Rudbeckia fulgida "Goldstrum"

Last year I did not trim my perennials back, and these ones in particular grew out of control, crowding out some other things. I have 3 planted in my garden and all have very hearty wood bases. Can someone please tell me how much to trim these back?
Thanks in advance..
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Ground level. They will take division every 2/3 years so you can spread more lovely yellow clumps around your plot or swop them. Regards Mike Gilmore WinsfordWalledGarden, SW England, USDA Zone9a

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Thanks much.. Im always afraid of destroying my perennials...
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