plantfinders Guide to Cacti & Other Succulents have another table of growing and flowering times. They note that some tropicals such as Rhipsalis and Lepismium have a short summer dormancy, and they note that Micranthocereus is a winter-grower and flowerer, and has a long summer rest.
The leaves of my Pereskias start yellowing off in November, and have all fallen, by January; very careful watering is needed until new leaves start in April. Over-water during this dormant period is to risk loss - as I once did. True, this one is not a 'winter grower', but it is an example of the need to know the resting periods for all of the plants that we grow.
BUT all that was for Pereskias in a greenhouse having minimum teperatures of 10 degrees. One plant kept indoors recently was in leaf all winter (and so got watered). So, for tropicals at least, there is a marked temperesture dependence.
An expert writing in the CSSA journal informed us that Pediocactus bradyi and Sclerocactus, along with some of the other North American cacti and Mammillaria tetrancistra, should be kept dry from April 15 to July 30. I was aware that the rains come at the begining of August; I was on a field trip just west of Pheonix on August 8th 2005, expected a hot dry desert, but was surprised to run into the tail end of a thunderstorm.
More publicity about dormancy is required else novices – and others – will continue to suffer the disappointment of unnecessary loses. When all is said and done, though, there is no substitute for careful observation of the plants in the collection.
Steve Hammer, in his The New Mastering the Art of Growing Mesembs, in Section 1.1, 'Respecting the Annual Cycle', says:
Mesembs have an annual cycle which varies little from year to year. The sequence is genetically fixed; it does not go haywire when plants are brought from the southern to the northern hemisphere, though such plants are temporarily confused. Captive mesembs have no memory of, or loyalty to, the month in which they or their parents flowered. They are loyal only to day-length and should match the growing pattern of their forebears, but at half a year's distance. In a given horticultural ambience, many plants even flower on the same date year after year and go dormant at the same point.
DSG. Wantage, UK