Despite 'watering was not missed' I still believe that scorch is mostly due to lack of water. There are a number of reasons why trees dehydrate.
1. Water supplied is not sufficient. Especially during change of seasons when water demand is increasing due to longer days, warmer temps as well as increased plant growth demanding more water. I find that I need to adjust how much and how often I water as spring progresses.
Another reason for insufficient water can be due to not watering thoroughly. When soil is dry the first water tends to run through or around the potting soil without actually wetting it properly. I discovered this many years ago when I started doing autopsies on deceased plants and was surprised to find the outside of the soil damp but the middle still bone dry even after what I considered adequate watering. Light watering may be OK through cooler months but may not be enough as the weather gets warmer or as the roots start to fill the soil. Allow the first water to soak in then water again to allow that water to soak in better. Japanese bonsai growers teach to water 3 times - once for the pot, once for the trunk and again for the leaves. My take is this is just a way to remember that a single watering may not be sufficient.
In warmer weather I water twice a day. Evening watering gives the tree something to keep going with after a hot day (for the tree) at work. The following morning water should soak in much further and set the pot up for another long day in the sun.
Not the case this time as these tridents were only potted up this spring but as the pots fill with roots there's less and less spaces for water in the pot. Eventually there's no way to apply enough water to get a root bound tree through a hot day. Pots in trays of water can help in these cases but I've learned to repot regularly to save the heartache of losing trees to dehydration.
2. Pot is too small or soil does not hold enough water to hold enough water to last all day. I've stopped using really shallow trays and the trees seem to do far better. My potting mix also has good amounts of pine bark which allows it to hold a bit more water. Really quick draining soil mix has certain advantages but also has disadvantages.
3. It seems that some trees just cannot transport enough water to maintain leaf hydration in really hot weather. Japanese maples can get scorched leaves despite there being plenty of moisture in the pot. Obviously they were never adapted to hot, tropical climates or desert conditions. More shade can sometimes help but trying to grow trees in areas they are not suited too is often futile.
How much tissue dies will depend on how severe the dehydration. Trees are able to shut down progressively to save themselves from total death. Sometimes it is just a few leaves, sometimes all the leaves die, sometimes the thinner shoots or a few branches also die back and in really severe cases the whole trunk dies.
Trimming won't make any difference to how much of the trees have been damaged. The trees can compartmentalise so dead sections won't go further. Just let nature take its course and watch for new buds then you can decide where to cut back to healthy living sections.
Sounds like a little more shade could be helpful at least until you can work out proper watering. In some areas there's no choice but to keep some trees in more shade than would otherwise be desirable.
If only one side of a group has been affected there must be a reason - watering not sufficient for the entire pot? One side of the group gets more sun? More trees on that side competing for limited water? have a good look with an open mind and see if you can work out why only one side has been affected.
If it makes anyone feel better, I also got caught out a few weeks ago and several of my shohin tridents are now growing new set of leaves. Sudden change from almost constant rain to a run of hot, sunny days caught me unaware.
for your tridents to bounce back quickly. If not, there are plenty more as replacements next winter.
