Potting Bench Urban Legends

Every plant enthusiast keeps a mental library of horticultural rules-of-thumb. Some of these rules we learn from other gardeners, some we read about, and some we come up with on our own. Most of these tips and tricks are, in fact, perfectly sensible: you can start new jade plants by sticking a leaf in soil, you shouldn’t water a cactus immediately after transplanting it, you can control mealy bugs by dabbing them with rubbing alcohol. However, some frequently encountered horticultural rules don’t actually hold up to close scrutiny. Cactus and succulent growers are particularly concerned about drainage, and so many C&S urban legends involve insuring that excess water doesn’t stay in pots.

Urban Legend #1: Shallow pots drain better than deep pots.

All else being equal, this one is dead wrong. Soil holds onto water by capillary action, and a certain amount of pressure is needed for excess water to be forced out of the drainage hole of a pot. That pressure comes from standing water in a layer of saturated soil at the bottom of the pot. The depth of the saturated layer that can be supported before water starts draining off depends on the characteristics of the soil, not on the shape of the pot, so in a shallow tray the layer of soil that stays wet after watering is closer to the soil surface and takes up more of the soil volume than in a deep container. Of course, all else is rarely equal, and a shallow pot might actually dry out more quickly that a deep pot, because it holds less soil and presents more soil surface to the air for water to be lost via evaporation. But for efficient initial drainage of water, deeper pots are better.

Urban Legend #2: A layer of gravel or crocking at the bottom of a pot improves drainage.

If your concern is the unimpeded flow of excess water out of the soil after irrigation, a layer of “drainage material” is counterproductive. It’s a basic principle of soil science that a soil layer that is either less or more porous than the layer above it, will act as a barrier to the downward flow of water. Thus, a layer of highly porous gravel or potsherds actually impedes the movement of water in a pot, and leads to the development of a “perched water table” (a layer of soggy, saturated soil) above it. Another way of thinking about it is that filling the bottom of the pot with porous material essentially turns a deep pot into a poorly drained shallow pot. In certain situations, a pot with lots of crocking may dry out faster than one with just soil, but only because the total volume of soil is reduced, not by improving drainage. Minimal usage of stones or potsherds over the drainage holes of a pot is often desirable, however, simply to prevent soil from pouring out.

Urban Legend #3: Adding sand to a soil mix makes it well drained.

This one isn’t always false, but it isn’t as universally true as you might think, either. Some mixes that are heavy on sand are indeed very well drained, but I’ve also experimented with sand-based mixes that water would just sit on top of for half an hour before percolating into the soil. The type of sand makes a difference: sands with larger, sharp-edged grains tend to promote water movement, while sands that include a high percentage of very fine material, or with weathered, rounded grains (like beach sand) can clog a soil mix. Strictly from a drainage-improvement perspective, the best sands are probably the courser types used for sandblasting or filtration systems. However, drainage is not the only consideration when deciding on a soil mix, and sands with some silty, fine material can offer benefits, too, such as sequestering the nutrients and water needed for plant growth. So, experiment with the materials that are available to you, and don’t get thrown off track by horticultural urban legends.

Growing Conophytum burgeri

Last month, I introduced Conophytum burgeri—Burger’s Onion—and promised to follow up with some information on the cultivation of this South African desert oddity. I’ve been growing these plants successfully since my undergrad days, and while the Onion is not really an ideal subject for the mesemb newbie, it’s quite possible to grow and flower one on a windowsill, if certain conditions are met.

Conophytum seedlings at about two weeks. These are C. hammeri, a close relative of C. burgeri.

Burger’s Onion is first and foremost a sunlight pig. If you are going to attempt windowsill cultivation, make sure you can secure a spot in a south window that gets at least six hours of direct sun per day (an airy, well-lit greenhouse makes things so much easier…). In winter, I don’t think it would be physically possible for a C. burgeri plant to receive too much sun here in New England. In summer, a potted plant might have trouble if it cooked in direct sun all day, but shade can still be minimal. Ideally, the plants should spend winter in a place that gets quite cool at night, into the 50s or 40s.

Conophytum burgeri’s habit of hiding inside of the dried remains of its old leaves can lead to trouble. In the wild, I suspect that older leaf sheaths are weathered away about as fast as they accumulate, but under glass the annual layers of dead tissue just keep stacking up. Some growers apparently peel their plants entirely in the autumn, to expose the living leaves directly, but I feel that this creates an unnecessary risk of sunburn in the spring. When I have some time to kill, every year or two, I do like to pick away all but the innermost layer of sheathing on my plants, to let in light and air.

The watering regime for C. burgeri plants should be on the stingy side. For most cultivation situations, this means a light watering every week or so, when the soil is quite dry, during the winter growing period, and nothing but a bit of mist every few days during sunny weather in summer. If the plants get too much water, they puff up and split, and are likely to rot.

The soil mix for Burger’s Onion should be lean, with little or no organic matter: I use poor sandy-loam subsoil, liberally cut with course sand, perlite, pumice and/or Turface (fired clay granules). Mature plants get almost no fertilizer. A splash of quarter-strength Miracle Grow (or something similar) once a year is about as much as you can get away with without risking bloating, cracking, and rotting.

Conophytum burgeri plants can be obtained occasionally from Mesa Garden or Steven Hammer’s Sphaeroid Institute, and seed is usually available at Mesa or from the Mesemb Study Group. Growing from seed is rewarding, but slow: even under ideal conditions, it will take three or four years to get a plant the diameter of a dime. Germination is quick and easy for seed sown on the surface of moist soil in August or September, but the minute seedlings have to be coddled along for their first year of life. Water when the medium is just barely dry, and spray lightly with dilute fertilizer every few weeks. Otherwise, seedlings and adult plants prefer similar conditions: bright sun, cool nights, and plenty of fresh air.

Conophytum bachelorum in bloom

Back in September 2005, I sowed seeds of Conophytum bachelorum, which is almost unknown in cultivation, for many years represented by a lonely self-sterile plant in a private collection in the UK. Its native haunt, a single quartzite ridge "east of Port Nolloth, South Africa" was rediscovered in 2003, and in 2004 various botanists and I visited the locality, and gathered a few seeds (for the gory details, see: M.R. Opel, 2004, Bachelorum's Pad, Mesemb Study Group Bulletin 19:81-82). As far as I know, this two and a half year old seedling at the University of Connecticut Ecology and Evolutionary Biology Greenhouses is the first plant of its kind to flower in the Americas.

The flower snuck up on me: the largest of my C. bachelorum seedlings are still only about half the size of mature plants in the veld, and I thought for sure that the first flower would be in 2009 at the earliest. This is the right time of year, though. Unlike most conos, C. bachelorum blooms in spring, while the old leaves are senescent (as seen here) or even totally dry. It does look like this will be the only individual to flower for me this time, so seed production will have to wait until some fellow bachelors reach maturity, possibly next year.

Burger's Onion

The genus Conophytum comprises about 100 species of compact succulents (or “sphaeroids”) in the mesemb family (Aizoaceae), native to dry areas in South Africa and Namibia. Possibly because the plants tend to be slow growing and a bit finicky in cultivation, “conos” are never available from The Home Depot or local nurseries, where one can often find their cousins, Lithops, for sale. Conos aren’t much known at the species level even among succulent plant enthusiasts, except by specialists (such as me; I wrote a dissertation on the little guys), and so far as I’m aware, only one species has ever earned itself a widely known common name: Conophytum burgeri, a.k.a. Burger’s Onion.

Conophytum burgeri is an oddity among oddities. Plants of the species consist of rounded conical masses of translucent, gelatinous tissue (actually a fused pair of succulent leaves), colored pink, reddish, and green, attached to a shallow root system. One rarely sees the gumdrop-like plant body itself, however, because it hides inside of a sheath—like the skin of an onion—composed of the papery remains of leaves from past years. Burger’s Onion can grow to be the size of the pointed half of an egg, but that takes a decade or longer.

Conophytum burgeri in cultivation at the University of Connecticut, October 2007.

Burger’s Onion grows naturally on a patch of white and rose quartz pebbles, outside of the mining town of Aggeneys, in Bushmanland in northwestern South Africa. The pale sheathing that surrounds the plants blends in with the pebbles, and undoubtedly reflects excess light and slows water loss. Seedlings, which start out smaller than pinheads, actually germinate underneath bits of quartz, which apparently allows in enough light to support growth, while shielding the delicate young plants from the harsh Bushmanland climate. This sort of behavior has been reported from time to time for C. burgeri and other sphaeroid mesembs from arid quartz fields, but I almost didn’t believe the reports until I witnessed it in person while gently turning over bits of rock on a visit to Aggeneys in August of 2000.

Aggeneys (pronounced something like Ahk-ken-ays, to the ears of this Afrikaans language poseur) is one of the driest corners of a dry country. Regional natural history expert Irma Burger (daughter-in-law of Abraham Burger, after whom the onion is named), grew up in the area, and tells me that there was a stretch of several years when she never saw rain. Showers and thunderstorms are rare and scattered, occurring mostly in summer. Burger’s Onion tends to be active in winter, when cooler temperatures and shallow moistening of the soil from dew apparently create the most favorable conditions for growth.

The native range of C. burgeri is literally the size of a couple of tennis courts; fortunately it seems to be situated away from mineral resources that might inspire digging and blasting, yet well within mining company territory that is fenced off, with locked gates that exclude the elsewhere all-too-common enemies of delicate dwarf succulents: goats, sheep, unscrupulous collectors and drunks with 4X4s. The mining company is also responsible for coining, or at least popularizing, the name Burger’s Onion: years ago, the mine used the plant as the subject for a poster in an environmental awareness campaign.

Next month, I’ll write a bit about growing Conophytum burgeri in New England. It’s doable, but not exactly easy.

Introduction

Last year, I was elected/conscripted to be vice president of the Connecticut Cactus and Succulent Society, a fine organization which everyone living in southern New England, with an interest in desert plants, should join. As part of my duties, I've taken to writing a monthly column for the CCSS newsletter. The newsletters are available to download as PDFs at the CCSS website, but I thought it might be useful to post some of my more general-interest columns in a more accessible format.

The backbone of this blog will be my newsletter writings, possibly padded out with extra text or photos that didn't make the cut for the newsletter itself. So, while it won't be worth checking every day, there should be new material here at the beginning of each month, at least. In addition, I'll probably have a random post now and then dealing with other botanical topics, when the urge strikes to write something that doesn't fit in with the contents of a cactus club newsletter.

I'll try to keep the jargon to a level suitable for the amateur grower or armchair botanist, but there may be occasional outbursts dealing with the evolution and epidermal micromorphology of the Mesembryanthemoideae, or some such. Overall, though, Burger's Onion should be accessible to the interested layman. Welcome!