Any time during the growing season you like. I give all of mine a dose of Osmocote in early spring as they are just starting to leaf out but supplement with Dyna Grow periodically through the growing season as well. I use the 7-9-5 GROW or Bonsai formulation but the 9-3-6 Foliage Pro would probably work equally as well.

If I were relying only on the Dyna Gro, I'd use it weekly at half strength.

I typically stop fertilizing in late August. But first frost here is not expected until late November so plenty of time to harden off before winter.

I do up to late summer as well. You don't want a flush of growth that will freeze and die back. I have used the 7-9-5, and the only reason I use it is because i have it and want to use it up. The P is too high IMHO. I prefer the 9-3-6 and Pro-TeKt. I full dose in the spring at the first sign of bud swell, then in Maintenance dosage or 1/2 maintenance dose as the season progresses.

I recently re-potted the tree 1.5 weeks ago. Can I fertilize it now or wait?

I would. It is not getting any nutes to speak of from the potting mix. But I would do it at half strength.

I use FP 9-3-6 for all my trees.

When maples start to push growth in spring, each bud breaks to push a short internode before the next pair of leaves appears. The next internode is usually extremely long, up to 6" or more, except on cultivars known for very short internodes. Look at your trees to confirm. This is a serious problem with regard to how it affects the form of your plant. In short, it wrecks it. Your trees should be pinched as soon as the first pair of leaves can be separated far enough to allow a pair of scissors to get at the internode. This results in 2 maturing leaves and a pair of buds in each leaf axil. As soon as the new growth starts moving in the new leaf's axils, you should remove the first pair of leaves that occurred after bud break. Not to worry, the tree will back-bud automatically and replace those leaves. When new leaves appear after the first pinch, pinch again immediately. The best time to fertilize is when most of the second sets of leaves are mature or almost mature. If you fertilize too early, you get a lot of long, rank growth that doesn't do anything for your composition.

Pinch the central leader where the pencil points as soon as the 2 leaves opening can be teased apart with the tip of your scissors to get at the central leader.

You'll get a new branch from the axil of each leaf that remains. When the new branches are evident, remove the 2 leaves you see above. When the 2 new branches have pushed the next set of leaves, pinch again. After that, you can use defoliation techniques to slow down the overly strong branches so the weaker branches get their share of the energy. If you see a branch high on the tree that's thicker than other lower branches, remove 1 leaf from every node to slow it down and force energy to other branches. Build your trees from the bottom up. Maples want to put 2/3 of their total energy supply into the top 1/3 of the plant, so you can build a top any time you want. Best to do it after your lower branches are formed.

Al

While I dislike disagreeing with Al about anything (and generally never do), I have to say that I have never done any pinching on any of my maples, ever. And I grow all of them in containers. None have ever produced long internodes between foliage sets....a most, there is only an inch or two. I do see it sometimes on inground trees but never on any of my container plants. In fact, I seldom touch them with pruners or otherwise alter their natural growth habit.

I wonder if the requirements for growing JM's as bonsai put more emphasis on this technique than just growing the plant free form in a large container...........?

Al, please feel free to comment if you think I am not doing something more that I should..........

My dwarfs don't generally get pinched, they stay small on their own. The ones that are not dwarf/miniature get pinched.