Hollyhocks can grow to eight feet or more in height and produce large flowers in shades of purple, pink, and yellow. A major problem with hollyhocks is a fungal disease called rust which can survive on infected plant parts from year to year. Other pests such as weevils, caterpillars, and slugs can damage or kill your hollyhocks.
Rust is a very common infection that's caused by the fungus Puccinia malvacearum. You can help to prevent initial rust infection by giving the plants good ventilation. Avoid planting hollyhocks too close together as crowding causes high humidity and low air circulation, which is the perfect breeding ground for rust. Water the soil around the plants rather than the plants themselves because the rust spores can attach easier to damp or wet leaves. Avoid overly wet and overly dry soil conditions. These will weaken your hollyhocks, making them more susceptible to rust. Any time you prune your hollyhocks, immediately dispose of the cuttings. Never leave them on the ground, as the dying foliage may attract fungus.
The first sign of a rust infection is reddish brown spots on the underside of leaves near the bottom of the plant. The top side of the leaf can show a larger orange or yellow spot, sometimes with a red center. The more spots there are, the more chlorophyll is destroyed and misplaced. Hollyhock growth will appear deformed and stunted. In severe cases, hollyhock leaves will begin to fall off.
Halt the development and spread of rust by removing infected leaves as soon as you identify them. Heavily infected plants may need to be disposed of altogether since they will likely infect other hollyhocks. Do not use the leaves or any infected plant material for compost. Regularly treat this persistent infection with topical fungicides according to the manufacturer’s instructions. Many fungicides are made specifically to treat rust. Many gardeners have had luck with a lime sulfur solution. Make sure to treat the undersides of the hollyhock leaves as well. Always treat your hollyhocks after rain that may wash away fungicides. At the end of the season, remove all hollyhock growth down to the base and destroy it. Do not use the hollyhock seeds elsewhere.
TIP: Rachel Klein, our gardening expert, advises: "If you've had problems with rust in previous seasons, you may want to consider starting preventative applications of fungicide in the spring when the hollyhock's first leaves appear."
Hollyhock weevils are tiny insects that multiply rapidly. They are a dark grey color with reddish legs, usually growing to about 1/8 inch in length. Weevils multiply quickly and can produce many generations each year. The insect drills into the plant stem and flower buds for food. Weevils that are around when the seeds have developed will drill through the seed pods to get at the seeds and destroy them. Unless massively infected, a plant can survive a weevil attack—though the flowers, if they survive, will be damaged. For cases of light infestation, place a cloth underneath your hollyhock and shake it vigorously to dislodge the weevils. Quickly dump them in a bucket of soapy water. Introducing helpful nematodes into the garden can help to kill them off. Nematodes can be bought at garden supply stores or ordered online.
TIP: Rachel also suggest: "An insecticidal soap can be effective against weevils, but a specific pesticide is the only real solution for extensive infestations. Find a pesticide that lists hollyhock weevils specifically and use it as instructed."
Cutworms is the name used for the larvae of many types of moths. They become active early in the spring and can do heavy damage to newly emerging plants. They can grow as long as 2 inches and come in a variety of colors ranging from grey to pink, green, and black. They tend to be active only at night and when they are not moving, will typically be curled up. Cut worms will eat the leaves between the veins, turning them into skeletons. They can also do serious damage to new growing stems, often causing collapse.
Mature hollyhocks are not very attractive to cutworms, but you do need to protect any new growth. Because cutworms travel across the surface of the soil, you can erect a fine mesh fence to keep them off new plants. A long strip of material about 12 inches high and set into the soil at the bottom will be an effective barrier. BT (Bacillus Thuringiensis) is a biological control that will paralyze the cut worm's intestinal tract. Sprinkle BT on the soil surrounding your hollyhock, and reapply after rainfall. This will keep cutworms from ever reaching them. Be careful not to get any BT on the leaves of the plant, as this may effect beneficial insects as well.
TIP: Rachel notes that: "Gardeners have also had luck sprinkling coffee grounds and crushed eggshells around their hollyhocks to discourage cutworms. Parasitic nematodes will hunt and kill cutworms. You can purchase nematodes from a garden supply center or order them online."
Slugs and Caterpillars
Slugs and caterpillars will attack hollyhocks, but if the numbers are small the plant will be able to cope with minor damage. Kill slugs with a simple application of table salt. Unless you can identify the caterpillar as being from an unwanted creature, it is best to leave them alone or move them to a less important plant.