Pretty but dangerous (to live-stock, that is)! In a good year, usually in autumn, the large, deep blue to violet or occasionally white, iris-like flowers of these two species appear in eye-catching displays in the arid central interior of southern Africa. Unfortunately their appearance in grazing lands causes anxiety rather than pleasure to farmers because they are extremely toxic to stock.
The plants are perennial geophytes, the leaves and aerial stems dying back to the large dark-coated corm during the period of dormancy following flowering and seeding.
The two species are not easy to distinguish: the leaves are 3-5 or 6 in number, long and strap-shaped, flat or slightly U-shaped and slightly twisted spirally; in Moraea polystachya they are about 6-12 mm wide with unthickened margins, whereas in M. venenata they are 10-30 mm wide, often with the lowest two close together to form a cup-like structure at the base of the stem and with the margins thickened and yellowish.
The flowering stem averages about 0.8 m tall in M. polystachya and 0.25 m tall in M. venenata, although well-grown plants can be much taller. The flowering stem is many-branched at the top, and although each flower lasts only one day, a succession of flowers is produced so that the flowering period may last for six to eight weeks.
The upright flowers are relatively unspecialized in the genus, with an outer ring of three large more-or-less spoon-shaped tepals, and an inner ring of three smaller and narrower tepals. In the outer or both rings, the tepals curve outward or downward from the point where they broaden. As mentioned above, flower colour ranges from dark blue through violet (with pinkish tones) to white with fine blue lines, to pure white. One or both tepal rings have yellow to orange nectar guides at the base of each tepal. M. polystachya has smaller flowers, with the outer tepals about 50 mm long, in contrast to those of M. venenata where the outer tepals may be up to 60 mm long. In the centre, is the self-coloured (same colour as tepals), complex expanded style structure, similar to that of the cultivated iris, which crowns the ovary and hides the stamens. In M. polystachya the ovary usually protrudes from the floral bracts and develops into a rounded capsule, 11-16 mm long; in M. venenata, the ovary is concealed by the floral bracts and develops into an oblong capsule, 16-20 mm long. The capsule splits open when mature to release many small angular, pale brown seeds.
Neither Moraea polystachya nor M. venenata is threatened or protected in any way; in fact the opposite applies and these plants are generally viewed as undesirable in agricultural land.
Distribution and Habitat
Moraea polystachya is widespread in central South Africa and Namibia, but its range overlaps that of M. venenata in Namibia and Northern Cape, South Africa. The average annual rainfall ranges from 100 to 700 mm, and is probably considerably less in a 'normal' year. In part of their common range, the sparse rainfall occurs in summer, while the remainder of the range receives rain in winter. The habitat is well-drained flats and slight slopes, with collectors often referring to the presence of calcrete deposits. Light frost occurs, but in mid-summer (when the plants are dormant) the temperatures may regularly exceed 40º C.
Derivation of name and historical aspects
Moraea polystachya was described (as Iris polystachya ) by Thunberg, the 'father of Cape botany', in 1782, from a collection he had made in the Eastern Cape 'in the area of the Sundays and Fish Rivers'. The species name polystachya, meaning many-branched, refers to the inflorescence, which differed greatly from the few-flowered species of the Western Cape already known to Thunberg. Its toxicity was mentioned by Ecklon, another famous botanical explorer, as early as 1830. M. venenata was only described from Namibia in 1924, but has had a chequered taxonomic history as it was not always recognized as a separate species from M. polystachya. The species name venenata means poisonous, referring to the very toxic nature of the plants.
Another closely-related, large blue-flowered species is Moraea speciosa, which occurs on the southern and western margins of the Karoo and could be confused with M. venenata when not in flower, on account of its several broad leaves with thickened margins. When in flower, it is not possible to confuse them: M. speciosa has pale blue, cup-shaped flowers with large nectar guides and which usually face to the side when open. The style structure is very different. Two distantly related species, M. amissa and M. gigandra, have already been discussed in this series.
As mentioned above, in the northern part of the common range, the plants flower mainly in autumn (March to May), but in the south, flowering is more usual in winter (June to August) or even spring (September, October). The complex style structure is known to be specifically adapted for bee pollination, but unfortunately no specific observations on these two species are available.
The plants are thought to increase in number due to overstocking of grazing land. It is interesting that stock animals born and raised in lands infested with these plants are able to recognize and avoid them. Cattle deaths often occur in the winter months when there is no other fresh grazing available. The dried plant material is also toxic and when accidentally incorporated in hay can have disastrous results. A few cases of fatal human poisoning have been reported, where the corms were mistaken for those of edible species, such as Moraea fugax. The toxin has been identified as a cardiac glycoside (heart-poison).
Uses and cultural aspects
One label attached to a herbarium specimen reports that the 'Hottentots plait the leaves into mats' but this information has not been confirmed elsewhere.
The two species are economically important due to their toxicity. Southern Africa abounds in toxic plants and much time and energy has been devoted to identifying the plants and their toxins by such government organizations as the Veterinary Research Institute at Onderstepoort, Pretoria.
Growing Moraea polystachya and M. venenata
Moraea polystachya is easy and rewarding to grow, but because it has a tendency to become weedy in cultivation, grow plants in pots or in a rockery where they can be controlled. For the best show, mass the plants together.
Plant the corms at a depth of two to three times their diameter in a well-drained soil mixture. Water lightly until after sprouting, when watering can be increased, but never over-water. Fertilize sparingly. Protect the corms from freezing during dormancy.
Sow seed in late summer or autumn. Seed germinates readily, and the same rules for watering and fertilizing apply. Seedlings flower in about three years.
Possible pests are red spider mite and aphids.
No information is available about growing Moraea venenata.
References and further reading
- Du Plessis, N. & Duncan, G. 1989. Bulbous plants of southern Africa : a guide to their cultivation and propagation. Tafelberg, Cape Town.
- Goldblatt, P. 1986. The moraeas of southern Africa. Annals of Kirstenbosch Botanic Gardens 14.
- Obermeyer, A.A. 1962. Moraea polystachya. The Flowering Plants of Africa 35: t.. 1385.
- Van Wyk, B-E., Van Heerden, F. & Van Oudtshoorn, B. 2002. Poisonous plants of South Africa. Briza Publications, Pretoria.
- Watt, J.M. & Breyer-Brandwijk, M.G. 1962. The medicinal and poisonous plants of southern and eastern Africa. Livingstone, Edinburgh & London.
National Herbarium, Pretoria