Same here so I think they must do. I have a Johnson's Blue that appears to have seeded and made a lilac one, and some thuggish pink ones that I think might have bred with Herb Robert, the flowers are so much smaller than the parent.

Oh gosh, yes they do, they are very promiscuous. The offspring can be very different to the parents.

Thanks for your replies. Now I know that they cross pollinate, I might even try to do some experiments with other varieties.

Got a picture of the offspring?

Unfortunately no, unless I look in my garden rubbish bag! I was going to take a photo the next day, but when I went out to do it, the poor geranium was looking very sad. I delved through the leaves, only to find the stem with part of the root had been severed on ground level. With a bit of luck it will grow again from what's left in the ground, and I also planted the piece of root I salvaged. The offspring's parents are Johnson's Blue and pale pink Endressii, I think. The result was a very dark blue, Endressii type flower.

Sounds lovely - can't resist the blue ones. I've just found a small flowered white one with pink veining that looks like a cross from Himalayan White.

Your hybrid sounds interesting! I wonder if your one plant is truly 'Johnson's Blue' as this popular, old hybrid is a cross between pratense and himalayense and is supposed to be sterile. One of the reasons that 'Johnson's Blue' flowers for a long period is because it does not produce seed. But then again, perhaps the pollen is viable?