AusBonsai

Hey guys and gals,
Just a quick question....ive bought 5 Acer Palmatums from the local bonsai nursery, lovely red trunks and bright red leaves.
Im going to plant them into a forest style pot in August, but for now they will remain in their individual pots. Id like them to thicken up a bit but not sure on best method....

With maples, will allowing them to grow tall thicken the trunks or is another method better?

Prune the roots short and flat to make it easier for your eventual forest planting. To thicken, just let them grow without pruning. There is no other way.

You are not going to get much thickening between now and August though.

Growing tall will thicken the trunks but at some stage you need to cut them back down to bonsai size. Growing one tall trunk will leave you with a stump with no taper and a big cut at the top - not much of a start to great bonsai. I try to grow multiple leaders on my trees so that after pruning there will be several smaller cuts that should heal quicker and also I can prune so there is taper in the bit of trunk I'm going to keep.Even though it takes a little more time initially the following development stage when you grow new apex and branches and heal the cuts takes far less time. I think this slower start actually saves time and gives a better result in the end.

Note that Japanese maples are far less forgiving of fast growing and pruning than tridents. Japanese maples are prone to unsightly thickening and developing odd angles where new shoots grow after pruning so tread carefully with this species. I firmly believe these should be developed more slowly for best results.