Acer palmatum 'Viridis' - a very common form of green-leaved dissectum. Not sure what your climate is like in SA, but in temperate zones, this tree will turn shades of glowing orange or scarlet in fall before dropping its leaves entirely and going winter dormant. Regardless of how complete the period of dormancy, it will naturally drop its leaves in winter.

It's a little bit hard to diagnose what may be wrong from remote but here are a few thoughts:

Morning sun and afternoon shade is the preferred siting in all but the hottest areas or those with very strong summer sun (closer to the Equator). In those, mostly shade would be better.

This is generally considered a small form of J. maple. Graft height will determine the ultimate height, but spread will be variable and can be indeterminate to a large extent. Growing a J. maple or other small tree in a container is very common and can be very practical in climates where moving to protect from adverse weather conditions is advised. The container size sounds about right. Because of the root confinement, maples grown in containers will typically grow much slower and stay smaller than those grown in the ground.

Container planting media is pretty important. You need a very well-draining medium, yet one that is moisture retentive. Generally, a compost-based medium will breakdown and compact rather quickly and than can adversely affect drainage of the container. A potting soil that contains a good percentage of perlite or pumice and bark fines as well as other material like humus, peat or screened compost will provide excellent drainage yet hold its structure. With a quality potting soil and the right sized container, repotting should not be needed for several years

In hot or very dry weather, daily watering of containerized maples is pretty much SOP. Unless your soil is not up to par or drainage is not ideal for other reasons (don't use a tray or saucer and make sure pot is lifted slightly), it's hard to overwater in summer.

Because watering is so frequent, nutrients leach very quickly from container plantings and need to be replaced. Typically, a slow release fertilizer is applied at planting - or on established containers, at the beginning of the growing season - and often supplemented during the growing season with a dilute liquid fertilizer.

It is also possible that your tree is affected with some sort of disease pathogen. Root rots are pretty common with J. maples that are under stress or experience significant root disturbance and since many of these are soil-borne pathogens, it is quite possible that the plant or the root ball was infected before you purchased it.

Is there an agricultural bureau or horticultural office in your area that you could take the little guy to for analysis? Here, that service is often provided by better nurseries or county agricultural extension offices. An experienced professional who could examine the tree in person should be able to determine exactly what the problem is.

I'm by no means an authority on JMs, but check your soil before you water.

In Iowa we have very HUMID, hot summers. The high air humidity can slow a plants transpiration of water - less water moving out from the leaves = less water being moved through the plant from the roots/soil. So the soil can stay very soggy despite the high heat.

I've known a lot of people to overwater their potted plants when it's hot and humid, the soil stays too soggy and contributes to root rot, the plant wilts from the root rot and they keep pouring water on it because it's wilting. It's a nasty cycle that frequently kills plants. I've seen it happen very quickly in hot weather - maybe the 'rot' spreads faster then?

Now, note that I said "plants" not JMs. I'm a newbie with JMs and this comment is meant more as a 'plants in general' sort of suggestion. People who live in very dry, hot climates can water everyday and very often need to. But if your area is hot AND humid check you soil moisture before you water. It's easy to do and you don't want to be constantly adding water to saturated soil.

Gardengal48 has some great suggestions - don't you love having experienced people to learn from!!!!

Can you find anyone growing JMs in your area? Maybe a local garden club? Can you check with the nursery where you got it? if you can find someone local who's having success with JMs they may have ideas or suggestions specific to your climate.

Great advice above. If you can't find a professional to look at your plant, I would suggest checking the soil at the bottom of the pot for moisture.

If it is more than just a bit moist, poor drainage and the dreaded root rot COULD be the problem. Of course I can't tell exactly what type of consistency your rich compost has, but in my experience these can hold a bit too much water. If this is the case I would suggest not watering until all of the soil is dry for a few days. Be sure to leave the tree in shade during this period.

I would also consider repotting it later this fall when temperatures at night are just above freezing. Like mentioned above I would suggest a good draining potting soil, similar to the azalea mixtures that are common.

If the soil is draining fine, there could be other pest issues. I would once again check the soil for problems there, as well as the leaves.

There is also the possibility that the tree has experienced a bit of transplant shock, and the heat could be amplifying this. In the deap south of the US some of our trees experience similar leaf yellowing and burning.

I have seen a 'waterfall' (which is another green dissectum) that experienced similar burning during the heat the past two summers. The tree appears to be healthy and is well established, but all of the leaves had some burning and browing before slowly falling off in early fall with very little fall color except for some yellowing.

Thanks so much for your replies, it's helped clear up some misconceptions. Comparatively to the US, I would have to say our weather is fairly temperate. It never snows and in the middle of winter it's rare that temps go below 41.At east now I won't panic when it does lose it's leaves in winter.I'm starting to think that maybe, as Gardengal suggested, that the root ball might not have been healthy at purchase. Reason being I popped into the nursery that I purchased it from and they had a bunch of JM with similar issues that had been put aside. I think I'm going to go back there and have a talk to the supplier.

Alternatively, I'm also wondering about the soil that it's in...we have an azalea mix potting soil here, perhaps waiting another month or so until we are in autumn and repotting it might not be a bad idea....I just hope it lasts that long.

Thanks for the replies...I'll keep you posted

>Acer palmatum 'Viridis' - a very common form of green-leaved dissectum.A. palmatum Dissectum Viride Group (there are multiple clones) is name used overseas instead of A. palmatum var. dissectum 'Viridis' used here in USA. All Japanese maples subject to vascular complaints and blighting off, if your plant came home with an infestation it may continue to dwindle. If you have an equivalent to our Cooprative Extension Service (such as an Agricultural Ministry) in operation there perhaps they offer a diagnostic service that can process samples of your plant and tell you what, if anything is attacking it.

Here in Texas my JM's have experienced much higher and prolonged temps and many of my JM's scorched, although I'm getting much much better at managing it. I've had many JM's suffer and recover from what you describe.

We have a second problem here and it's alkaline water and soil. The leafs will scorch quicker when chlorotic, and I believe that the mychorizzae are also effected.

There are several things I try to do: maintain constant moisture, an acidic environment, maximize root mass.

My current soil mix includes a top quality potting soil, pine bark, peat, shale, lava and green sands, alfalfa meal, kelp meal, cottonseed meal, wormcastings, polymer crystals and mychorizzae. I mulch heavily with pine needles and pine bark.

I've had excellent success with periodic applications of seaweed extract and compost tea that is supplemented with mychorizzae and humate. I stay away from rock phosphate because I read that it could destroy the mychorizzae population which is so critical to maples. I also read that some gas emitted by the perlite could be detrimental to JM's but I know perlite is widely used for JM's esp. with Bonsai.

I try to protect most all of my JM's from afternoon sun (although when conditions are right some have handled some serious sun and heat w/o adverse effect). JM's purchased in the summer here will most assuredly suffer leaf scorch and potentially drop leaves (even when in the shade) but they generally recover. Also, don't wet the leaves in full sun.

You didn't mention the caliper on the tree but if the soil is good and mulched the pot sounds large enough, although a little wider prob wouldn't hurt.