Pelargonium: Greenhouse Plants
C – cool, minimum of 7°C (45°F) / W – warm, minimum of 13°C (55°F)
South Africa and St Helena
Originally, because their seed pods all looked like cranes’ bills, a whole group of plants were lumped together in Geraniaceae and called Geraniums. However, Linnaeus later decided that there were important differences between the hardy border Geranium and what we should now call. Unfortunately, the old name of Geranium has stuck leading to some confusion.
Onlycotyledonis, the Old Man Geranium, comes from St Helena; all the other species are from South Africa. When the tea clippers called in at the Cape the captains, if they had time, would pick up a few species and deliver them back to various parts of Europe into the hands of the head gardeners of the big estate gardens. Thus, 21 different species chosen at random by non horticulturalists and hybridised by the gardeners gave rise to all the we have today. Not only did different varieties arise by crossing plants to get seed but by plants producing sports, or parts of the plant which are different from the rest, which can be removed and propagated to give a new variety. The first type of Pelargonium to arise as a result of hybridising was the Regal followed by the Uniques and Shrubland Pets which were originally used for bedding out. Finally the modern Zonal were bred which have proved to be the toughest and whose flowers last the longest. All the make excellent greenhouse plants. The Zonals make good bedding plants but to my way of thinking real satisfaction lies in growing a named collection of species and varieties under glass where they will bloom magnificently, unaffected by the ravages of typical British summers. There are also those which flower better in winter.
Cultivation depends to an extent on which type of Pelargonium you are dealing with. Regals are propagated bytowards the end of summer and grown on warm throughout the winter to flower in May and June. These plants should have the tips pinched out throughout their growth to give a good bushy shape to the plant and lots of flower heads. Zonal Pelargoniums like to be kept very much on the dry side and prefer a loam-based . Over-watering leads to the leaves turning yellow or sometimes bright red and dropping off. If plants are being overwintered in frost free temperatures they can be cut right back to remove all the leaves and kept almost completely dry throughout winter. Cuttings that might have been taken in autumn need a bit more warmth and watering to keep them going through winter. I pinch the tips out of cuttings at 5-8 cm (2-3 in) tall. Keep the species and scented leaved types growing all year round, being much more careful about the watering in winter. Occasionally, the taller ones will need to be cut back; best done in spring. Some of the species have tuberous roots or very succulent stems. These will become dormant during winter and should not be watered until they grow again in spring. Apart from Regals, cuttings should not be given humidity in which to root, preferring to stand on the open staging in pots of peat and grit allowed to become dry between waterings. Great care must be taken with cuttings especially of Zonals in autumn as they are prone to a fungal disease known as black leg. Treatment with the appropiate fungicide as a precaution may be necessary. The other main problem is rust which affects the Zonals. Rust spores overwinter on leaves; if your plants have been badly infected this is why you should cut plants back to remove all the leaves in autumn which should then be burned. The plants will break into growth the following spring. My personal favourites are the species which can be obtained either as plants or seed sown in spring at a temperature of 13°C (55°F). P. quinquelobatum has small curiously coloured flowers which manage to be almost but neither grey, blue nor green. A large clay pot of P. triste, which is tuberous, makes a fine sight with its carroty foliage and pale yellow flowers.