Probably they are in rich and moist soil. I have one which I planted nearly four years ago it never grew no taller 10 inch. So two weeks ago I dug it up moved to very poor soil. Let us see what happens?!!
I suspect you're being too kind and need to move it somewhere less rich and moist.
My best ones are in poor dry soil and self seed happily. There are also wild versions in the driest part of our plot which we leave uncut over summer for the wildlife.
They never did very well in my Belgian garden where the soil was rich and rain was plentiful.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
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My best ones are in poor dry soil and self seed happily. There are also wild versions in the driest part of our plot which we leave uncut over summer for the wildlife.
They never did very well in my Belgian garden where the soil was rich and rain was plentiful.