It may be fertilizer burn. But i cant be sure.

A soil with 50% peat might not be draining well enough, and becomes hydrophobic if it gets too dry. It could be holding too much water, or water could be running through a single channel in the interior of the pot. Also define "regularly watered".

Lift the pot up a few hours after watering. If it's very heavy, too much water. Very light is hydrophibic.

I have 5 JM are in "Al's gritty mix" which is equal parts pine bark fines, granite grit and turface and and are doing wonderfully. I would recommend no more than 10-15% peat in a long term tree in a pot. The peat will also lead to problems due to compaction a few years down the road.

Yeah, they aren't in there long term, but you're correct. Too much peat. I'm pretty confident that they just got too dry.

I plan on going to a similar mix myself. One that won't dry out as quickly as the gritty mix though.

I use what is essentially the 5-1-1 mix - primarily bark but with a small percentage of peat and grit/perlite. This works extremely well for JM's in my climate. Holds up very well until I need to repot, provides very good drainage and is sufficiently moisture retentive so that I do not need to water every single day in summer.

I've been using the same mix for going on 25 years and have never lost a tree to either drought to root rot, so it must be doing something right :-))

Actually you would be surprised at how long the GM stays moist. last week I didn't water mine for 5 days with temps were in the mid 90's without any issue.

Lol, yes I think you're definitely doing something right Gardengal!

The mix that had been suggested to me is very similar.

Another question: how do you determine what size pot to use? I currently have a 'Seiun kaku' in a pot that is probably bigger than what it really needs.

Okay, that makes good sense. I should be good for a while then. Though I plan on switching out my crappy soil for something like you suggested. I assume that should be done in later winter/ early spring?

If you are fully replacing the soil, I'd wait for dormancy. Put if you are just repotting, it can be done any time. I just up potted 3 JM's - 2 new ones and one that I bumped up to a larger container. No problems whatsoever!!

Gardengal, what do you think of replacing the peat in the 5-1-1 mix with Fox Farms Coco Loco coconut coir?

Probably a case of 6 of one, half dozen of another :-)) Coir and peat are almost interchangeable, except for the acidifying feature of the peat.

Ahh, I never would have thought of that. I believe I will just go with the normal 5-1-1 mix.

I only own a half dozen of these things. Some in the ground, some in pots. I error on the side of caution and try to not disturb them over the heat of the summer. In fact I leave their pots alone in some shade over the summer.

Maybe its a Missouri thing but my Japanese Maples seem on the edge of surviving my summers. Their leaves get crispy in the sun, they start to bleach out, then fall comes. Every five-ten years or so we have that perfect summer where it is more 80's not 90's and I can get away with anything. Maybe Tennessee is different.

Gardengal, is in the ocean moderated climate of the Pacific Northwest where soo many things grow so well.

Repot JMs just before or at the onset of spring budswell. Pot up any time but use caution if your soil doesn't drain as well as the plant would prefer. If you pot-up during periods of limited growth, which impacts how long it takes for roots to colonize newly added soil surrounding the root mass, take extra pains to avoid over-watering or prolonged periods of soil saturation.

There ARE a lot of tricks by way of which you can coax a large % of unwanted/excess water from hanging around in your tree's soil.

Soggy soil help.

Keep in mind that in a high % of instances, too much water in the rhizosphere is what's preventing plants from being able to move enough water to their most distal parts to keep them hydrated, which is why the symptoms of over-watering usually first appear at leaf margins and tips - trees dying of thirst in a sea of plenty.

Al