December 2010

Iberis sempervirens ‘Tahoe’ By Paul Pilon

Iberis sempervirens 'Tahoe' is one of the earliest and brightest flowering perennials growers can produce for early spring sales. It boasts large clusters of white blooms covering its low-growing narrow evergreen foliage.

In the garden, 'Tahoe' grows 8-10 inches tall when blooming and spreads 18-24 inches wide after a couple of growing seasons. It is an early spring bloomer and flowers several weeks before many other perennials and spring bulbs in the landscape. 'Tahoe' is very floriferous, with blooms that cover nearly all of the foliage resembling a patch of snow.

Candytuft grows well in sunny locations across most of the country and is hardy in USDA Zones 3 to 9. It makes a great ground cover that is not invasive; works well as a border, edging, or rock garden plant; and performs in containers or mixed containers. With its compact growing habit, early flowering and ease of production, 'Tahoe' is well suited for commercial growers.

Propagation

Iberis 'Tahoe' is easily propagated by seed. Growers commonly sow candytuft in small-celled plug trays (such as 288s). Multiple sow three to four seeds per cell and cover the seeds lightly with germination mix or vermiculite to help maintain adequate moisture levels during germination. The seed flats should be moistened and moved to a warm environment, where the temperatures can be maintained at 60-65¡ F for germination. It is best to geminate them in a germination chamber where uniform moisture levels and temperatures can be maintained. Using a germination chamber will also increase both the germination rate and percent germination.

Keep the media slightly moist, but not wet during germination. The seeds should be germinated in 10 to 15 days after sowing. Following germination, gradually reduce the humidity and continue growing them with temperatures of 60-65¡ F. Reduce the moisture levels somewhat, allowing the growing medium to dry out slightly before watering to help promote rooting. Fertilizers can be applied once the true leaves are present, applying 50-ppm nitrate nitrogen; after two to three sets of true leaves are present the fertilizer rate can be increased to 100 ppm. At these temperatures, iberis 'Tahoe' will be ready for transplanting in seven to eight weeks.

Production

'Tahoe' is most commonly produced in 1-gallon or smaller-sized containers. They perform best when grown in a moist, well-drained medium. Iberis requires a bulking phase in the late summer/fall prior to being over-wintered. Growers typically allow eight to 10 weeks for bulking them in the final container prior to the onset of cold temperatures in the fall. Plant a single plug in the center of the pot when iberis is to be grown in small container sizes (5-inch or smaller); when they are grown in large containers (1-gallon), it is best to plant two to three plugs evenly spaced to help fill out the containers. When transplanting, the growing medium of the pot should be even with the top of the plug.

To promote branching and fullness, it is helpful to pinch the plugs prior to transplanting. If necessary, an additional pinch can be done three to four weeks after transplanting. Allow four to six weeks between the last pinch and the beginning of the cold treatment.

Iberis requires average to slightly below average amounts of irrigation and does not tolerate really wet or overly dry growing conditions. Maintain the media throughout the production cycle with a pH between 5.5 and 6.2. When delivering nutrients using water-soluble fertilizers, feed iberis using 50-75 ppm with each irrigation or 150-200 ppm as needed. Controlled-release fertilizers can be used by incorporating 1 pound of elemental nitrogen per cubic yard of growing mix. Monitor the fertility levels of the crops biweekly to maintain EC levels of 0.75-1.0 with the 2:1 media extraction method. Providing high or luxury fertility levels will cause them to appear lush and may delay flowering.

With their compact growth habit, it is usually not necessary to control the plant height of iberis. Usually any need to control height is in the vegetative bulking phase. At this stage of production, it is usually best to pinch or lightly trim the plants provided the timing of the pinch allows for four to six weeks of active growth before overwintering them. If growth regulators are necessary, paclobutrazol (Bonzi, Paczol or Piccolo) at 15 ppm or uniconazole (Concise or Sumagic) at 2.5 ppm can be applied for toning purposes.

Insects and Diseases

Iberis can generally be grown without the occurrence of significant insect feeding injury. There are a number of disease organisms known to infect iberis including Alternaria leaf spot, Botrytis blight, downy mildew, powdery mildew, Phoma and Pythium root rots, Rhizoctonia damping off, rust and Xanthomonas leaf spot. Although this seems like an overwhelming list of diseases, iberis can generally be grown without the occurrence of plant pathogens. To prevent the occurrence of these diseases, it is best to manage the environment by providing the proper plant spacing and adequate air movement, controlling the humidity, monitoring the salt levels of the growing mix, and providing proper irrigation practices.

Early signs of these insects and diseases can be detected through routine scouting. Control strategies may not be necessary unless the scouting activities indicate actions should be taken. There are a number of good products on the market for controlling these problems once they arise or for using with preventative programs.

Forcing

Due to its early bloom time, 'Tahoe' is typically grown for early to mid spring sales. As mentioned above, it is best to plant them in the late summer or fall the year before they are to be sold to allow adequate time for them to bulk up. Omitting the bulking phase before vernalization will result in small and often unmarketable plants. 'Tahoe' does have an obligate cold requirement for flowering; provide at least eight to 10 weeks of cold temperatures (28-41¡ F) to properly vernalize them. Iberis are classified as day neutral plants and will flower under any photoperiod following vernalization.

Compared to many perennials, iberis will bloom very quickly and are often in flower before they are needed. To delay flowering, it is best to grow them cool (50-60¡ F) in the spring. At these temperatures, 'Tahoe' will flower in four to six weeks.

Availability

Iberis sempervirens 'Tahoe' is brought to the market and is available to growers through Syngenta Flowers (www.syngentaflowersinc.com). Plugs can be acquired from C. Raker & Sons, Inc. (www.raker.com), Northwest Horticulture (www.northwesthort.com), and Swift Greenhouses, Inc. (www.swiftgreenhouses.com) or from several reputable plant brokers.