Diosma oppositifolia is an aromatic shrub with attractive foliage and a long flowering season; it is a useful filler shrub for fynbos and water-wise gardens.
Diosma oppositifolia is an erect, multi-stemmed, densely leafy, aromatic shrublet up to 1 m high. The leaves are small, needle-like to lance-shaped, stalkless and dotted with glands. The leaves are alternately opposite, which means that they are situated directly opposite each other on the stem, and that each pair is arranged alternately to the next pair so that they form four rows up the stem.
Small, white, star-shaped flowers occur almost throughout the year but mainly during spring to summer (Sept.-Jan.). They are produced in pairs or in small, flat-topped clusters at the tips of the branches. The centre of each flower is bright green: this is a shallow, wavy-edged, 5-lobed, fleshy nectar disc.
The fruit is a 5-chambered capsule with horn-like tips producing one seed per capsule. As it matures, the capsule dries, then cracks open and expels the seeds in a process known as ballistic dispersal.
Diosma oppositifolia is not a threatened species.
Distribution and habitat
This species is frequent on sandy flats and mountains, in sandstone, granite or limestone soils from near Darling to Bredasdorp and Agulhas, including the Cape Peninsula.
Derivation of name and historical aspects
Diosma is derived from the Greek dios meaning divine, and osme, meaning scent or odour, referring to the scent the leaves emit when crushed. The species name refers to the arrangement of the leaves. Diosmas, like all buchus, contain oil glands in the leaves, which give each species its distinctive fragrance. The genus Diosma contains 28 species that all occur in fynbos, most in the southwestern Cape with a few east as far as Grahamstown.
Diosma oppositifolia is a resprouter with a woody rootstock. When a fire occurs, the above-ground part of the plant is killed but the underground rootstock survives and the plant resprouts after the fire. The flowers are visited by insects and bees.
Uses and cultural aspects
Bitter buchu does not have the same medicinal properties as the 'real' buchus, Agathosma betulina and A. crenulata, but has been used as a similar though inferior buchu when the other species are not available. It gives an acrid, bitter taste to an infusion. The leaves are used in brandy or vinegar tinctures for sprains and bruises. In former times the KhoiSan people used the dried and powdered leaves of a wide range of buchu species mixed with animal fat to clean and perfume their skin. Flowering stems can be used as a filler in floristry or to add an unusual fragrance to a bouquet where handling releases the scent in the leaves. It favours sandy conditions and can be used to stabilize sandy slopes and dunes.
Growing Diosma oppositifolia
Diosma oppositifolia can be planted with other buchus in a scented garden, or used as a filler shrub in the fynbos garden or water-wise garden. It also makes an attractive container plant for stoeps (verandas) and terraces. Unlike many other fynbos plants, this buchu grows equally well in acid or alkaline soil allowing it to be grown in a wider range of soils.
Diosma oppositifolia, like most buchus, is best grown in full sun, in well-drained, well-composted soil. Prepare the bed by digging over the soil, adding well-rotted compost and a slow-release fertilizer. Water the area before planting. Plant out in groups of three to five, 20 to 30 cm apart. They are slow-growing plants and should not be planted too close to fast-growing plants as they will get overwhelmed. A layer of mulch should be added after planting and annually to keep soil and roots cool in summer, retain soil moisture and reduce weeds. In the winter rainfall areas, buchus are best planted out during the rainy season: autumn-winter-spring. Plants require good watering in winter and moderate watering in summer. Do not allow plants to dry out and avoid cultivating the soil around the roots of these fynbos plants. They are water-wise plants and once established in your garden they will survive long periods of drought and will withstand a fair amount of frost.
Diosma oppositifolia can be propagated by seed sown in autumn or by tip cuttings taken in summer (December to February). Tip cuttings yield a flowering specimen faster than germination from seed.
Seeds develop after flowering, ripening as the seed capsule darkens and should be harvested before the capsule dries, splits and dispserses the seeds. Seeds collected from fully ripe capsules have a higher germination percentage. If seeds are harvested too early, the embryo will not ripen and the seeds will not be viable. The best harvesting method is to tie small netting or muslin bags around the flowerheads to catch the seeds. Fresh buchu seeds are sown in autumn (March to April). The seeds are sown into a tray containing a well-drained medium of equal portions of sand, loam and compost. Use some of the medium to cover the seeds and water. Place in a covered area with good light and air circulation. Keep medium damp. Germination occurs in one to two months. Young seedlings are pricked out when four true leaves have developed and planted into 0.5 litre bags containing a fynbos medium. Feed plants regularly but sparingly with a well-balanced fertilizer. Pinch out the growing tips of the seedlings to encourage bushy growth. Flowers are produced after two years.
After flowering, plants develop new shoots or branches, which are ideal for tip, stem or heel cuttings. Collect plant material early in the morning to reduce stress. Cuttings have the advantage of producing a larger flowering plant quicker than seedlings. Tip cuttings, 50-70 mm, are taken from the current year's growth. Prepare cuttings by making a clean cut below the node and remove a third of the foliage. Dip the base of the cutting in a rooting hormone. Firmly place the cuttings in a medium of 50% bark and 50 % polystyrene, and water cuttings with a fungicide. Ideally these cuttings should now be placed in a well-aerated propagation unit with a bottom heat of 24º C. Rooting occurs in 9 to 11 weeks. Carefully pot the rooted cuttings using a well-drained, humus-rich, fynbos potting medium (2 parts leaf mould, 1 part coarse sand). Plants will be ready for planting in seven to eight months, ideally during autumn to spring. Feed regularly with a well-balanced nutrient. Yellow leaves can be treated with an application of iron chelate.
References and further reading
- Gold, M. 1992. The buchus: cultivation and propagation. National Botanical Institute, Cape Town.
- Manning, J. 2007. Field guide to fynbos. Struik Publishers, Cape Town.
- Maytham Kidd, M. 1983. Cape Peninsula. South African Wild Flower Guide 3, Botanical Society of South Africa, Cape Town.
- Smith, C.A. 1966. Common names of South African plants. Memoirs of the Botanical Survey of South Africa No. 35.
- Trinder-Smith, T. 2006. Wild flowers of the Table Mountain National Park. South African Wild Flower Guide 12. Botanical Society of South Africa, Cape Town.
Justin Wigget & Alice Notten
Kirstenbosch National Botanical Garden