Mine only blooms once a year.

Even if it only blooms once a year, it may need potting up to do it's best. 12" seems pretty small to me, especially for a 3 year old clematis. Mine are all in 18"-22" pots by that age.
Judith

Agree with the above. 'Henryi' has a single (and somewhat short) bloom season in early summer. And I would definitely pot it up to a larger sized container - I consider an 18" diameter an absolute minimum size for a containered clematis. The soil just heats up too much in a smaller pot and it is difficult to keep the roots sufficiently cool and evenly moist.

So pls gimme a list of very free /long flowering Clems for pots in fullsun!
Tks...!

How about trying one of the patio clematis? They would be perfect for a pot on a sunny deck. There are several different ones, but here is one I have:

Cezanne
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I grow it in a 18" foam pot (overwintered in my attached shed) It's on my back deck, which gets sun until about 1:00pm and does very well. Cezanne only grows 4'-6' tall.
Judith

Sorry Kris, Cezanne only grows 3' or so tall. It's in a pot, so I guess, during one of my more senior moments, I was judging from the ground up :)
Judith