Buxus microphylla and high temperatures

High summer heat keeps on prevailing here.... We are having 96 - 106 F. the last weeks and temperature is going on keeping this schedule...
I have two buxus microphylla in training (nursery stock) and one of them has yellow leaves here and there for the last 20 days but they become more. The other one is just starting to show the same problem. Both of them have been growing very well since last February when they had been repoted with a careful root trimming and root reduction (one or two wedges).
I have put some slow releasing organic fertilizer back in early June and I have discontinued liquid fertilizing long time ago due to high temperatures.
Will you please give me your opinions about this problem?
Thank you in advance.

In hot, dry Athens I assume you keep them well watered. I think I'd also keep them in at least deep afternoon shade. I find mine need repotting annually here, but we're a good deal more humid than you are.

Air quality can sometimes be a problem in the Athens area, so if you live in a part of the city where traffic is heavy or if the winds bring emissions from an electrical power plant in your direction, NOx may be an issue.

You almost always find bright yellow (sometimes turning red) leaves in the summer. Even evergreen leaves have to die some time. But if the entire tree begins to turn a mottled yellow green, it may be getting too much sun, and -- despite the heat -- a light dose of fertilizer.

JimLewis wrote:In hot, dry Athens I assume you keep them well watered... ...I think I'd also keep them in at least deep afternoon shade.
Answer is "yes, I do" to both your points, Jim.
... ...and -- despite the heat -- a light dose of fertilizer.
I need to verify... You mean that I have to lightly fertilize the trees even with these temperatures. Do I understand correctly?

Thank you very much.

I have a few boxwood and some seem to be more susceptible to sun burn than others. Moved/relocated plants can be burned faster as well if the leaves didn't have time to acclimatize to the new sun/heat exposure.

As discussed, heat can and will shut down your plant's root system in turn shutting down new (leaves, etc.) growth. Added shade or protection is preferred. Some of my boxwood stopped growing but after relocating them under shade, they started producing new leaves again. Like you, we are around 96-106*F here right now.

Good luck!

Thank you, Dario!

Poink88 wrote: ... ... Some of my boxwood stopped growing but after relocating them under shade, they started producing new leaves again... ...

Mine keep producing some new leaves on some branches (along with the yellowing problem) but at a distinctly slower rate.

The only time I've had extensive yellowing on the leaves of one of mine was when I got too much sunlight combined with our usual summer heat. Probably in the ground they're fine, but in pots (in our summers) they need shade, like Dario said.

Recently an older very experienced person in our area had the same problem with yellowing leaves and very slow or no growth. Last summer was brutal, and I think the problem started there, showing earlier this year. It seems to me that they act like conifers at times, showing the effects of stress after the event happened. This one old B. microphylla was getting strong sun until early afternoon with tree canopy in late afternoon and just kept yellowing. It was watered more to keep cool, but was kept in the strong light. One half of the trunk has just died, and since it has been moved to deep shade, the other half is finally growing some green leaves. My guess is that the plant was stressed by too much sunlight & heat, and the "cooked" plant just couldn't take up nutrients and water, maybe getting root rot.

I have a couple dozen boxwoods and try to give them sunlight when the weather is cool (not often!), but when it gets hot, they go into full shade. Never had a problem with this so far, and all the foliage is a nice dark green. It thought I'd share since our weather is probably so similar.

I greatly appreciate your response, Jim.
Thank you very much!
I'm afraid that in conclusion things are more complicated for my buxus...??? Because I live in an apartment and all my trees are at the balcony where the sunshade canopy is kept down over the pots from early in the noon until late in the evening...

But I'd like to hope that the answer is what Jim said above

JimLewis wrote: ... ...You almost always find bright yellow (sometimes turning red) leaves in the summer. Even evergreen leaves have to die some time... ...

I need to verify... You mean that I have to lightly fertilize the trees even with these temperatures. Do I understand correctly?

Yes. If it is in the shade.