During the past two years, I've noticed insect damage on the leaves of my hardy hibiscus. The leaves were skeletonized, & when I turned the leaves over, they were covered with tiny greenish-yellow 'caterpillars'. I did some research on the internet, & found out that the culprits are not true caterpillars at all, but the larvae of an insect called a sawfly. (Mallow sawfly -Atomacera decepta) .Unfortunately, they have six generations a season, so you must be vigilant in treating the pests.
At the first sign of damage to the leaves, spray the UNDERSIDES of the leaves with a foliar insecticide spray. (Ortho Systemic Insect Killer; Liquid Sevin; Bonide Eight Vegetable, Fruit & Flower Insecticide(contains permethrin);Bayer Advanced Guard Multi-Insect Killer.)
I've read that Liquid Sevin can cause phototoxcicity to the plant, but I used it last year & had no problems. I grow 23 different cultivars of hardy hibiscus & they are spectacular when in bloom.I bought one, 'Red Flyer,' from Plants Delights nursery last year, and , under ideal conditions, is said to grow 10 feet ACROSS.
Hardy hibiscus are easy to propagate. After the plants emerge from the ground in late spring, let them grow about 12 inches high & then cut off the growing tip of the main stem. This will cause the plant to form side branches. When the plant reaches 18 inches in height, pinch out the terminal buds on the side branches. This will cause additional branching, the result being a great number of flowers. To propagate, cut some of the side branches (7 to 9 inches in length), about the end of July.
Dip the cut ends of the branches in Rootone(or any hormone rooting substance), & insert the cutting in a 4 1/2 pot filled with a sterile potting mix. (do NOT use garden soil.) Water the cutting until just a small amount of water emerges from the drainage holes. Label the cultivar, & put two drinking straws in the potting mix on opposite ends of the pot. Cover the pot & cutting with a plastic bag &
place a rubber band around the bottom of the pot to contain the plastic bag. You have just made a miniature greenhouse. Place the pots under a fluorescent light fixture timed to be on for 14 to 16 hours a day. Your cutting will form roots in 2 to 3 weeks. After acclimating your rooted plant to the out of doors (in shade first, then in sun), you can plant the cutting out in the garden, or put it in a coldframe for the winter. If you plant the cutting directly into the garden, mulch it for the first winter.