Garden Q&A: What are these dark spots on my perennials and herbs?

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Q: Some of my perennials and herbs are developing dark spots. Is this a fungus? The spots are block-like and many appear on the same leaf, with no yellowing.

A: This is the feeding damage of the four-lined plant bug, a native insect that starts to appear relatively early in the growing season. (Image records of adults on Maryland Biodiversity Project begin around mid-May and continue into early July.) Juveniles are red and black while adults sport stripes of yellow-green and black with an orange head. Belonging to the true bug group, insects like these pierce leaf cells with a strawlike mouthpart, inject an enzyme to break down tissues, and suck up the resulting slurry of cell contents. Unsurprisingly, this causes the tissue around the feeding site to collapse and darken as it dies and dries out, leading to the angular spotting that mars their host plants.


Fortunately, despite being a bit of an eyesore, this nuisance insect doesn’t really cause much harm as far as the long-term health of the plants, and no action needs to be taken. By the time any damage is widespread enough to draw attention or warrant intervention, the culprit is probably already gone or at least will be done feeding soon. There is only one generation per year, so they won’t revisit the plants later in the season. New foliage produced by the plant will look better and help cover-up older scars. You can snip off any heavily damaged leaves if you prefer, but don’t cut the stems themselves back if you can avoid it as that might interfere with flowering later on.

You can learn about these and other commonly-found plant bugs on our Plant Bugs on Flowers webpage.

The four-lined plant bug along with its characteristic feeding damage.

Q: What does herbicide damage look like? I’m not sure if lawn treatments harmed any nearby perennials or shrubs.

A: The symptoms of exposure will depend on the particular herbicide used, because their active ingredients work in different ways. Some interfere with the formation of chlorophyll, so can cause a bleached or yellowed look on growth emerging after exposure. Others interfere with plant hormones and cause excessive or stunted and malformed growth, such as twisted stems, puckered leaves, or foliage that never reaches its normal size and shape. One of the symptoms we see most often in shrubs is a pronounced change in leaf appearance, usually as a paler, deformed, and crowded bunch of leaves at the end of a branch. Similar exposures in perennials and some shrubs (particularly roses) sometimes look like those of a virus.

If the exposure wasn’t too great, plants will grow out of the damage and you can trim away the weird growth. If too substantial, though, the plant may be doomed for replacement. To avoid this in the future, always follow herbicide label directions closely and use caution when making applications near any desirable plants. Drifting spray mist or drips can contact their foliage, and over-application around tree and shrub roots can be absorbed and affect the canopy or weaken roots. Water-soluble products may run into other garden beds following the flow of stormwater that hasn’t had time to soak in.

The soil testing we use to get a nutrient analysis and acidity (pH) reading will not screen for herbicide residues. While environmental testing labs exist that might be able to detect these chemicals, they would need to know exactly what they are testing for and such tests are not inexpensive. Even if a lab confirmed the presence of herbicide, it wouldn’t change the fact that you have few options for salvaging a damaged plant beyond waiting for the chemicals to degrade on their own over time. Observation of the foliage on plants you suspect were exposed is the simplest way to determine if they may have been affected.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at Click “Ask Extension” to send questions and photos.