The types of vinca you grow dictate when and how to prune the potentially invasive plants. Perennial periwinkles (Vinca major, Vinca minor), suitable for U.S. Department of Agriculture plant hardiness zones 7 through 9 and 4 through 8 respectively, spread by leaf nodes that root wherever they touch the soil. Madagascar periwinkles (Catharanthus roseus) -- also known as annual vincas -- are hardy in USDA zones 10 through 11 but typically grown as annuals. They spread from seed, which they produce with abandon.
Pruning Perennial Vinca
To keep perennial periwinkles looking their best, prune them back every two to three years. After they've finished blooming in spring, shear them to a uniform, 4- to 6-inch height.
Kneel down at the edge of the periwinkle planting and hold the shears with their blades open like oversized scissors.
Slide the blades into the planting, horizontal to and 4 to 6 inches above the soil.
Shear the top of the periwinkle, keeping the blades as level as possible to create an even surface.
Rake up and bag the debris for disposal, and wipe the shears off with a clean towel.
Pour enough rubbing alcohol to cover the shears' blades into a shallow container. Soak the blades for at least five minutes, rinse them in clean water and air dry them before reuse.
Pinch back perennial periwinkle stems any time they begin creeping beyond their designated area. Simply grasp a stem between your thumb and forefinger and pinch it off to the desired length. Pinching back encourages bushy new growth.
Deadheading Annual Vinca
Annual vinca spends all its energy on flowering to produce seeds for the next generation, then stops blooming. Deadheading is the process of pruning its faded flowers before their seeds mature. Regular deadheading encourages the plant to bloom right up until frost.
To deadhead, grasp a spent flower below its base and above a set of leaves. Holding it between your thumb and forefinger nails, gently snap it free.