Dregeochloa pumila

(Nees) Conert

Family : Poaceae
Common names : Tom Thumb grass, Kleinduimpiegras (Afr.)

Dregeochloa pumila Dregeochloa pumila in its habitat. Note that the centre (inner canopy) of the plant is dead resulting in a circular outer canopy. Note the succulent leaves and compact growth.

The Tom Thumb grass (Dregeochloa pumila), a peculiar, little known grass from the coastal southern Namib Desert in South Africa and Namibia, is the only known grass with succulent leaves and perhaps the smallest of all our grass species.

Description (from live plants as well as Barker 1990)
A dwarf perennial forming small tight mats up to 200 mm in diameter, sometimes dying back in the centre with age and leaving an outer circle. The succulent leaves grow opposite each other in two rows (distichous) with an ascending spreading orientation. The leaf blades are clearly channelled or folded, up to 25 mm long, but usually much shorter (about 10–15 mm) and 1.6–3.5 mm broad, keeled below; distinctly succulent; the epidermis is dark green to grey-green, densely covered with fine minute hairs (pubescent). The leaf tips are rounded and minutely spine-tipped (apiculate). Flowers appear during the summer, and are usually borne in a short raceme, or sometimes a panicle up to 30 mm high bearing up to 9 spikelets. The spikelets are up to 10-flowered, the glumes are about 13 mm long, up to 7-nerved and also minutely hairy.

Dregeochloa pumila in fruit
Dregeochloa pumila in fruit on Kortdoringberg.

 

succulent grass Dregeochloa pumila
The succulent grass Dregeochloa pumila in its habitat growing together with Crassula muscosa. Note the succulent leaves and compact growth.

Flowering time
Winter to summer (July–December).

Status
Locally abundant and not threatened. Coastal diamond mining might pose a threat in future.

Distribution and habitat
Distributed along the Namib Coast (up to 15 km from the coastline, from Alexander Bay (South Africa) to Luderitz in southern Namibia. It grows in a semi-maritime habitat in very sandy regions in open desert among succulent plants, often among quartz or granite rocks. The vegetation is typical Succulent Karroo, in which succulents (especially of the family Mesembryanthemaceae) are dominant. The vegetation in which it occurs near Alexander Bay was however recently re-classified as Western Gariep Plains Desert.

Habitat of Dregeochloa pumila
The habitat at Kortdoringberg near Alexander Bay.

Habitat of Dregeochloa pumila at Luderitz
The habitat of Dregeochloa pumila at Luderitz: coastal rocky parts with ample sandy soil on granite bedrock.

Derivation of name and historical aspects
The name Dregeochloa honours Franz Drège (1791–1867) a well known German horticulturist and collector of botanical specimens. Drège who made a major contribution to the knowledge of South African plant species, and discovered this species in the Richtersveld, in the northwestern Northern Cape, in September 1830. The specific epithet ‘pumila' and the common name refer to the dwarf size of the species. This grass was originally named by the botanist Nees, as Danthonia pumila. Conert realized that it belongs to a different genus and created the genus Dregeochloa in 1966 to accommodate our species. There is only one other species in this genus: the non-succulent D. calviniensis from the dry Nama Karoo regions of the Northern Cape.

The grass family (Poaceae) is one of the largest and most widespread groups of flowering plants. Grasses range from dwarf species, as described, to giant bamboos, up to 37 m high and growing up to 91 cm in a single day (= 0.63 mm per minute).

Ecology
The Tom Thumb grass is fog-dependent and grows in a dry, hostile environment. Temperature remains cool throughout the year except during onshore bergwinds when temperatures can suddenly rise considerably. The average daily maximum is more or less 19°C and the average daily minimum about 10°C. Frost is absent from the habitat.

Fog settles on the minutely hairy leaves or on rocks and runs into crevices providing additional moisture. Tom Thumb grass grows in full sun in very windy conditions. Its small stature (alpine growth), as with many other succulents, allows it to make use of the surrounding soil and rock for absorbing the warmth of the sun's rays. The main growth occurs during the rainy season (May to September). Rainfall in the southern Namib occurs mainly during winter (May to September) and is associated with cyclonic cold fronts. It ranges between 15 and 50 mm per annum. The seeds are wind-dispersed.

At Kortdoringberg near Alexander Bay (southernmost distribution range), Dregeochloa plants grow amongst quartz rocks and pebbles and loose reddish wind-blown sand on a gentle west-facing slope. The area is very rich in succulent plants, some of which are endemic to the region. They include Aloe krapohliana var. dumoulinii, Crassula brevifolia, C. nudicaulis var. herrei, Euphorbia ephedroides, Cotyledon orbiculata var. orbiculata, Euphorbia hamata, Adromischus roaneanus, Pelargonium carnosum, Othonna clavifolia, Pelargonium echinatum, Cheiridopsis verrucosa, C. brownii, Conophytum saxetanum and Tridentea herrei. Non-succulent plants include Dianthus namaensis and Rhyssolobium dumosum. Lichens are also very prominent on rocks and on plants.

Near Luderitz on the Dias Peninsula (northern populations) plants grow on low dome-shaped granite outcrops. Plants grow in shallow soil and in crevices and share the habitat with succulent plants such as Brownanthus marlothii, Sarcocaulon patersoniae, Conophytum saxetanum, Cephalophyllum sp., Crassula deceptor, C. namibensis, C. elegans, C. muscosa var. muscosa, Drosanthemum luederitzii, Othonna furcata and Tylecodon schaeferi. Semi-succulent plants include Pteronia spinulifera, Eremothamnus marlothianus, Didelta carnosa and Gazania schenckii.

Uses and cultural aspects
No uses have been recorded. 

Growing Tom Thumb grass (Dregeochloa pumila)

Propagation
Dregeochloa is easily propagated by stolons planted in sand. It can be taken at any time of the year but preferably during spring or summer. Make sure that the container provides sufficient drainage. Rooting is rapid and the young plants can be transferred to a container or a permanent site in a sunny position. With less light plants rapidly grow longer spear-shaped leaves which loose their succulent nature.

Cultivation
The Tom Thumb grass is best grown as a container plant. It is so small that it would disappear on a rockery. It is best treated as a winter-growing plant but should be gently watered throughout the year (due to its dependence on regular fog). It thrives in a cool situation and is best grown in a sandy or gravelly soil. It is very fragile and should be handled as little as possible. Plants should soon fill out a container. They can be fed with a mild organic fertilizer.

References and suggested reading

  • Barker, N.P. 1990. Dregeochloa in G.E. Gibbs Russsell, L. Watson, M. Koekemoer, L. Smook, N.P. Barker, H.M. Anderson,M.J. Dallwitz (ed. O.A. Leistner). Grasses of southern Africa National Botanical Gardens, Botanical Research Institute, South Africa.
  • Fish, L. 1997. Poaceae Barnhart (Grass family) in G.F. Smith et al. (eds), List of Southern African succulent plants. National Botanical Institute/Succulent Society of South Africa, Umdaus Press, Pretoria.
  • Gunn, M. & Codd, L.E. 1981. Botanical exploration of southern Africa. A.A. Balkema, Cape Town for the Botanical Research Institute.
  • Mabberley, D.J. 1997. The plant book. Cambridge University Press, Cambridge.
  • Mucina, L. & Rutherford, M.C. (eds). 2006. The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19. South African National Biodiversity Institute, Pretoria.

 

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