Hollyhocks are a favorite ‘old-fashioned’ flower. The tall stalks and large, colorful flowers are a great addition to a country or cottage style garden. They are easy to grow (aside from rust!) and varieties range in height from 2 to 8 feet tall and flowers are available in many shades of pink, purple, white, yellow and red. Apparently (not verified), one hollyhock grower from Arizona had a plant that exceeded 14’ tall!
Photo credit: www.ourstate.com
If you grow hollyhocks, you are probably familiar with hollyhock rust. This disease, caused by the fungus Puccinia malvacearum, can cause serious damage to the plants if control measures are not used. The first sign of disease is tiny yellow spots on the upper surfaces of leaves in association with rust-colored pustules on the leaf undersides. Spores are produced in the pustules and these can cause new infections on leaves, stems and green flower parts. Spores are spread by wind and splashing rain. Because spores and new infections are produced throughout the growing season when weather is favorable, disease is most severe later in the season when plants can become quite unsightly.
Rust spots (upper surfaces) and spore pustules (lower surface). Photo credit: www.extension.umn.edu .
The rust fungus also infects the weed round-leaf mallow (Malva rotundifolia), which can serve as a source of spores. Removal of the mallow in the vicinity can help reduce infection of your hollyhocks. The fungus overwinters in infected plant debris, so thorough cleanup of infected plants should take place soon after flowering ends. Spread may be reduced by removing infected leaves early in the growing season and should be done when plant surfaces are nice and dry. If needed, fungicides can be used preventively for control. Active ingredients including chlorothalonil and sulfur are recommended. Sulfur may cause plant injury if applied at temperatures greater than 85°F. Whenever using pesticides, read and follow all label instructions carefully and use only as directed.
Round leaf mallow (Malva rotundifolia). Photo credit: http://njaes.rutgers.edu
Other problems that can sometimes crop up on hollyhocks include other fungal leaf spots, a stem canker, and powdery mildew. Occasional insect pests include the hollyhock weevil, stem borers, and aphids. Not sure if the hollyhock weevil has occurred in Connecticut or New England…it’s been reported in western states and is a native of Europe.
By J. Allen