#5: Tree Dahlia (Dahlia imperialis)
says, "I've had this plant for around 6 years, it does well by my south wall but living over 53 degrees latitude north it doesn't get the day length requirements soon enough for the flower buds to open before the plant gets frozen. It has often got through frost to -5C, the leaves have had some frost on them and survived it, but an extra degree to -6C or different frost conditions usually kill it.
In the UK it grows to around 11 feet tall by my wall, I have grown more plants from it and put in other places in the garden but they never thrive, although some have lived. Buds in the last three years have looked so promising, this year some plants further south in England made flowers in mid November but mine were about to be frozen by mid December so I cut the stem to put in water, there's only one promising bud which I am not holding my hopes for. Getting to mid December is a miracle in itself, unless we happen to have very mild weather it would struggle to flower in very low winter temperatures here. I keep trying and hoping though!
Wind can be a problem when the new stems are still soft, I have had some blow over but with age they go woody. My plant survived in the ground after the very hard winter 2010/11 in the UK, with 7 weeks continually below freezing and with temperatures down to -17C.
New plants can easily be grown from young shoots taken with a little of the woody base. I have also grown them from the stem near the base after winter, which was still 'green' inside although the stems are hollow. If you have a live stem, cut a length either side of two nodes with a length of stem between them, place just under the surface of compost laid on the side in a large pot. They will root and make new plants from the nodes, so with two nodes you will get two plants which can be cut apart once the roots are sufficiently formed and repotted individually. I did this in a cold greenhouse where the pot got plenty of sun. If grown in the ground in a greenhouse I might get more chance of flowers, I now have a greenhouse with broken glass in the roof so next year maybe I will grow another plant to put in that greenhouse even though it gets some shade, worth a try!"