Japanese maples and hostas can coexist quite well...........Japanese maples have very mannered root systems and will cohabitate with other low growing plants - hostas, ferns, epimediums, Japanese forest grass, groundcovers - easily and very companionably.

Having said that, my suggestion is to always plant anything you desire to grow along with the maple at the same time you plant the maple. JM's have very delicate root systems that are easily damaged by any sort of cultivation. And damaged roots can be an access route for some very serious soil pathogens that could kill the tree.

It depends on how large and well established the tree is as to planting distance away from the trunk you can get away with. If at all possible, I'd stick with the 30" distance........any closer and you risk encountering and damaging the tree roots.

With more upright JMs the hosta would be fine. Cascading JMs tend to overgrow the hosta in time to the point where the hosta barely show (ask me how I know). To move the hosta later involves disturbing the JM's roots which is to GG48's point. In either case, the hosta and JMs would grow fine but hosta value/impact ultimately decreases with cascading JMs without pruning yearly or so.

tj

I made the mistake of adorning potted JMs with hosta or heuchera. They looked lovely. But when the time came to repot or put them in the ground (which is my goal) I found it almost impossible to extricate one from the other. So I didn't.

I just put them whole shooting match either back in a pot or directly into the ground. Maku Jaiku with hosta 'Bold Ribbons.' Tamyukama with heuchera 'Color of Beauty.' Corillium with heuchera 'Obsidian' and 'Sunspot.' Omuryama with hosta 'Red Dragon' and Johin with hosta 'Purple Passion,' for example.

After 4-7 years of these goings on, I can report all three types are thriving.

If your JMs are weeping, stake them to grow higher, then when the time is correct, trim them up from underneath. In the meanwhile, with 15" of clearance, you should really be fine.

I have a collection of red hostas. Things with red stems, or deep purple blossoms. Red Dragon, Cherry Bells, Fire Island, etc. Right now, a Green Cascade and Waterfall are shading those strongly, in that I have to peek under their limbs to check on them. But both are happy in this odd spring weather.

(Forgive me if my JM spellings are incorrect. I just got up from a 14 hour shift and am way too lazy to get up and check them.)