The scoop on hardy hibiscus, their common pests |
Jessica Walliser, Columnist

The scoop on hardy hibiscus, their common pests

Jessica Walliser
Jessica Walliser | for the Tribune-Review
Hardy hibiscus return to the garden year after year, but their foliage is a favorite of sawfly larvae.

Question: When can I cut back my hardy hibiscus? Also, something devastated the leaves on my hardy hibiscus this summer. How can I prevent this from happening again?

Answer: Hardy hibiscus (Hibiscus moscheutos) are show-stopping late-season bloomers for sunny gardens. These big beauties are hardy down to -20 degrees F, making them a great fit for Western Pennsylvania gardens. There are many different varieties with large, 5-to-6-inch diameter blooms in shades of pink, red, white, rose, burgundy and bi-colors. Depending on the variety, the plants grow between 3 and 6 feet tall with dozens of stems per clump. With a bloom time in late summer, hardy hibiscus make a wonderful addition to perennial beds and border.

Hardy hibiscus require very little in terms of care. They thrive in average soils, and they shrug off our wet clay soils like a champ. To keep the plants a bit shorter and denser, you can pinch off the growing point of each stem in early June to encourage branching and more flower production.

These perennials can be cut back either in the fall after a hard frost or first thing in the spring. No matter which time you choose, cut each stem down to 4 to 6 inches tall. I enjoy looking at the unique seed pods in the winter time and prefer to cut mine down in the spring. Remember that the new growth is very slow to poke up through the ground in the spring, so don’t worry if May arrives and there’s nary a sprout in sight. They’ll be there. Sometimes it just takes several weeks of warm temperatures to wake them up.

Hibiscus sawfly larvae are a very common pest of hardy hibiscus. These small insects are caterpillar-like in appearance, but they are not true caterpillars. Though many species of plants are attacked by various species of sawfly, the one attacking your plants is specific to relatives of hibiscus.

With tiny green bodies that measure a mere 1/8-to- 3/4-inch and light brown heads, hibiscus sawfly larvae are extremely difficult to spot.

They are most frequently found on the undersides of leaves, so flipping over the leaves and examining their backs is essential for proper identification of this pest. Start searching for the larvae next June and try to get a handle on their population before they completely skeletonize the leaves and cause them to drop.

While hand-squashing is effective, it’s also time consuming. Sawfly larvae damage will never kill your plant; it just makes it look not-so-hot. The damage they cause is mostly aesthetic. If you feel you must have hole-free foliage, turn to an organic product control with the active ingredient spinosad. Spinosad is a fermented bacterial product that is labeled for use on many common garden pests. Always follow label instructions carefully and apply any products only when bees are not active (early morning or late evening). For spinosad to work against sawfly larvae, the tops and bottoms of all leaves must be covered.

Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden,” “Good Bug, Bad Bug,” and her newest title, “Container Gardening Complete.” Her website is Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.

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