Thinning and deadheading are two measures that add to the good looks of your garden. Thinning refers to selectively eliminating plants or stems. Deadheading is removing spent stems and blooms from a plant. Both help shape and control the density of plant foliage and blooms. The end result is a more attractive and healthier garden.
If your garden contains mildew-prone perennials, such as phlox and bee balm (Monarda didyma), you must ensure adequate air circulation to deter the formation of the fungus. This is simply a matter of periodically cutting enough stems to the ground so that the remaining ones are not crowded. Such surgery in no way harms the plant.
Thinning must be done regularly, however, because once mildew sets in, it is hard to control without resorting to chemicals. One easy way to thin plants is to inspect new shoots in the spring. If they appear crowded together—as is often the case with phlox—simply cut out the woody center of each clump.
Deadheading is a grim-sounding term that describes cutting off the unattractive dead heads of flowers in your beds and borders. While deadheading is not essential, it certainly provides great rewards by prolonging the bloom period of most plants, preventing self-seeders from seeding, and ensuring freshness and neatness in the garden.
Most plants are genetically programmed to produce seeds. Once seed is produced, the plant's function is completed and it can appropriately wither or simply settle in as a foliage plant. If you cut the flower before the seed sets, however, the plant must produce another flower in order to fulfill its goal. The glory of modern breeding is the creation of sterile cultivars; these literally do not know how to stop producing flowers. If you wish to reduce deadheading in your perennial garden, choose sterile cultivars.