Acer osakazuki problem

I have an apparent die back issue with an Acer Osakazuki. It is grown in a container, potted almost exclusively with a regular multi-purpose compost. It was bought as a 1 metre tree in spring, before coming into leaf.

It's in a fairly sheltered location although it does get some direct sun, especially at this time of year (June/July) and is exposed to wind. In recent weeks certain stems/branches have been dying back, starting with the leaves prematurely turning red and then shrivelling up. Initially this was only affecting a few lower branches but recently appears to be more widespread (pictures below). I thought it may have been down to wind or sun damage but the affected parts of the tree don't seem to be the ones with the worst exposure. Also I understood Osakazuki were more tolerant of sun/wind than other cultivars. Am hoping it's not more serious - eg. verticulum wilt. I don't think it is, but it's hard to diagnose from the few pruning cuts I have made to remove the dead wood.

Some of the branches do appear to have a form of powdery white mildew on them, which I have sprayed with a regular fungus treatment for ornamentals but to no apparent effect.

I live in South West London, UK. Grateful for any ideas, thanks

 

Difficult to tell for sure, but it might just be the plant making an adjustment to its new growing conditions by shutting down some of the branches; natural dieback on otherwise healthy trees tends to have the same red then shrivelled appearance. The above ground part looks relatively large when compared to the size of the rootstock, which may have been of an adequate size to support the top in the protected environment of a polytunnel or shadehouse at the nursery, with an automatic watering system, but may now be struggling when exposed to wind and the present UK heatwave conditions.

As for more general advice, I find 'Osakazuki' needs a large container to do well, it might be a good idea to consider repotting once the heatwave has passed. Also, "almost exclusively regular multi-purpose compost" is not the best potting medium for long term root health of Japanese maples, if you can amend this with about 30% by volume of coarser material such as perlite, grit/gravel, bark chips etc. your tree will thank you for it.

 

Thanks for that maf, really helpful. I had a nagging suspicion about the pot size - and also the potting mix - so glad for your advice on that too.

I'll keep an eye on it, but would be gutted if it died - the colour of the leaves that have succumbed is so nice in its red stage, that it'd be great to see it in full autumnal mode.

 

Another benefit to a larger pot is that it will lessen the chances of the tree blowing over and allow more oxygen into the roots with a greater surface area. Your margin for error narrows with a smaller pot, because it can go from too wet or too dry in a much shorter time. It also makes the root zone more susceptible to larger and faster temperature swings. Of course there are also downfalls to going with a pot that is too big. So it's important to keep things in balance.

I would monitor the soil conditions between now and when you decide to re-pot. Sometimes 100% general purpose compost can become very saturated and limit the roots ability to breathe. This will cause the leaves to dry up, which gives the impression that it's too dry, when in fact it is actually too wet and the roots are lacking oxygen.

 

Also really useful, thanks for your advice JT1

 

I am going to stick my novice nose in here. Something I have observed with potted maples. Some I have looked miserable in pots and when I put them into the ground, they flourished (Watnog, Ruby Ridge, to name two recent ones put into the ground). Now, either my soil or something was not to their liking, or it was something I couldn't figure out. I would try a different pot, different mix, etc. Still nothing. I finally decided that some, especially larger cultivars, just don't like being in a pot. My Osakazuki has always been in the ground and is getting rather large. It is an outstanding maple, the fall color is electric, but I would think in a pot it might not be really happy. I would agree the larger pot would be a good choice and maybe, if you have room, try it in ground.
Kay

 

I have lost more trees to putting them in pots over the yrs. The only luck I've had is large planters like half whiskey barrel planters. They seem to do well in them.

I have a very rare witches broom in a whiskey barrel and its been in there over 5 yrs now. This spring I decided to change out the potting soil and come to find out the bottom of the barrel had rotted and the tree was rooted at least a foot below the planter! She is all back in a new planter now. Doing great!

 

Thanks once again for everyone's input. As it happens I went to the RHS Hampton Court flower show 2 weeks ago and spoke to a lady from a specialist Acer nursery there, and showed her my pictures. She similarly advised more watering - she told me she was watering hers twice a day in the hot weather we've been having here in the UK - and noted that the pot was too small.

Since then I have been watering more regularly and the premature colouring and withering has stopped. I also plan to re-pot it once the hot weather has passed and the lady from the nursery advised I do that by initially placing existing pot inside a much larger one and filling around it and top-dressing, until it's time to re-pot properly.

Apparently I should not cut back the stems where the leaves have died back as there should be new growth once the plant heads into its next growth phase.

I would love to have it planted in the ground but I just don't have the space, so it seems a much larger pot and a better potting mix are the way to go.