Q: Red and black bugs have appeared in my vegetable and flower gardens, both on plants and fence posts. They tend to group together. How do I protect my plants?
A: These are boxelder bugs, a native insect commonly seen in summer and autumn (or winter too, if they wander indoors for shelter). Outside, they may appear alarming, given their abundance and tendency to aggregate, but they don’t cause serious damage to plants. No intervention is necessary, and I would not recommend the use of any insecticide to control them.
Named for their fondness for boxelder, a widespread native maple species on which they suck plant sap for part of the year, they can also wander over many other plants. Fortunately, though, they are merely a nuisance. Shoo them away, if you prefer, using a hose blast of plain water, or brush them off structures with a broom, but otherwise, just ignore them.
Q: How often should I be watering new plantings of drought tolerant species? Or native plants?
A: As often as their roots get too dry several inches below the surface of the soil. The frequency of this will depend on the weather, your soil type, and other growing conditions. If you feel the soil and it’s already somewhat dry to the touch at that depth, then watering is probably needed.
What is “too dry?” That will depend on the plant species. The substantial storage roots of yucca and the succulent foliage of sedum will put up with much drier conditions than a buttonbush or summersweet, shrubs of wetter habitats where they grow wild. Gardeners sometimes assume that a native species in a garden setting can be more hands-off, but during the establishment phase at least, or if it’s being grown in less than optimal conditions, this is not necessarily true.
Plants that are drought tolerant need to be established in order to put up with inadequate rain, and they still have their limits. Keep in mind that wilting is not a reliable indicator of dry roots; over-watered plants or those suffering from stem borers or infections can wilt even when the roots are damp. High heat can also cause wilting. Hydrangeas and impatiens are notorious for wilting on a hot day, especially if growing in containers .
Monitor the watering needs of new plantings (drought tolerant or otherwise) for the first two years they are in the ground and longer if you had a large tree or shrub installed. The older the plant, the longer it’s going to take to re-establish. While the “tough love” approach of giving them the bare minimum needed to survive might appeal for trying to save water or encourage roots to dig deep to get their own moisture, there can be risks with this treatment. Several insects, mites, and pathogens will take advantage of stressed plants, and drought can be one of the main precursors for dieback.
Botryosphaeria canker, for example, is a fungal disease that can cause serious damage to shrubs and trees that have been stressed by dry soil. The list of susceptible species is long and includes rhododendron, viburnum, redbud, holly, redtwig dogwood, fruit trees, camellia, lilac, St. Johnswort, crapemyrtle, rose, mountain laurel, arborvitae, cherrylaurel, and juniper.
If you need to prioritize watering, check on the most recently planted specimens of any plant type first. Their roots haven’t spread yet and have limited access to moisture. Anything growing in containers will also need close attention, since pots dry out and heat up faster than soil in the ground. Trees, shrubs, and vegetables that fruit (peach, blueberry, cucumber, etc.) use a lot of water to mature their crops. Irregular watering can be a key factor behind tomatoes developing blossom end rot.
The longest-lived parts of your landscape, trees and shrubs, get dibs on irrigation. It may go without saying, but any species you’re growing that do best in evenly moist soil should also be given attention, including turtlehead, azaleas, rose mallow hibiscus, silky dogwood and helenium.
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.