What sort are they? Rhododendron ponticum is a terrible invasive species in parts of the UK. If you know the species it would be a good idea to investigate whether it is invasive where you live before encouraging it.

Not sure what kind, they were here when I moved in. Doesn't look like ponticum, big white flowers (and I'm in the US, but rp may be invasive here, too. don't know.) Would love to have more!

Where is 'here'?? I would not consider self seeding to be a common situation with most rhododendrons. Even here in the PNW where rhodies grow naturally (it is the WA state flower), it is unusual to encounter seedlings springing up. Can't say as I've ever seen one and there are typically multiple rhododendrons in every garden!

Could you post a picture just to confirm the id of both the parent and the offspring?

I'm in CT...and i'd love to post some pix, if i can figure out how to stop getting the "your post is too long" message. Grr...

What would be useful toward identification of the kind demonstrating this behavior would be any shots you might have on hand of the parent plants in flower.

Looks like they are coming up in shaded moss over a stony soil, which is probably a particularly suitable substrate. And unlike out here you have a rainy summer climate, presumably more favorable to survival of small rhododendron plants.

Not that a little reseeding of what is very likely Rhododendron x superponticum (R. ponticum of gardens) hasn't been noted in Seattle.

At any rate you don't have to thin them, they will just make a thicket - as do rhododendrons in nature.

Yep, the only place I've seen self-sowing rhododendrons was not far from CT, in the Long Island garden of a well known collector. His were complex hybrids involving R. fortunei, I'm guessing these involve the native-to-parts-of-CT R. maximum, or possible the so-called 'ironclads'. Really not surprising in the right moist spot of a CT garden.

Parent plant has white flowers so I don't think it's the evil ponticum. I may move some..I've got plenty of rocky, moss-lovin' soil on the fringes of my garden and wouldn't mind spreading this embarrassment of riches.

Is the larger rhodie on the right a seedling, parent or other? If a seedling, has it bloomed yet?

Embarrassment of riches? Yes. I'd wet my pants from excitement if I saw rhodies reseeding in my garden.

These are all seedlings. None have bloomed yet. I transplanted a few..we'll see if they thrive elsewhere in garden. Fingers crossed.

I love my mountain laurels..wish they would selfseed. The leucothoe are doing it too.