Is it a Geranium? Is it an Erodium? No it's a Pelargonium!
One of the most confusing generic names to any gardener is Geranium. Anyone familiar with temporary planting schemes will know the popular Bedding Geraniums. This is the rather misleading common name for hybrids of the genus Pelargonium (mainly derived from Pelargonium zonale). To distinguish them, true Geranium are often called Hardy Geraniums. Most species and hybrids of true Geraniums are able to tolerate several degrees of frost, and therefore can live outside all year round, even in cold climates. So you cannot talk just about a geranium, you have to clarify whether you mean a Bedding Geranium or a Hardy Geranium, ie. Pelargonium or Geranium respectively.
Fortunately, the two genera are quite easy to tell apart. Geraniums have flowers with 5 more or less identical petals, usually arranged in a flat or bowl shaped. Pelargoniums also have 5 petals but they are arranged 2 above and 3 below. The 2 upper petals are usually larger and broader than the lower 3 (indeed in some species the lower 3 are even missing). So Geranium flowers have radially symmetric flowers (actinomorphic), whereas Pelargonium flowers have a single plane of symmetry (zygomorphic).
This brings us on to Pelargonium incarnatum, which grows here at Phillipskop Mountain Reserve (Phillipskop Mountain Reserve is 246ha (608 acres) in size and occupies the southerly slopes of the Klein River Mountains just to the east of Stanford, on the Western Cape of South Africa). In almost every genus there is one species that refuses to obey the rules. This species of Pelargonium has perfectly radially symmetric flower. So how do we know this is a Pelargonium and not a Geranium (for there are a number of true Geranium species that grow in South Africa)? One way is to count the stamens. A true Geranium will have 10 fertile stamens (have anthers with pollen) whereas Pelargoniums will always have 7 or fewer. The remaining stamens in Pelargonium are modified in some way but always lack an anther. In Pelargonium incarnatum one can clearly see just 5 fertile stamens.
Source: Pelargonium Incarnatum
Posts are made by Brenda Archer or Sharon Pearce - both are past Presidents of the San Diego Geranium Society!