Garden Q&A: Why is my boxwood making noise?
Q: I don’t think I’m imagining it, but there is a subtle rustling sounds coming from my boxwood shrubs. Do I have rodents in there, or are birds foraging under the branches that I’m not seeing? This is bizarre.
A: What you’re hearing is fairly common, which is somewhat reassuring but not great news. It’s caused not by birds or rodents in this case, but insects. Boxwood Leafminer is the most common insect pest of boxwoods in our region and the most damaging one at that.
This adult insect is a small orange gnat. Its yellow larvae feed on boxwood leaf tissue, between the upper and lower leaf layers. (It’s amazing that anything fits in that flat space, much less has enough to eat to mature without leaving that one leaf.) As the larvae finish development in early spring, after more-or-less hibernating during winter, their feeding en masse can create a rustling-like sound if you listen closely and your surroundings are fairly quiet.
Mild damage is not enough to bother treating. Heavier infestations can cause premature shedding of the older leaves, leaf discoloration, and a blister-like or rippled texture to the leaves from the empty tissue inside. A tiny hole eventually appears in the leaf underside when the adult gnat emerges.
Any leaves that were torn open were pecked by birds looking for snacks. Adult gnats are also easy prey for nibbling songbirds as well as, I imagine, hungry hummingbird migrants since they eat small insects.
Fortunately, there is only one generation per year for this species. Adult emergence timing depends on temperature trends. On average, they appear in late April or early May in central Maryland. If you can interrupt its re-infestation of the shrub, you can suppress them effectively for an entire growing season.
To avoid pesticides, try trimming back the branch tips to remove the larvae living in the leaves since they can’t escape. I think pruning in late spring makes the most sense (so, after the leafminer adults have laid eggs, not before they emerge), because you don’t want the boxwood’s tender new growth in response to pruning to be injured by a late frost.
How far to trim may depend on the shrub’s current growth habit. Those that have been habitually over-pruned into manicured shapes tend to have a thin layer of outer foliage and a twiggy, bare interior. In that case, you’d have little to work with and might need to denude the plant to force it to gradually regrow. Shrubs with more foliage along the stems can have the outer third of the leafy portion of the stem cut back.
Dispose of the trimmings, though if your home compost pile gets hot enough to kill insects, you can put the clippings there.
Q: What plants do you suggest for a mostly shaded area that’s drier due to tree roots? I’m looking mostly for groundcover-type plants, and preferably native.
A: “Dry shade” can be challenging, though fortunately there are multiple species that tolerate those conditions well. Below are some starter ideas for plants that tolerate (or prefer) afternoon shade as well as the drier conditions that come with living underneath a tree or in areas with limited soil. They are either native to Maryland or to the eastern U.S.
Some creep as they mature, while others clump but self-seed into colonies to cover more ground over time. Not all of them will flower showily or for a long period (especially if heavily shaded), but they will at least help to hold the soil and out-compete most weeds once established.
- White Wood Aster (Eurybia divaricata)
- Partridgeberry (Mitchella repens)
- Black Huckleberry (Gaylussacia baccata)
- Coral Bells (Heuchera americana)
- Green-and-Gold (Chrysogonum virginianum)
- Pussytoes (Antennaria, several species)
- Wild Ginger (Asarum canadense)
- Wild Bleeding-heart (Dicentra eximia)
- Beardtongue (Penstemon digitalis)
- Sedge (Carex) – multiple species, many native and some not
If the site can be irrigated periodically or isn’t super dry, you can also consider the plants below, though a couple of them struggle in areas with high summer heat.
- Wild Stonecrop (Sedum ternatum)
- Golden Ragwort (Packera aurea)
- Barren Strawberry (Geum fragarioides) – so-named because it doesn’t fruit
- Allegheny Spurge (Pachysandra procumbens)
- Partridgeberry (Mitchella repens)
- Foamflower (Tiarella cordifolia)
- Violets (Viola) – many species
- Green-and-Gold (Chrysogonum virginianum)
- Creeping Phlox (Phlox stolonifera)
- Woodland Phlox (Phlox divaricata)
- Coral Bells / Alumroot (Heuchera americana)
From a practical standpoint, installing young plants will be simpler and less potentially damaging to tree roots than planting more mature specimens. Since a planting hole should ideally be dug twice as wide as the current root ball size, this involves disturbing a lot of root real estate for the tree, if an extensive area is being planted and if pots are a typical nursery container size of a quart or gallon.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.