What caused brown spots on my boxwoods?

Thanks to landscape architect Tait Moring for this great question! The boxwoods in this hedge are exhibiting sporadic areas of yellowing/browning leaves throughout the hedge. The damage appears to start on the tips of the leaves in most places, then proceeds in from the leaf margin.

Tait also noted that this hedge gets a lot of reflected heat from a white stone patio, so actually, he already knew the answer himself!

And we can confirm his suspicions: these are classic symptoms of sun burn, and heat buildup in the leaves.

I have some ‘Baby Gem’ boxwoods myself. Two of my plants border a mostly shaded sidewalk and never have any issues, but one borders my driveway, which is in complete sun for the entire day, and has the same damage as shown here.

While you can’t do anything about the reflected heat, you can alleviate some of the heat buildup in the leaves by watering the plants more frequently during the summer. In order to take in water from their roots, plants must lose water from their leaves. This process also helps with heat buildup. Much like sweating helps to lower our temperature slightly when we get too hot, for plants, water loss has a cooling effect. And when it’s really hot, water loss occurs quickly, depleting the leaf’s supply faster than the roots can replenish it.

When the leaves can’t replenish the water that’s evaporating, they desiccate from the edges towards the center, much like the drying up of a lake. If it’s really hot where these plants are, you may not be able to completely eliminate the problem, but watering often, maybe even daily, during super-hot periods of time, will mitigate the damage.

And boxwoods are resilient, so as soon as temperatures cool down, the plant will begin to recover. Just shear off as much of the damage as possible at the end of summer, which will encourage healthy new growth to take its place.