Thursday, August 22, 2013

Welwitschia Propagation

Female Welwitschia in cultivation at the University of Connecticut, mid-July. Note the pollination drops of sugary liquid at the tips of filaments protruding from between the cone scales.

Welwitschia mirabilis is a distant relative of the conifers, native to the Namib desert in Namibia and Angola. Welwitschia is notable for a number of reasons, most obviously its fat, stumpy trunk and the single pair of strap-like leaves that last for as long as the plant survives, which is thought to be up to 1000 years or possibly more. The trunk of a Welwitschia never produces branches (aside from ephemeral reproductive shoots) or offsets, and the only way to propagate the plants is by seed.

Welwitschia matures at around 10 years old under my conditions, although if given a more generous root run and warmer temperatures they can grow considerably more quickly. Adult plants are either male or female. From my observations, male plants seem to be more common than females, but that's based on a small sample size, and I haven't run across any surveys of sex ratios in the wild. In both cultivation and their southern hemisphere native range, Welwitschia cones are initiated in the spring and become fully formed and receptive in mid-summer.

Male Welwitschia with pollen-producing cones. Male cones are smaller that the females, only a little larger in diameter than a pencil.
Welwitschia seems to be pollinated by insects and not by wind, which is somewhat unusual for a cone-bearing plant. During the early afternoon the female cones secrete a drop of nectar from whisker-like extensions of their ovules. The male cones bear little whorls of pollen-producing microsporangia, and at the center of each whorl is a nectar gland, which may itself be a modified, vestigial ovule. The cones of cultivated plants in Connecticut attract a variety of insects collecting nectar and pollen, including flies, wasps, bumblebees and yellow jackets. In Namibia, various flies (including the common housefly, which also shows up on my greenhouse plants), bees and wasps have been observed transferring pollen from male to female plants. Neither wind-dispersed pollen nor the Welwitschia Bug (Probergrothius sexpunctatis), both of which were once suspected of being important to Welwitschia reproduction, actually seem to be involved in pollination of plants in the field (Wetschnig and Depisch, 1999).

A large wasp visiting male cones on a cultivated Welwitschia.

Successfully pollinated female cones ripen in autumn, about two months after pollination. Ripe cones shatter into a pile of papery scales and winged seeds, which seem to be adapted to wind dispersal.

Female cones in September releasing winged seeds.

I've had good luck sowing the seed right after it was shed, in September or October, in a sunny, warm spot, in a pot of well-drained soil mix for desert plants. I mostly use biodegradable fiber pots, which can be sunk into the soil of a larger pot when the seedling needs more room, minimizing root disturbance. The seed germinates well within a few weeks in moist soil. The young seedlings are very susceptible to damping off, for some reason; once they make it past their first few months they seem to become resistant to such problems. I've tried the usual cultural remedies for damping off--strong sun, allowing the soil surface to dry as much as possible--without much success, and will probably resort to the use of a fungicide the next time I sow a batch of Welwitschia.

Welwitschia seedling at a few days old. The cotyledons (seedling leaves) start out a bronzy-yellow color, and take some time to turn green.

Welwitschia seedling at age six months, with its first and only pair of true leaves emerging at right angles to the cotyledons.

Reference: Wetschnig, W. and  B. Depisch. 1999. Pollination biology of Welwitschia mirabilis Hook. f. (Welwitschiaceae, Gnetopsida). Phyton 39: 167-183.