Zelkovas, like maples and elms, are good candidates for defoliating. What's your main goal for defoliating? (uploading a photo could be helpful)
Defoliation can be used to:
- reduce leaf size
- balance vigor
- improve ramification (see related post for details about what this means)
Defoliation can also be partial. If the tree is young and better ramification is the goal, you can completely defoliate the tree. If the tree is more developed, removing the large leaves and leaves near the outside of the silhouette but leaving smaller, interior leaves will strengthen the interior areas.
It's very stressful for the tree, but if your tree is healthy and you've been feeding this spring, you'll have prepared it well.
After defoliating, leave the tree in the sun and mist or overhead water regularly. Don't over-water as the tree won't need as much water until the new leaves appear.