Answer: To be honest I am very surprised to hear your burning bushes suffered from mites. These shrubs are usually quite pest free. The defoliation you described would be symptomatic of heat and moisture stress, possibly caused by poor rooting. Poor rooting can be caused by planting with encircling roots left intact or planting into a smooth sided hole in clay soil where the roots continue to grow in a circle in the planting holes as though still in a container -- this then strangles the plant, with symptoms often starting in midsummer when it turns hot. You can check this when you remove the shrubs, and see. It could also be caused by a disease such as verticillium wilt, possibly brought on by overwatering and poor drainage. Or, if you have been using de-icing salts, it is consistent with salt damage to the roots.
You might want to check with your county extension to try to get a firm diagnosis as to what has happened before you decide what to replace them with. They may also have some suggestions for replacements based on a more detailed understanding of the planting area, the growing conditions and your design goals.
As far as a replacement, I would not recommmend boxwoods for that type of location, as evergreens they will suffer terribly in the winter from the reflected sun and heat. I would suggest you consider a juniper or possibly try again with the burning bush. Both of these will tolerate a hot sunny spot and grow in clay as long as the site is well drained. This means it should not be sopping wet -- if your gutter drains there, for example, it is not going to be suitable. It would be best if the spot can be a little raised over the surrounding grade or slightly sloped, just a few inches is enough.
You should also loosen the soil over a wide area (three feet across) and as deep as the container, leave the sides of the hole rough and jagged rather than smoothly
polished with your shovel. Also plant them a tad high, say half an inch higher than they grew in the container. Make sure any encircling roots are cut or untangled and directed outward at planting. Notice I am not suggesting adding amendments, this can create a sump where the hole drains faster than the surrounding soil and causes water to collect in the bottom of the hole. Next, replace the soil and firm it, then water to settle air pockets, then mulch with several inches of organic mulch.
Keep the soil evenly moist (not wet) for the first growing season or two to encourage good rooting. This means you may need to water to supplement rain. The soil should be damp like a wrung out sponge, not saturated and not bone dry. To know if you need to water, dig into the soil with your finger. If it is quite damp, do not water yet. When you do water, water very slowly and thoroughly so it soaks down in. Clay soil is interesting in that it will stay damp for a long time, but it is difficult to re-moisten once it has gotten dry, so do not let it go too long. On the other hand, overly wet soil will damage the roots so do not water unless it is needed. There is no set schedule, it depends on the soil and the weather. After watering, wait five or six hours and then dig down to see how far your water has soaked in; it can be surprising.
Also keep the plants mulched year round with several inches of organic mulch. It should be placed in a flat layer over the root area; do not allow it to touch the stems or trunk. This will help maintain a more even soil moisture level and hold down weeds, it also helps feed the soil as it rots down slowly over time.
Good luck with your shrubs!
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