Pelargonium denticulatum with its attractive, strongly scented leaves and pinkish purple flowers is an interesting addition to the mixed herbaceous border, adding texture, fragrance and colour.
Pelargonium denticulatum is a well branched strongly aromatic plant with sticky finely divided leaves. The leaf margins are irregular and finely dentate with sharply pointed teeth. The leaves are characteristically hard and rigid and are densely covered with glandular hairs. The inflorescence consists of three to seven pink to purple flowers. The two posterior petals have dark red to purple markings and the anterior three have narrow claws. A claw is a narrow proximal part of a spathulate petal. This species flowers from April to November with a peak in September and on occasion the following January. The peduncle (the stalk of the inflorescence) bends downwards after flowering. The fruit, schizocarp, splits into 5 separate parts, mericarps, with the basal part about 6 mm long and a tail of about 35 mm long.
Pelargonium denticulatum is listed on the Red List of South African plants 2009 as Rare. A taxon is Rare when it meets any of the four South African criteria for rarity, but is not exposed to any direct or plausible potential threat and does not qualify for a category of threat according to the five IUCN criteria.
Distribution and habitat
Pelargonium denticulatum is restricted to a small region in the southern Western Cape. It is known from populations in Herbertsdale and Mossel Bay. These areas receive most of the rainfall during the winter months, frequently in the form of fine drizzle which may carry on for quite a few days. This species is confined to mountains or their immediate surrounding area. The microclimate in the area is significantly moister than in neighbouring lower lying areas. It is usually found in ravines and in close proximity to streams.
Derivation of name and historical aspects
Pelargonium denticulatum belongs in the family Geraniaceae, a large cosmopolitan family of approximately 11 genera and 800 species in subtropical and temperate regions of the world. The South African genera in the Geraniaceae family are Monsonia, Sarcocaulon, Pelargonium, Erodium and Geranium. There are approximately 270 species of Pelargonium which occur in S, E and NE Africa, Asia, St Helena, Tristan da Cunha, Madagascar, Australia and New Zealand, most of which (± 219 species) occur in southern Africa, with a large concentration of more than 135 species in the southern portion of South Africa between Nieuwoudtville in the west and Port Elizabeth in the east. Pelargoniums are often incorrectly called geraniums. The genus Pelargonium gets its name from the resemblance of the shape of the fruit to the beak of a stork, pelargos in Greek, while the Latin species name, denticulatum, refers to the finely toothed leaf margin. This species has the common name of tooth-leaved pelargonium. In 1789 the species was introduced into England from the Cape by Francis Masson, a Scottish botanist who was sent by the Royal Botanic Gardens, Kew to collect live plant specimens.
Robert Sweet, an English botanist and horticulturist, published a description and colour illustration in Geraniaceae 2: t. 109 (1822). He mentions that the species was grown in Kew until 1821 but there was no evidence to suggest that Nicolas Jacquin, a Dutch and Austrian botanist, obtained the material, which he used to describe the species in 1797, from Kew. In Vienna the Museum of Natural History has several specimens of Pelargonium denticulatum. These specimens do not have any inscriptions in Jacquin's handwriting. There is also no indication that he used any of these specimens for his original descriptions.
The seed is adapted to wind dispersal: it is light in weight and has a feathered, spiral, tail-like attachment. When the seed lands and there is adequate water in the soil, the tail acts like a drill, twisting the seed into the soil so that the seed can affix itself in the ground and thus avoids being blown away, or carried away on moving animals.
Uses and cultural aspects
Although Pelargonium denticulatum has no cultural use, horticulturally this attractive shrub is ideal for any garden and will do well in a sunny position. The essential oils of Pelargonium species that have scented leaves such as P.graveolens, P. tomentosum, P. odoratissimum and P. denticulatum have been found to have good antibacterial activity.
Growing Pelargonium denticulatum
Pelargonium denticulatum can be propagated from both seeds and cuttings. Cuttings can be taken at any time of the year from a healthy strong plant, from softwood or herbaceous growth. The use of a rooting hormone will speed up rooting development, especially for the softwood cuttings. Place the cuttings in river sand in cold frames and keep them moist. The cuttings should root within 3 weeks. Give the rooted cuttings a 2–4 week weaning or hardening-off period and transplant them into a well-drained potting soil mix and place them in a sunny position. Feed with an organic based fertilizer. As soon as the plants have produced a sturdy root ball they can be planted out into the garden.
Sow seeds in late summer to early autumn. The seed should be sown in a light, well-drained potting mix. Sow the seeds evenly in the seed tray and cover them with fine white sand or fine-milled pine bark. Water the seeds gently but thoroughly with a fine rose spray and place them in light shade with no direct sun. The seeds will germinate in 2–3 weeks. Pelargonium denticulatum is a tough shrub suitable for any garden. The shrub is able to grow in most soil types and prefers full sun. In the Western Cape the best time to plant is during the rainy months of winter to allow plants to establish themselves before the warm, dry summers. Plants can be maintained in the garden for several years, but need to be cut back after flowering to keep them neat and vigorous.
References and further reading
- Raimondo, D., Von Staden, L., Foden, W., Victor, J.E., Helme, N.A., Turner, R.C., Kamundi, D.A. & Manyama, P.A. (eds). 2009. Red List of South African plants. Strelitzia.
- Van der Walt, J.J.A. & Vorster, P.J. 1981. Pelargoniums of southern Africa, vol. 2. Juta, Cape Town.
- Website: The American Herb Society http://www.herbsociety.org/pelargonium/pdenticulatum.php
- Website: S.P.N. Mativandlela, N. Lall and J.J.M. Meyer, Antibacterial, antifungal and antitubercular activity of (the roots of) Pelargonium reniforme (CURT) and Pelargonium sidoides (DC) (Geraniaceae) root extracts. http://www.up.ac.za/dspace/bitstream/2263/1825/1/Mativandlela_Antibacterial (2006).pdf
Kirstenbosch National Botanical Garden