Tall and showy, hollyhocks (Alcea rosea) add a splash of vertical color to any landscape, especially when planted along fences or walls. The effect is somewhat lessened, however, if the plants are plagued by pests. Some pests are merely a nuisance, while others can completely destroy the plants. Sprays -- including just a strong spray of water -- can rid your hollyhocks of destructive insect pests.
Hollyhocks are biennial. Although somewhat difficult to care for, hollyhocks actually self-seed rather aggressively but won't bloom until the second year of growth. Hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, they grow best in well-drained soil and full sun or part shade. Hollyhocks are plagued by several insect pests, including Japanese beetles, several species of thrips and two-spotted spider mites.
Japanese beetles love the tender foliage of hollyhocks. These large bugs will chew holes into the leaves until they look like fragile pieces of lace. Spider mites also attach to the leaves of the plant, often leaving fine, white webbing behind. Thrips, on the other hand, munch on the plant's tender flowers. Aphids cluster on new growth, including flower buds.
Environmentally Friendly Sprays
Many home gardeners are hesitant to use chemicals in the garden. Fortunately, soft-bodied insects -- aphids in particular -- can be knocked off hollyhocks with a strong stream of water, according to the National Home Gardening club. Aim a hose at the area where the aphids are clustered, and blast away. This will also work with thrips, although you might blast away the fragile flowers along with the bugs; so, start with a low-pressure stream, and increase the water pressure as needed. Spider mites, with their sticky webbing, and Japanese beetles may resist the water, however. For spider mites, a good solution is an insecticidal soap, which is less harmful to the environment than chemical-based sprays. Thoroughly coat the hollyhocks with insecticidal soap once a week for at least three weeks. More applications may be needed throughout the summer. The amount of soap needed will vary depending on which brand you choose; be sure to follow application directions carefully. Most are mixed with water to create a concentration of about 2 percent soap, according to the Colorado State University Extension.
Although most small insects can be controlled with water or insecticidal soap, Japanese beetles are another story. The best, most environmentally friendly way to control them is to pluck them off the hollyhocks by hand and drop them in a bucket of soapy water to drown them -- but this is time-consuming and not always practical. If you need to be able to spray your plants to quickly get rid of the beetles, your best bet is a chemical-based insecticide. Most wear off after about two weeks, which means that repeated applications will likely be necessary. Protect your skin when applying the spray, and follow the directions on the package because application amounts and instructions vary depending on which brand you choose.
- Cornell University: Hollyhock
- University of Illinois Extension: Hollyhock
- Colorado State University Extension: Insect Control: Soaps and Detergents
- National Home Gardening Club: Hollyhock
- University of Illinois Extension: Two-Spotted Spider Mite
- University of Illinois Extension: Japanese Beetle
- University of Illinois Extension: Thrips
- Photo Credit Jupiterimages/Photos.com/Getty Images