Two possibilities. First, there has been a lot of discussion over the years about just how perennial certain coreopsis really are. There are both annual and perennial species, they hybridize fairly easily, and until a variety has been around for a while, its real tendencies may not be known.

If, OTOH, these are proven reliable varieties like Moonbeam, I'd stop cutting them back in the fall.

If they are planted in an area that stays wet throughout the winter, they are probably dying from root rot.

Bonnie

I agree with Bonnie. It's too wet.

Yes, most of the standard varieties should survive the winter in Lower Michigan. Some of the "newer" forms that involve C. rosea ('Sweet Dreams', 'Heaven's Gate', etc.) are likely less hardy, as well as most of the even newer forms such as 'Autumn Blush', 'Red Shift', etc.

As mentioned, your issue is likely winter wetness.

Has anyone else experienced even good ol' 'Moonbeam' getting tired-looking (less vigorous/profuse) after only 2-3 years? I'm beginning to wonder if most/all the genus is generally short-lived. Certainly the C. grandiflora types are, although they self-sow readily.

Coolplantsguy, Yes, in my zone, C. 'Moonbeam' deteriorates after a few years. It's recommended that you divide it every 2-3 years to maintain its vigor.

My Moonbeam also faded out on me. I kept wondering why I seemed to be the only person in the world who couldn't grow this "easy" perennial. Now I don't feel so bad, lol.

It was in a bed with vole problems, but I don't think it was the voles. Usually with voles, the whole plant up and dies because the darn critters eat the roots. This Moonbeam just got less vigorous over about three years until last year I had one little stem of it.

:)
Dee

I planted two or three different ones about five years ago. One of them came back every year they were like a weed, I was pulling some out in the Spring they were so plentiful, gave some to my neighbor (She wanted them) and she cussed them out after awhile, I told her they were like a weed. Finally this last Spring they must have given up because I didnt see any. Promised myself never again. Must be a certain variety. Anyway thats my experience with them. Good luck!!! Tricia

My moonbeam didn't make it through the summer. The Full Moon, on the other hand, is still nice and green at the base and threw out lots of seeds that seem to have already sprouted with our warm weather last month.

The only variety I've had true success with for years is Zagreb. VERY strong and healthy plant and transplants easily when divided. The less fussing you give it, the better. Moonbeam does better for me if I don't cut it back in the fall. Zagreb doesn't care one way or the other.

I have Zagreb and Threadleaf Coreopsis that come back every year. Planted in heavy wet clay but they are very hardy and beautiful.

This is what is says on a very reputable website 'Divide coreopsis and its cultivars every two or three years, otherwise you risk losing them in winter.'