Runners on Japanese Mapel

[PowerTap]

I got this Japanese Mapel from a club member who was giving them away because he had to much stuff that he started from stock. My plan this year was keep it alive with food and fertilizer until I could see it naked in the fall. I don't really have a plan for it beyond that.

But now it's July and I have some shoots that are just running way out of control. I know I'll cut them eventually because they have long internodes and are to tall generally.

Is there a case for letting them grow all summer to add trunk caliper or cut them now?

 

[pandacular]

I have read that Acer palmatum likes to put out runners before starting to "mound" or add branches in the interior, and the as a corrolary to this, trimming runners too far back too early can cause you to lose out on that internal ramification.

You should definitely wait for someone more experienced than me to chime in though!

 

[Shibui]

All growth will add extra thickness to the trunks below which is usually a good thing. However need to be wary of strong growth from close to the tips of JM. They are very susceptible to overthickening at the ends of branches and top of the tree. That's even more of a problem if there are several growing close together.
Not clear from the photos where all the shoots come from so you will need to check and make decisions. If there's any growing close together or if any sign of inverse taper I would recommend cutting off right at the base. Those shoots will probably already be too thick to be useful way up in the canopy anyway. You may even need to remove some of the old branch they are growing from if those are already thickened but that can probably be left until fall pruning when it's easier to see what's what.