I grow the plumbago here in south Fla. I thought it needed a lot of sun and heat to grow and bloom. So..it might need more light and warmth to get it to bloom. Plus a shot of a bloom booster fert.??
The mandevilla is a pain in the neck here in south Fla. To get it to bloom here, we have to cut it way back to get rid of the aphids and white flies after the blooms die off. Then it grows right back,blooms, gets aphids or white flies again and by then I give up. It always looks skimpy not matter how much I pinch it back,trim it, or cut it back to nearly the ground. So up north,I can only imagine you have the same problem. But up there it probably costs a small fortune, here all the colors are only $9.99 at most for a gallon size plant. So when they look sickly, we just throw them out and buy new. Up there, I would keep it on a preventative spray program and fert. it a lot to get it to grow and bloom.

Thanks for replying. I guess I'd just be better off moving to Florida! What you said makes good sense. I get suckered in because the plants look so beautiful at the greenhouse. I shouldn't complain - at least I got one good year out of the mandevilla. Those white flies are horrendous aren't they? I'll give them both a couple of good shots of Bloom Boost and hope for warmer weather. Maybe I'll see a flower or two before the snow flies!

Guess I just had beginner's luck. I wintered my dipladenia (mandevilla relative) in the house and then stuck the pot back outside this spring and it has lots of blooms on it. That's fortunate because they are over 20.00 here and I don't know anything about whiteflies. Brandy