Xerophyta retinervis

Xerophyta retinervis, or “Black Stick Lily,” is a flowering plant from the high-altitude interior of eastern South Africa, with tufts of coarse leaves emerging from a clump of fat stems with a fibrous coat. The stems are often charred by fires in the veld, giving rise to the first part of Xerophyta’s common name. The “lily” part of the name comes from the large cobalt violet flowers that seem to emerge from the stems and open overnight, and always catch me by surprise when they appear in spring. I have hardly ever caught the early stages of flower development, but it is less than a week between no visible flower buds at all and full bloom.

Xerophyta is part of the family Velloziaceae, which is a small, Southern Hemisphere group of monocots (plants like palms or grasses that typically have parallel leaf venation and a single seedling leaf). The Velloziaceae are related to Pandanus, the screwpines, a genus of tropical swamp and coastal trees.

Xerophyta retinervis is poikilohydric, like Myrothamnus, which I wrote about in a previous entry.  That is, it is a “resurrection plant,” which can become completely dehydrated, then soak up water and start growing again when conditions are favorable. In the greenhouse I’ve seen this happen once, accidentally: the leaves curled up and turned yellow, and the plant looked like it was finished, but it perked up again and looked fine a few days after watering.

The desiccation and resurrection process in Xerophyta has been the subject of a certain amount of research, in part because of interest in improving the drought tolerance of crop plants, and it turns out that the leaves actually break down all of their chlorophyll when drying, then rapidly synthesize new photosynthetic pigments when water is available again. In cultivation, it’s probably safest not to test this remarkable ability too much, and keep the plants well-watered during the summer growing period, and barely moist enough to keep the plant from dehydrating in winter. No matter how I treat Xerophyta, it does tend to drop a lot of its foliage in the cooler months.

 References:

Tuba, Z. et al. 1994. Planta 192: 414-420.

Clark, W.D. et al. 1993. Annals of the Missouri Botanic Garden 80: 987-998. 

Myrothamnus: an African Resurrection Plant

Resurrection plants—or plants that are poikilohydric, in biology-speak—are found growing in seasonally arid sites all over the world. One local Connecticut example is Rock Polypody (Polypodium virginianum), a woodland fern confined to thin to nonexistent soils, often on top of glacial erratic boulders, where it gets very dry very soon after rain. Whereas succulent plants hold onto stored water to survive drought, resurrection plants lose water readily and simply shrivel up when the going gets tough. However, they have the remarkable ability to rehydrate themselves, restart their metabolism and resume growth when moisture is available again. During a drought, a resurrection plant like Rock Polypody will get to the point where it is as crunchy as 10 year old rosemary from your spice rack, yet retain the capacity to spring back to life with the first rain.

A. Dry and seemingly expired Myrothamnus flabellifolius.

The majority of resurrection plants are mosses, ferns and other spore-bearing plants, but there are a couple of flowering plants that are poikilohydric, including Myrothamnus, which is apparently the only woody shrub that has the ability to resurrect. There are two species in the genus: Myrothamnus flabellifolius from arid mountains in Sub-Saharan Africa, and M. moschatus from Madagascar. These two species have a taxonomic family all to themselves: the Myrothamnaceae. A couple of years ago, I obtained a packet of seed of M. flabellifolius from Silverhill Seeds, and now have a good crop of plants to experiment with.

Myrothamnus seeds are tiny, so I planted them in a pot of loose peat moss and perlite mix, right on the surface. The plants come primarily from summer-rainfall areas, so spring is probably the best time of year to get them started. Germination is easy and quick in a seed pot kept in a moist, well-lit location.

Seedlings should be kept moist, in a sunny spot with good air circulation. Initial growth is slow, but as is the case with many plants, growth accelerates as the seedlings gain some size, and in two or three years Myrothamnus can become respectably shrubby. I’ve tried the plants in several types of soils, and they do fine in a range of mixes, with probably the best results achieved in the same airy, acidic mix I used for starting the seeds. Regular applications of dilute, water-soluble fertilizer (e.g., 20-20-20 at half the recommended strength, every 2-3 weeks) seem to be beneficial. The plants do best in full sun.

It’s generally not a great idea to try to test the abilities of resurrection plants in cultivation: they tend to wind up permanently dead if they are dried out too often, or possibly if they are dried out in the wrong way (weather, water quality, soil, and root run are probably quite different in a pot and in the wild, and affect the way dehydration occurs). It’s safest just to keep the plants growing, and as far as I know none actually require the growth –> pseudo-death –> resurrection cycle in order to stay healthy. Still, it’s tempting to play around with desiccating and reviving the little guys, and since I’ve wound up with quite a few Myrothamnus plants, I recently decided to experiment.

Left out in a sunny, dry greenhouse with no watering, my test Myrothamnus was thoroughly dry after about four days. The pleated leaves fold up and turn almost black as they dehydrate, and the plants look pretty sad when the process is complete. I let the plant sit out in the sun for two weeks before taking pity on it and breaking out the watering can. Overnight, the lower leaves expanded and turned green, and in subsequent days a wave of reviving shoots moved up the plant from the base to the apex. A few branch tips had not resurrected several days later, but even these perked up during an afternoon outside in a gentle rain. More extensive studies have shown that Myrothamnus can stay dry for six months with no ill effects, and still recover after nine months, although leaves are lost (Farrant & Kruger 2001). Dry plants give up the ghost for good only after a year baking in the sun. All of this raises an interesting possibility for the desert plant enthusiast with limited winter growing space: you should be able to grow Myrothamnus plants outside all summer, dry them off and toss them into the back of a closet for the winter, and revive them in spring.

B. The same plant as image A., a week after watering.

I've had some minor problems with insect pests on Myrothamnus. Mealybugs and aphids have recently been getting into the plants, so I have experimented with a new type of organic pest control, unfortunately applicable only to poikilohydric crops: let everything stay bone dry for a couple of weeks. It seems to have worked: resurrection plants spring back to life with a little water after an artificial drought; mealybugs not so much. As a bonus, some weeds in the Myrothamnus pots also croaked.

Literature:
Farrant, J.M. & L.A. Kruger 2001. Longevity of dry Myrothamnus flabellifolius in simulated field conditions. Plant Growth Regulation 35: 109-120.