I have a heaven tree that has barely any leaves on this year. It's about 11 years old and I never had this problem before with it.
Assuming that you're talking about the common "tree of heaven," ailanthus altissima, all over North America it's considered one of the most invasive and pernicious of weed trees. In other words, almost nothing bothers it short of heavy doses of herbicide, cutting down, poisoning of roots, and digging out of stumps. If you're talking about some other kind of tree, an established plant that suddenly starts to lose leaves is likely being affected by some change in environment (water and light being the two major ones,) or by pests or pathogens. You could get samples of the leaves and soil analyzed through your local Master Gardener program. This link will help you locate one: http://mastergardeners.org.uk/get-involved/find-your-nearest-master-gardener/
Hey, so this seems to be spreading like mad in the garden. The roots are deep and spread widely. Does anyone what this is and how to kill it? I'm in the UK (Dorset). Cheers
This is a perfect example of introducing non-native species to an area, and the invasive nature of such actions!
Ailanthus altissima, or the "Tree of Heaven" is no angel. Controlling this, and its offspring will be just short of impossible. I would recommend contacting your local agricultural authorities, and extension services in your area for a way to solve this, as it is going to require a good bit of professional help.
This article will offer a start to controlling this tree: https://www.gardeningknowhow.com/plant-problems/weeds/tree-of-heaven-weed-control.htm