We have one, but since we pollard it severely for the huge elephant's ears leaves, it does not actually flower. Supposedly, if one damages a root it suckers and then one can propagate it that way. I have never tried that. The book also says cuttings in Summer. Tried that without success. They are available as trees from The Dingle Nursery in Welshpool, but are quite expensive.
The seeds need light to germinate so if you coverd them with compost that may have been why they did not come up.
Oh and another thought. They do not flower like that until they are fully mature trees, say 60 foot plus?
