i live in zone 9 in louisiana. dahlias grow fine here. i would suggest you contact your local country agent you can find them in phone book or web. they will know what cultivars grow best in your area and can advise of the care requirements. rather than hit or miss, get the info from them. or if you can purchase a book that is specific for your part of the state. i don't know about tuberous begonias nor columbine.

Sunny43 -- I love all you have mentioned! Dahlias do not seem to hold up well to the heat, humidity and/or salt in mid summer and languish by July in my yard. Columbine and begonias might flourish if we can mimic cooler microclimates conducive to growing. At least that's what I'm going to attempt this year. :-) ONE more time.

Houston gardner here, zone 9. There are a few Texas native columbines that you try, mainly Hinkley's Columbine (yellow) which is often available at local nursery's specializing in native Texas plants. It also has a delicate and lovely smell. I get these to stick around and often reseed. I give them part sun but not too shady and don't overwater either. These natives are pretty sturdy despite their fragile looking foilage. There is a red one too which survives our summers: also a native.

I have never had luck with Dahlias but begonias are easy.

I stand corrected after a little more research. Your question was more complicated than I imagined.

As many gardners here will tell you, most Dahlias don't work, but the good news is there are 3 that can...Dahlia merckii, Dahlia coccinea and Dahlia imperialis (for shade). This is from Scott Ogden ("Garden Bulbs for the South"). He also say no to all tuberous begonias EXCEPT Begonia grandis (evansiana).

He does recommend RHIZOMATOUS begonias however; check out the link below to a site with tons of different kinds of these! It also shows Begonia grandis (evansiana).

What is frustrating about gardening down here is that we hear about someone having success with something, like Dahlias, and we try the wrong kind (that do well up north). After they burn up by June we swear them all off not realizing that there are indeed some SPECIFIC kinds that will do well here. I'm going to look for those Dahlia's he mentions now...I had sworn them off forever after a fruitless experiment with the wrong kind many years ago. Glad I read your post!

Here is a link that might be useful: all kinds of Begonias if you drop down