Could they have come up underneath a big piece of the stone, bent over as they pushed against it and then grew enough to dislodge it? I had a hyacinth come up like that - it had pushed aside the corm of a bedding cyclamen that I'd planted in the top of the pot.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
In the sticks near Peterborough