Geraniums and Pelargoniums are both very closely related; both are in the Geranium family (Geranianceae), with some species of Geranium having leaves superficially like those of Pelargonium, and some Pelargoniums dying back for winter like Geraniums. There is sometimes a degree of confusion between the two genera... add the fact that 'geranium' is often still used as a common name for Pelargonium and things can get tricky!
In flower the two are usually easy to tell apart; Geranium flowers have several planes of symmetry (they are 'actinomorphic') while Pelargonium flowers only have one ('zygomorphic'). Let's take for our example probably the best known Geranium of modern times, Geranium 'Rozanne'.
Although the leaves of Geranium 'Rozanne' look similar to many Pelargoniums you can see that the flower can be divided symmetrically several times, as below.
Now let's compare this with a Pelargonium flower, using Pelargonium 'Lemon Fancy' as our example.
As you can see, the flower of this Pelargonium can only be divided symmetrically once, as shown below.
Of course sometimes plants like to make life difficult; below is Pelargonium 'Paton's Unique', a variety of Pelargonium with several planes of symmetry (an actinomorphic flower)...
...while Pelargonium 'Appleblossom Rosebud' is just trying to be awkward!
It doesn't matter how much these modern varieties of Pelargonium break away from the typical zygomorphic flowers of Pelargonium species their lineage can still be traced back to 'proper' Pelargonium flowers, keeping them firmly and unmistakably in the genus Pelargonium.
So now you know.
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