Diosma hirusta ' Blue Downs' belongs to the Rutaceae family commonly referred to as the citrus family and is easily identified by the presence of volatile oil glands crushed or when brushing against this bush. D. hirsuta' Blue Downs' is a selected form of D. hirsuta known as the wild buchu.
The 'Blue Downs' form is an attractive single-stemmed shrub with a round growth habit and grows to a height of 0.5 to 1 m. It forms a neat, upright shrub with beautiful greyish green foliage, which is characteristics of this 'Blue Downs' form and makes it stand out amongst other plants in the garden. The leaves are alternate, linear, 5-10 mm long, and pointed at the tips. A close look at the new foliage reveals fine white hairs on the leaves and on new branches/stems. The new branches or shoots are cream-coloured to light brown, whereas the older stems are dark brown. Tiny, white, 5-petalled, cup-shaped flowers, each 5-7 mm across, are borne in clusters of 5 or more at the tips of branches. Flowering time is from September to November.
Plump dark green fruit develop on bushes after and during flowering: one can easily spot the oil glands on the fruits, which release a fragrant aroma when touched. The fruit is a 5-chambered seed capsule bearing one black and shiny seed per chamber. As the fruit ripens or matures, it turns from light green to a darker green. The seed capsule can also be tested for ripeness by pressing the seed capsule between your fingers: if it feels hard and not soft, it can be collected.
Diosma hirsuta occurs naturally on rocky sandstone and clay slopes and on sandy and often gravelly soils from the Cederberg Mountains to Humansdorp. This grey leafed form was collected on at Blue Downs on the Cape Flats.
Derivation of name and historical aspects
Diosma is derived from the Greek meaning Dios, divine, and osme, odour (smell referring to the scent when leaves are crushed). The genus Diosma is comprised of 28 species found in the Western Cape. D. prama, false buchu, bears masses of tiny creamy white flowers and forms a single-stemmed, rounded shrub.
Anthony Hitchcock collected this buchu in 1986 on a field trip with Prof. Ted Schelpe, Barry Louw and other botanists to view Satyrium carneum and Satyrium corifolium growing at Blue Downs near Cape Town. There were large fields of these two orchids, which were unfortunately lost when the area was developed.
Uses and cultural aspects
Diosma hirsuta 'Blue Downs' is an ideal garden plant for its aromatic leaves and beautiful flowers and can be used as a filler plant in a mixed fynbos bed, as a border or hedge plant, by planting it is masses, or a container plant. The cut flowering stems can be used in a mixed floral display or in bouquets.
Bees, butterflies and other insects pollinate buchus. Shiny, black seeds are scattered over a distance from its capsule on ripening. Seed is easily collected at the bottom of the plant. This phenomenon is known as ballistic dispersal.
Growing Diosma hirsuta 'Blue Downs'
Diosma hirsuta ' Blue Downs' as well as most of the species found in the Rutaceae family are best grown in your garden in full sun, in soil that is alkaline to acid, well-drained and composted. Prepare the bed by digging over the soil, adding compost and a slow-release fertilizer. Water the area before planting. Plant out in-groups of 3 to 5, 20 to 30 cm apart. They are slow- growing plants and should not be planted too close to fast-growing plants. A layer of mulch is added after planting and annually to keep soil and roots cool in summer, retain soil moisture and reduce weeds. Buchus are best planted out during autumn, winter and 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.
Buchus are used as accent plants or in a mixed fynbos bed with companion plants such as Erica, Restio, Phylica species, Pelargonium betulinum, herbaceous perennials such as Lobelia valida, Geranium incanum, as well as other buchu species such as Agathosma lanceolata (honey buchu), Adenandra uniflora (china flower) and Coleonema album (white confetti bush).
Buchus can be successfully grown from seed or cuttings. Viable seeds develop after flowering when the seed capsule darkens and should be harvested before the seeds are dispersed. Seed collected from fully ripe capsules has a higher germination percentage. If seed is picked too early, the embryo will not ripen and the seed will not be viable. Fresh buchu seed is sown in autumn (March to April). The seed 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 seed 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 into 0.5 litre bags when four true leaves have developed, using a fynbos medium. Feed plants regularly but sparingly with a well-balanced ferilizer. 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 24ºC. 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 7 to 8 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.
- Goldblatt, P. & Manning, J. 2000. Cape plants. A conspectus of the Cape flora of South Africa. Strelitzia 9. National Botanical Institute, Pretoria & Missouri Botanical Garden, Missouri.
- Kidd, M.M. 1983. Cape Peninsula. South African Wild Flower Guide 3. Botanical Society of South Africa, Cape Town.