Boxwood yellowing in mid-summer--what could be the problem?


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Hi everybody!

We had a long row of boxwood shrubs planted in the middle of May this year by professional landscapers. When the shrubs were planted, they were wonderfully lush and healthy with thick, green foliage. (See pictures) However, since being planted, the foliage color has gradually become more and more yellow, and the plants do not appear to be nearly as full as they were 3 months ago.

Let me give you some background info on our climate:
We live in central Japan. The weather is pretty hot and humid during the summer. From early June to mid-July we have a rainy season with about 3-5 days of rain each week. It has been about a month since the rainy season ended, and there has been almost no rain with just about every day being sunny, reaching temperature of 33-35 degrees Celsius (91-95 decrees Fahrenheit) at the hottest part of the day. The plants are in full sun on the south-west side of our yard. Since the rainy season ended, we've been watering them about every other day at nighttime, making sure that the ground around the shrubs is saturated.

What could be causing our boxwoods to be yellowing? Are they dying? Any help would be greatly appreciated!

May 15th:
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August 13th:
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Watering at night is never a good idea. It can open up a host of problems like fungus, blight, etc. Try switching your watering times to the morning hours, and see if any signs of improvement occur. Do you know what type of boxwoods they are? They look something like a few I had many years back, which didn't tolerate full sun at all.
 
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Thanks for the tip! As you can tell, I'm a complete garden newbie! I will try switching my watering times to early morning and see if things improve.

I'm not exactly sure what species of boxwood this is (we were never told), but it looks like Buxus Sempervirens is the most commonly used boxwood here in Japan.
 
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Thanks!

The species could also be Buxus microphylla var. japonica, AKA Japanese boxwood.
 
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We found the culprit!

It turns out we have a cydalima perspectalis (box tree moth) infestation. I found this one:
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The larva often eat just the top layer of the leaves, so it's often hard to spot that the leaves are actually being eaten.
Looking at the underside of the plant, I found quite a lot of damage. These larva leave web-like trails and droppings everywhere. I thought this whole time it was just spiders...

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By the way, we confirmed that the boxwood species is buxus sempervirens.

I will be looking further into how to get rid of these pests before our boxwoods get killed...
 
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Nice catch catym! I am quite impressed by your tenacity. Most people would have given up, and just pulled them. Let us know what you use to treat the problem, as I have never heard of boxwood moths. Are they common to your area?
 
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Our landscaper came out and took a look at the plants. He said the yellowing is likely caused by the unusually hot/dry summer, but the plants are otherwise pretty healthy. However, he said that we should address the moth issue as soon as possible as they are causing the die-back of the leaves and suggested a caterpillar insecticide. We will be spraying today.

Apparently these moths are native to East Asia and pretty common! They only go for boxwood plants, though. It looks like we will probably have to treat this problem regularly : (
 
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These miners over winter as a partially grown larvae in the boxwood leaves. Warm weather in the spring helps the larva finish growing and become a pupa. A few days before the adult emerges, the pupa wiggles out of the mine to the surface of the leaf. A fly emerges from the pupa case. The flies are about the size of a gnat. After mating, females lay slightly more than two-dozen eggs inside the upper tissue of new leaves. Several weeks later, the larvae emerge and begin feeding. It is not uncommon to find multiple larvae in the same mine. Larvae grow slowly through the summer. Examine the leaves and stems often for signs of pests, such as spots on the underside of the leaves. Remove affected leaves and treat the shrub with insecticidal soaps or other appropriate pesticides to get rid of pests before they severely damage boxwoods.
 

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