Hi Wengren
I have been growing bottlebrushes for a couple of years now, growing cuttings and training into standards as well as bush plants.
They can withstand the cold and some frost, I left one out this year and it survived with only a frw crinkled leaves. During the sumer they like to have plenty of water but in the winter most of them go in the greenhouse and I let them dry out a bit. Some of them have gone a little yellow but I have taken them out of the greenhouse now watered and given a little feed (fish, blood and bone). If left they should recover and when new shoots start to apear I will cut out any dead. Bye the way all mine are kept in pots, repotting to a larger pot as the roots grow.
Hope this is of some use to you