To discourage powdery mildew, choose sites that are sunny and have good air circulation. Maintain good air circulation by thinning clumps and providing room for plants to grow. Water early in the day and remove and destroy any infected foliage. Powdery mildew can overwinter on infected foliage and spread to new foliage the following year. Insects that can attack perennials include aphids, plant bugs, leafhoppers, mealybugs, scales, thrips and whiteflies, red spiders and other mites, caterpillars, grasshoppers, beetles, and leafminers.
Slugs are often a problem, especially in shaded gardens where mulch is routinely applied.
These are the primary problems that may arise with perennials. For assistance in identification and treatment of specific disease and insect pests of perennials see Extension Bulletins and Fact Sheets.