"Likes" full sun may be too strong a descriptor but a great many JM's will tolerate full sun, especially in more northerly locations. I am pretty far north as well (47th parallel) but not as far as you and most of my maples are in full sun.

'Trompenburg' is well known for both its heat and sun tolerance so I am not sure it is the choice of cultivar that is at issue. I'd be more concerned about the sizing of your container, the potting medium and the frequency of watering. Containerized plants in full sun will just need more frequent watering than those in more shade. The shade reduces soil evaporation. I'd guess your trees were just drying out too fast in the sunnier location. The drooping or sagging and slight curling are classic signs of early drought stress.

I grow all my JM's in containers and we have also been having an extended run of unseasonably warm and dry weather. I started watering them in April, which is unheard of in the PNW! Usually that doesn't become necessary until at least the end of this month.

Container is 3 times the size it was delivered in so that is not an issue, also I use 50% john innes no.3 compost mixed with 50% ericaceous compost. The soil is never dry, ever, it has 20mm slate chippings on the surface to keep moisture in, I always water to keep moist but not wet, also drainage is good.

I am not familiar with UK soil mixes so it is difficult to comment on what you are using. I can say that growing trees and shrubs most successfully in containers longterm requires a very textural, fast draining potting soil - something formulated only for container growing, not something one would add to inground soil. These are typically soil-less mixes - no loam, no compost: really no organics at all - and are bark based.

I might add that depending on the size of the container you received them in and the dimensions of the root ball, an initial container 3X larger may be excessive, particularly if you are using a rather moisture retentive potting mix, which it appears you might be. Excessive moisture surrounding the roots can produce nearly the exact same visual symptoms as underwatering can produce.

The Container Gardening forum might be helpful in this situation for further clarification but I am linking you to a discussion that originated on that forum about the importance of preparing or selecting a proper container soil mix.

Container Soils

john innes no.3 is made for final transplanting of established plants and trees into pots. I mixed in 50% ericaceous compost as JM's seem to like this. I don't think the compost is overly moisture retentive, but I made sure it is keeping a little moist in by adding a top layer of slate chippings. It will be moist this time of year here as it's the beginning of spring so lots of rain.

I need to stress that the only different thing that I had to do was move the plants into shade, after that they were ok. Which has puzzled me because they should have been fine in sun

We have a small issue with regional terminology :-) 'Compost' here in the states refers to an entirely different material! But I did some quick research on John Innes#3 and the one thing that jumps out at me immediately is that it is soil-based. Without seeing the material in person, it is difficult to draw conclusions but I would suspect that your mix is rather finely textured. Certainly the loam soil base and the sand would indicate that and the composted waste products would generally not contribute enough textural size or durability to offset. A very finely textured potting medium is not ideal for the longterm growing of woody plants in containers. It doesn't offer the porosity that is necessary for fast drainage and proper aeration, which are critical factors for container growing.

I really do urge you to read through the thread that I linked to. The success of your trees long term is diredlty related to the quality of their growing medium. As a long time container gardener myself, I can attest to its importance.

Finally, let me say I don't think there is anything wrong in your choice of trees. As I stated previously, 'Trompenburg' IS highly rated for both sun and heat tolerance. I strongly believe the issues you are having with your trees is most closely associated with how they are being grown - the potting mix and the watering practices.

Proof is right in front of me, I re-potted them 6 months ago, during winter while they were dormant, then at the beginning of spring they were put in front of my house, they were still fine, then over the past 4 weeks the sun in the front of my house got quite intense and warm here, for spring. The trees leaves drooped a lot and slightly curled.

I put them in the back garden again so they were out of the sun and then they were ok within 2 weeks. The potting mix and water has not changed from being in the front of the house to the back of the house, the only thing changed is the amount of sun, at least 14+ hours direct sun (front of house) to 3-4 early morning sun hours in the back garden.

In the future I will add some composted wood bark to the compost medium to make it more textured (when I next transplant them).

I'm thinking maybe I wasn't even sold "Trompenburg" ?

Can you identify JM's just by photos?

Hi DJ,I agree with Garden Girl,the full sun is probably showing up a weakness in your potting mix,together with the recent strong winds it wouldn't surprise me at all.It's not just the amount of water but the lack of air pockets that make it harder for the plant to get the water.Here in the UK a lot of maple growers who don't wish to use a gritty mix favour John Innes No.3 mixed with pine bark 50/50.As long as they're careful not to over water they seem to have good success....and yes,Trompenburg is one maple we can usually identify from photos.

ok, i'll put up some photos tomorrow, i'll add some composted bark in the mix next time

..... looks like folks in the know are all in accord. If your plants wilt in the sun while the soil is still moist, you can scratch under-watering off the list of potential causes. That turgidity returns as evening approaches or soon after the plant is moved to shade is a clincher, reinforcing the idea that a too dry soil is not to blame. Roots function most efficiently when there is lots of air in the rhizosphere (root zone) and the soil is damp/moist, not wet/soggy. Water/nutrient uptake is an energy-driven process that requires O2 to convert (literally 'burn') stored carbohydrate to produce the energy required for the process. If what you're experiencing is media-related, it's probably because the soil is too water-retentive and your plants can't absorb enough water due to anaerobic (airless) conditions in the pot.

Al

Here's some photos, the 2 larger trees where sold to me as Trompenburg, I have no idea

what the middle smaller one is? What do you think? Crimson queen or bloodgood?

Above is a close up of the unknown small JM

Below is the same unknown small JM

Below is a close up of the Trompenburg? Leaves, still looks like a bit of droop there but nowhere near as bad as it was

What age would you say the 3 plants are? The Trompenburgs are supposed to be several years old, that's all I know. The person I bought them from said they got them from Holland and they kept them for several years. So what do you think? 8 years?

Hard to say, if they've been in containers all that time. Container culture does naturally stunt growth to some degree because of the restricted root run. They seem quite small to be 8 y.o. - I'd say closer to 4 or 5.

If you repotted them six months ago while they were dormant, the rootball is still small and isn't able to draw enough moisture to the leaves when it's warm and sunny. That, coupled with the info above, is causing your problem. Give the roots more time to expand and I think they will be able to handle more sun.
Mike

ok, i'll have to wait till next year to re pot into a larger pot and then add wood chip as the trees have all grown leaves now and I don't want to harm them, I cant seem to find pine bark so they'll have to do with any woodchip / composted bark that I can find

Look for reptile bedding at pet stores. It is exactly the right size and consistency to use for this purpose.

is orchid bark ok?

As long as the piece are no larger than a dime (or your thumb nail for you Brits :-)), yes, orchid bark is fine. Don't want extra fine either - that defeats the purpose of the bark.

I'll use the fine beech chippings, they are a bit cheaper.

You don't want to use chipped hardwood to save a buck. They present a considerable number of potentially serious problems as any significant fraction of container media. Stick with pine or fir bark.

Also, the size of your plant's root system isn't an issue unless it's rotting or impaired by poor soil and/or over-watering. If you root prune a dormant tree, the inactive buds won't come back online until there is enough root mass to support the growth. Roots always precede top growth. If the root work was severe and the tree 'just barely made it' through. It's STILL not a problem because the net result of that scenario is the death of distal and/or weakest buds. After those buds die and growth resumes, the tree simply builds on what living organs remain and you prune off the dead growth; but it sounds like nothing so severe is in play here.

Al

Just got some pine wood chippings (not bark), It will have to do, its not all small pieces like a "dime" as its random chipping waste from the local sawmill. It will mix in ok and help a lot with drainage, plus it'll add the benefit of composting down into plant food over the years.

You must not have been paying attention, but as you wish ....... you're in queue for some heartache. Count among the issues:

Severe N immobilization (tie up)

Soil (structural) collapse

A pH spike during the composting process

Very high soil temps due to the exothermic heat given off during rapid composting of the cellulose in the sapwood/ heartwood

Best luck.

Al

Yeah, it looks like there is too much surface area involved. Better to add the wood shavings than not though. I routinely till in larger woodchips from a tree service and have had no yellowing problems in my ornamental garden for many years. The woodchips break down slowly. I till them in to a soil that is very good to begin with, and haven't used fertilizer for over thirty five years, except for potted stock, of course. Sawdust tilled in is a real nitrogen robber because of all the surface area that breaks down rather rapidly. Your pine chips are very thin and will break down almost as fast as sawdust. You might want to add a little fertilizer to compensate for that.
As a good analogy, I was in a bar in rural Montana that had rock salt in shakers with large holes in the cap. When I asked the bartender why rock salt instead of normal salt, he explained that normal salt was like flash powder and dissolved real fast leaving the beer flat. Rock salt dissolved slowly, giving a salty taste with out flattening the beer.
My neighbor tills in a lot of sawdust every year in his veggie garden. He adds ammonia sulfate when he tills to help it compost. The added heat provides him with warmer than normal soil and helps his veggies off to a faster start in the Spring. In effect, he gardens in a compost pile, and has a very good garden every year.
Mike

Apples are to orange safety cones as gardening is to container culture. No matter how the words are parsed, hardwood bark and sapwood/ heartwood should be left out of container media.

Excellent media for maples:

Al

Damn it, I walked 5 miles (8km for you in the US) with a 70kg sack of pine chippings! Now they are useless ha ha.

I'll buy 4 or 5 bags of this, then do a mix with 50% of it along with 25% John Innes No.3 and 25% ericaceous compost next year:

http://www.ebay.co.uk/itm/Orchid-Bark-Reptile-Substrate-5-Litres-10-Litres-CHEAP-FREE-POSTAGE-/131503532505?pt=LH_DefaultDomain_3&var=&hash=item1e9e38a9d9

Its 0.5mm - 12mm orchid bark chippings

Al, I'm clueless as to why hardwood bark and sapwood/heartwood should be left out of container mixes. You have probably explained it numerous times in the Bonsai Forum, but I don't hang out there.
The reason I ask is because I have a lot of plants in containers (250-300) and I make my own potting soil from variously aged woodchips and sand. The woodchips are a combination of softwood and hardwood, depending whatever is in the load my arborist friend brings me. Leaves and needles also. When potting up the plants I mix the woodchips with aged woodchips so there is proper aeration for at least a year, sometimes two. So far, I haven't differentiated between hardwood and softwood.
If there is anything I can do to improve things, I'm all ears.
Mike...near Seattle

Fir bark should be cheap where you live. Shasta Forest Products in N CA is a big distributor, so it shouldn't be to hard finding it - and I'm guessing the price should be a fraction of what I pay (when I can find it).

Sawdust and wood chips generally start off at a pH of around 3-4.5. Without lime, the chips initially drop further in pH. As the organic acids decompose, ammonia combines with hydrogen to form ammonium ions and the pH starts to rise - up to as high as 9.5. As the ammonium is used by microorganisms, hydrogen ions are again released and pH falls to more favorable levels. It's THAT pH spike I was talking about, and the significant amount of heat generated by the rapid decomposition. Remember that piles of wood chips and/or sawdust can get so hot that they spontaneously combust.

Al

I didn't know about the pH spike. Thanks, Al.
I treat my whole garden as one big compost pile. Not one that gets hot enough to cook seeds or anything like that. Just a slow breakdown. I've been applying about 30 cu. yds. a month for over 25 years on 10 acres. I till in as much as I can and use most of it as a mulch, top dressing, and it makes good trails.