Groundwork for June 16, 2013

Vinca is the most popular, most sold and most planted warm-season bedding plant in Louisiana. We sometimes call this plant periwinkle.

Vinca (Catharanthus roseus) is very drought tolerant and has a very long blooming season. It can also tolerate the highest temperatures we face during the summer growing season, says Allen Owings, horticulture specialist with the LSU AgCenter Hammond Research Station.

Great improvements have been made in vinca flower colors and varieties over the past 25 years. In the 1980s, gardeners had few choices in terms of vinca growth habits, flower colors or disease resistance. In the 1990s, new forms and new flower colors arrived with rapid expansion occurring from 2000-2005.

Vinca flower colors now include pink, deep rose, red, blush, scarlet, white, white with a red eye, lavender blue, peach, apricot, orchid, burgundy and many others. Some new vinca varieties grow upright and others are spreading.

Upright plants generally grow 18 to 20 inches tall with a spread of 12 to 14 inches. Spreading types, though, are more trailing or resemble groundcovers and reach no more than 6 to 8 inches tall with spreads of 18 to 24 inches.

Problems and diseases can show up in the landscape. Based on the number of people calling in this spring, it was a bad year for vinca.

We had very cold conditions compared with normal temperatures during March and April, and early planted vinca have suffered.

The main disease 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.

This fungus causes a disease most often seen shortly after planting in a landscape bed. But it also can be found later in the year.

Rhizoctonia is another disease common on vinca in Louisiana. It normally shows up in summer after plants are established. Plant pathologists can also find botrytis (gray mold) and alternaria (leaf spot) on vinca in summer and fall. To get the best performance out of your vinca in the landscape, consider the following LSU AgCenter recommendations:

  • Begin with good quality plants. Inspect plants obtained from the greenhouse grower or retail garden center for healthy roots.
  • Select a full-sun location. Vinca need at least 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. If beds are already established, all debris from the previous planting needs to be removed. Mulch also should be removed. Add a couple of inches of landscape bed soil prior to planting.
  • Avoid planting vinca too early — late April through early May is the ideal first planting date for spring. This year, vinca was best planted mid- to late May.

You can continue planting vinca through summer. The main thing to remember is that vinca love warm soil and warm nighttime temperatures. It is very important to avoid planting too early in spring.

  • 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 to 10 inches apart. The faster plants grow together for complete coverage, the higher the likelihood of disease moving through foliage later in the year.
  • Apply mulch to decrease splashing rain and irrigation water from soil onto the lower stems and leaves of the plants.

Bedding plants should be mulched to a depth of about 1 inch. Pine straw is the preferred mulch material.

  • Manage irrigation properly. This is the main culprit in plant decline in landscape beds. Vinca need very little irrigation once established.

Avoid regular overhead irrigation. Even if the landscape bed drains very well, an adequate volume once a week is the most irrigation that should be applied.

  • Don’t plant vinca in the same bed year after year. Rotate them with other summer bedding plants that like sunny locations, such as pentas, blue daze, lantana, angelonia, scaevola, verbena, melampodium or sun-tolerant coleus.
  • Varieties of vinca available in Louisiana include Pacifica, Cooler, Mediterranean, Victory, Titan, Nirvana and Cora series. Cooler and Pacifica are older varieties that still perform well under the correct management practices.

Mediterranean vincas are a spreading type and should be planted only in hanging baskets and containers. Titans have the largest flowers of all the vinca groups. The newer and more expensive Nirvana and Cora vincas have genetic resistance to phytophthora.

Summer plants

Summer officially arrives Friday. Why is this significant? Well, many of the spring plants that provided early season color have died or will languish because of the heat. But a number of plants actually look forward to our soaring temperatures and enjoy the summer landscape.

Some of these plants are native but the majority hail from locales spanning the globe.

Visit with me as I present my favorite summer plants and why they like to travel to Louisiana for their summer vacation at 6:30 p.m. Tuesday at the Central Library on Joor Road. The program is hosted by the Central Bloomers Garden Club, and the public is welcome to attend.

Got a gardening question? Write to Bob Souvestre, horticulturist with the LSU AgCenter, at Burden Center, 4560 Essen Lane, Baton Rouge, LA 70809, or email to bsouvestre@agcenter.lsu.edu, or call Master Gardeners at (225) 763-3990.