I think M. umbellata can be much hardier than people familiar with the much more commonly cultivated M. excelsa may realise, maybe it depends on provenance of the seeds. M. umbellata grow at 1000m above sea-level in the southern alps of New Zealand, for example in Arthurs Pass national park, where I think they have real winter conditions. I have had one for over three years in a large pot in my front garden, exposed to easterly winds on the Chiltern plateau, and it gets through the winter fine, even last winter, when most of the upper growth of my Callistemon was killed. The pot does sit on a sewage inspection cover, which might protect the pot from freezing too badly, but I don't think that would protect the upper part of the plant.

It was about 60cm when I got it from Trevena Cross garden centre, and is now about a metre, and looks like a big lollipop, or child's drawing of a tree. No sign of it flowering yet though. I have heard that it can take 20 years in the wild, though maybe in less trying conditions than I give it, it would flower sooner.

I have been advised to prune it "after flowering", which I presume means about August if it doesn't actually flower.