By LSU AgCenter Horticulturists Dan Gill, Allen Owings and John Young

One of the most popular warm-season annual bedding plants is vinca or periwinkle. It is well-known for its prolific and long blooming season.

The plant is heat- and drought-tolerant. It thrives under our Louisiana growing conditions. The vinca we use as a bedding plant is Catharanthus roseus.

Thirty years ago, periwinkle flower colors were 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 1 foot, although trailing types spread to about 2 feet.

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.

The culprit is a fungus called phytophthora, which 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 attacks on periwinkles actually are to the stems of the above-ground portions of the plants. 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. A severe attack can essentially wipe out an entire planting. We call this disease aerial blight.

Follow these practices to reduce disease problems on periwinkle:

– Select a full-sun planting location (eight hours of direct sun daily).

– Prepare the landscape bed to allow for drainage and aeration. Raise the bed at least 6 inches if drainage is questionable.

– Plant in soil with a pH level of 5.5.

– Avoid planting earlier than May. Soil and nighttime temperatures need to be warm prior to planting.

– 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 crowded plantings limits air circulation and can create conditions more favorable to disease development. Space transplants at least 10-12 inches apart.

– Mulch with pine straw to decrease splashing of rainfall and irrigation water from soil to the lower stems and foliage of the plants.

– 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.

– Don’t plant periwinkles in beds where aerial blight has been a problem in the past.

Now the good news. Recent developments in periwinkle breeding make these plants much better than they used to be. New varieties of periwinkle are resistant to aerial blight.

The Cora and Nirvana series are new additions to periwinkles and have genetics in the breeding that makes these varieties resistant to the fungus causing aerial blight. These have been evaluated in LSU AgCenter landscape trials at Burden Center in Baton Rouge and at the Hammond Research Station in Hammond.

You also can continue to grown Pacifica, Cooler, Titan and other series of periwinkle using the recommendations listed for success with these plants in your home landscape.

Come to LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is located near the intersection of Burbank Drive and Nicholson Drive (La. Highway 30) in Baton Rouge across the street from the new LSU baseball stadium. Go online to Louisiana Yards and Neighborhoods for additional information.

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Editor: Mark Claesgens