I presume you mean Kerria japonica, can be pruned right back to strong new growth after flowering, you may also need to cut back some roots as they send up a lot of suckers.

Hmm, button bushes - we need to know precisely what plant you've got there, button bushes means Santolina to me, not Kerria. If it is Santolina, prune back to a neat and tidy shape in early April, and then again around August time, after flowering. You can take up to half the length of the stems to keep them a nice rounded shape.

When I saw button bush I automatically thought Kerria japonica, {batchelors buttons}, I know santolina as cotton lavender, although Ihave never heard of Kerria japonica being grown in a pot, Iwould have thought it far too vigorous a plant for that, {although I suppose it could look nice for a while}.
I think it might be another example of not using the botanical name, and regional variations of common names. maybe alalsmith will be able to tell us which one it is