RE: Paulownia tomentosa

Joan,
 
I think the problem with Paulownia in oceanic climates like the UK is not winter cold but not enough summer heat. (That's one reason I moved here...) I'm growing two trees here in Saint Louis, Missouri, USA (Zone 6), and last year their 3-4 foot (1 m) stems were fully exposed to at least 15F (-10C) without damage. However, we have hot summers from May/June through September, which I believe ripens the wood so it is more frost hardy when winter comes. The region where they originate in China (quite widespread) has a similar, slightly warmer climate.
 
So, not feeding them too much might help (to stop the stems growing too soft), although they do like moisture and grow fast when they have it. I'm not sure if too much moisture would have the same effect as too much nitrogen. Maybe not. Yes, I would definitely consider lagging the stems with something insulating in winter, at least while you're trying to build up a framework of branches. Definitely don't cut them back, except to cut out any dead wood. When mine started leafing out this April, I rubbed out all the buds except the top two (all the nodes started growing shoots, the terminal bud being defunct). This seemed to work well and I now have two 17 foot (5 m) trees forking from the top of last year's growth. It's amazing how fast they grow. After a while, the strongest shoots started to branch again naturally. Maybe "apical dominance" would have kicked in anyway had I not restricted each tree to two branches, and the lower sideshoots would have stopped developing?
 
There's a park in St. Louis with several old trees of P. tomentosa, which are stunning in late April/early May when covered with flowers. They are deliciously scented too. We are just far enough north for the tree not to seed itself naturally; further south they can be a noxious weed, invading woodland, etc. I assume the seedlings are more vulnerable to winter cold than saplings and larger trees.
 
Nick Turland
Saint Louis, MO
USDA Zone 6
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