Hollyhock Diseases and Pests

The Hollyhock (Alcea rosea) is truly an old fashioned garden plant I like to grow for their tall spikes with showy flowers. They are the quintessential cottage garden flower and are great for filling in large areas like the back of a flower bed, along a fence or a wall.

Hollyhocks are biennial plants.  They spend the first year of their life building roots and storing energy, growing close to the ground in a circular rosette fashion.  After going dormant for the winter, they re-emerge, growing into a much taller flowering plant that will set seed and then die. Allowing the seed to fall to the ground will ensure more plants for following years.  This plant likes full sun and fertile, well drained soil.  Surround them with a 2 to 3-inch layer of mulch to keep weeds at bay, retain moisture and keep the soil cooler during the hottest days of the summer.

The major disease problem that hollyhocks face is rust, a fungal disease.  It starts as orange, powdery looking spots on the underside of leaves.  Swellings soon emerge within these spots.  As the swellings develop they release masses of reddish-brown spores covering a major portion of the underside of the leaf.   Leaves that are infected eventually turn gray or tan and die.  The reddish spores are easily spread by splashing water, rain and wind. Lower leaves will show the condition first and the disease will progress upward during the growing season.  The extent and severity will depend on weather conditions.  This fungus will overwinter in plant debris and if not disposed of, the symptoms will appear early the next spring in new growth when weather conditions are favorable.

You can help to prevent rust infection by giving the plants plenty of space and ventilation.  Place plants in a sunny, dry location so that moisture can quickly evaporate form the foliage.  Water the soil around the plants rather than the plants themselves as the rust spores will attach easier to wet foliage.  Remove infected leaves as soon as you have identified them and do not use the infected leaves or plant parts in your compost pile.  Treatment would include use of a fungicide. As in any use of a chemical, read and follow all label directions.  Start spraying in the spring as new growth starts.  You may need to spray several times to protect the young plants.  At the end of the season, remove all infected hollyhock plant material down to the base and destroy it.

When it comes to pests, the hollyhock sawfly is quite common.  The larval form of the hollyhock sawfly is a leaf skeletonizer that eats it way through the leaves, leaving them looking like swiss cheese.  The little green worms are up to ½” long with a black spot on their head and full stomachs.  The plant should be treated as soon as the first holes are seen with Sevin or Bacillus thuringiensis (Bt).  If you decide to forego treatment, the plant will still live, it just will not look very good. Hollyhock weevils are tiny insects that drill into the stem and flower buds for food.  Spider mites, caterpillars and slugs also plague hollyhocks.

As always, Happy Gardening!

More information is available online.  The Purdue Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.

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