Red/yellow/dropping leaves on boxwood
For the last several years, a boxwood in a public garden in Kalorama (Biltmore Triangle Garden) has had some of its leaves turn red, then yellow, then drop off. This begins in the early spring, then slows down somewhat as summer approaches, but of course by this time there are many bare twigs. Many online searches have not yielded a definitive answer, the problem is getting worse, so I’m hoping for an expert!
Yellowing leaves can have multiple causes, including stress from winterburn and natural shedding in the spring. Those leaves will drop but should be replaced by a flush of new growth, which we do see happening at the branch tips.
Any wood that is entirely bare and brittle/dead can be pruned out at any time.
Here is our IPM:Boxwood publication that covers the many issues that dogwoods can have: https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs...
We suggest reading through this page on general culture and Boxwood decline:
https://extension.umd.edu/hgic/topics/boxwood-decline and the link at the bottom that tells about a relatively new invasive disease that is of concern called Boxwood Blight. At the moment we would not suggest adding any new boxwood plants to your garden.