Ontario,

Texas talking here. Those are some beautiful trees. I'm assuming that you've called some of the better nurseries that sell this plant. I can't help you. Just trying to give you some encouragement in a forum that doesn't get much feedback. But, that's why I'd do. Here in Central Texas we have Extension Agents in each county that can answer questions like that... or should be able to. While you are not in the USA, does Canada have anything in place like that for you?

Good luck!

One more thought. The Brits are avid gardeners. They may have the answer you need.