Daniel Gill, Merrill, Thomas A.
By Dan Gill
LSU AgCenter Horticulturist
One of the most popular summer bedding plants is the periwinkle or vinca. Known for its prolific and long blooming season, it is heat- and drought-tolerant and thrives in our climate.
The vinca or periwinkle we use as a bedding plant is Catharanthus roseus. Two other plants used in landscapes, Vinca major and Vinca minor, are related to it and are also commonly known as vinca or periwinkle. Even though they go by the same common names, Vinca major and Vinca minor are different plants all together. They are evergreen, hardy vines that are used as ground covers in shady areas and should not be confused with the bedding plant.
Periwinkle has a long history of use as a medicinal herb. In the 20th century, researchers discovered the plant contains dozens of alkaloids. In the 1950s they discovered two alkaloids that are the source of anti-cancer drugs used today.
When I started gardening in the 1970s and 1980s the color selection for periwinkles basically was limited to white, white with a red eye and a rosy purple. Breeding work in the 1980s involved crossing Catharanthus roseus with other species to improve the color range, increase blooming, increase flower size and improve the growth habit.
The results have been spectacular. Periwinkles now come in pink, deep rose, red, scarlet, white, white with a red eye, lavender blue, peach, apricot, orchid, burgundy and many other shades.
Periwinkles generally grow from 10 to 18 inches tall with a spread of about a foot, although trailing types spread to about two feet.
But There Is Bad News
Unfortunately, since all of these improvements have been made, this once totally reliable plant has developed a serious disease problem. This disease can be so devastating to landscape plantings that many gardeners and professionals are limiting the use of periwinkles or eliminating them completely from their gardens.
There are some things we can do, however, to minimize the effects of this disease.
The culprit is a fungus called Phytophthora, and it has always been present in our soils. It is often responsible for root rots and crown rots, and it attacks many types of plants.
The disease’s attacks on periwinkles actually are to the stems of the above-ground portions of the plants. First, dark brown streaks or blotches appear on the stems followed by the wilting of the leaves on those stems. Often, sections of the plant wilt and turn brown while other parts remain healthy.
Attacked plants sometimes recover eventually, but they also may completely succumb and die. A severe attack can essentially wipe out an entire planting. We call this disease aerial blight.
What You Can Do
Here are some tips provided by the LSU AgCenter concerning what you can do to help deal with this disease:
–Select a full-sun location. Periwinkles need at least six to eight hours of direct sun daily for optimum performance.
–Properly prepare the landscape bed to allow for drainage and aeration. Raise the bed at least 6 inches if drainage is questionable.
–Avoid planting periwinkles earlier than May.
–Plant so that the top of the root ball is level with or slightly higher than the soil of the bed. Proper spacing also is important, because a crowded planting limits air circulation and can create conditions more favorable to disease development. Space transplants at least 8 inches apart.
–Mulch to decrease splashing of rainfall and irrigation water from soil to the lower stems and foliage of the plants. Bedding plants should be mulched to a depth of about 1 inch.
–Manage irrigation properly. Periwinkles require minimal irrigation and are frequently watered too much in landscape beds. Avoid overhead irrigation, when practical.
–Don’t plant periwinkles in the same bed year after year. Rotate them with other summer bedding plants that like sunny locations such as blue daze, dwarf lantana, dwarf pentas, scaevola, verbena, melampodium or sun-tolerant coleus, to name a few.
–Consider using the "Pacifica" series of periwinkles, which come in a variety of colors. These have been among the top performers in LSU AgCenter research station trials. The "Cooler" series also has performed well.
–Controlling this disease with fungicides is not always easy or reliable. We generally suggest home gardeners switch to other bedding plants when there is a problem. In some instances, commercial horticulturists may choose to treat a large planting to salvage an investment in labor and materials. Fungicides labeled for control of Phytophthora in landscape beds used for bedding plants include those containing aluminum tris (Aliette), etridiozole (Terrazole and Koban), metalaxyl (Subdue) and propamocarb hydroxide (Banol). Regularly repeated treatments may be necessary.
–Don’t plant periwinkles in beds where aerial blight has been a problem in the past.
Get It Growing is a weekly feature on home lawn and garden topics prepared by experts in the LSU AgCenter. For more information on such topics, contact your parish LSU AgCenter Extension office or visit our Web site at www.lsuagcenter.com. A wide range of publications and a variety of other resources are available.