Hollyhocks (Alcea rosea) are an easy-to-grow plant, which makes them a great choice for novice gardeners. They are true biennials, as they do not flower until the second year of their life cycle. Hollyhocks may grow to 5 feet tall and may require staking. However, hollyhocks have some very serious flaws that keep them from being a favourite plant.
Approximately ½ inch long with a metallic green body and bronze wings, Japanese beetles feed on over 300 species of plants. Hollyhocks are a secondary host to the Japanese beetle. In their adult stage, Japanese beetles will eat the leaves until they have large holes or are skeletal. It is critical to act as soon as you sight Japanese beetles. Manually remove them from your plants and drop them in soapy water to kill them. Once the first beetles appear, others will quickly follow and will be difficult to eradicate.
Hollyhock sawfly is one of the most common insects affecting hollyhocks. In its larval form, the hollyhock sawfly will eat the plant's foliage, leaving only leaf tissue patches and leaf veins in their wake. These translucent green worms are 1/2-inch long. Maturing larva spin silky webbing, covering themselves before they pupate. The adult sawfly emerges as a black fly with an orange thorax. To control hollyhock sawfly with pesticides, begin applying Sevin dust or Bacillus thuringiensis at the first sign of leaf holes. Without control measures your plants will be slightly damaged, but will survive.
Puccinia malvacearum is the fungus that causes hollyhock rust. This disease will disfigure the plant's leaves if the infestation is severe enough. The rust manifests as reddish pustules that resemble raised bumps on hollyhock leaves, typically on plants that are under stress. The tops of the leaves will develop multiple yellow spots. Left unattended, hollyhock rust will worsen as summer continues and kill all the plant's leaves by the end of the growing season. This infestation may overwinter in colder climates if it infects the plant's crown. While there is no cure for hollyhock rust, spray the plant with lime sulphur or fungicide containing chlorothalonil, sulphur, myclobutanil or neem oil to limit the spread of the disease. Repeat often to control spread of the pustules. You may also try removing the first few leaves that appear in spring to reduce overwintered spores. To create an environment that reduces the hollyhock's chance of rust infestation, feed your plants with a generous amount of compost and water them well, as healthy plants are less susceptible.
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