每日吃瓜

I have a 17 yr old English oak grown in a large pot from acorn. I started to reduce it in size last year and am going to try root pruning it in spring ‘19 to get it in smaller/shallower pot, but wonder if I can defoliate this deciduous species in summer to try to reduce its natural large leaf size and get the foliage in proportion to the tree?

If you repot in spring. Then i would defoliate in late summer. No problem with that.

Be carefull defoliating, certainly if you now do a root reduction. In general, defoliation is only done in the more final stages of tree development. In most cases, thinning of the canopy, removing the largests leaves is much better for the trees health, and the densification of the canopy. Leaves will reduce as the canopy gets denser and the rootball smaller.

If I judge the situation right from your post, your tree is not in a state that you want to reduce foliage mass drastically.

I would never do re potting and root pruning in the same year as defoliating., and I never remove all leaves I think it is far too stressful on the tree.