We recently planted two types of rhubarb in patio pots next to one another; one had come from a family garden, the other was a new plant purchased at a local garden centre.

They both seem to be growing very well, but we noticed yesterday that a sprout had emerged from the edge of a nearby drain! And today another has appeared in the crack between our paving slabs and the wall of the house. Closer inspection revealed they'd also developed a fondness for growing in gaps in the paving behind the pots in which they were originally planted; we're getting quite a family!

Is this normal? What's especially worrying is that the ones growing in the concrete are developing at a much quicker rate than those in the pots. Is it likely they've morphed into some kind of weed?!

We're very much novice gardeners and any advice would be much appreciated!