Perennial Solutions: Iberis sempervirens ‘Whiteout’ By Paul Pilon

Candytuft has been a perennial favorite for its reliably early spring color. In spite of its early flowering and popularity with consumers, iberis has been known among many growers as a hit or miss crop. Many growers have struggled to produce consistent crops from year to year. The recent introduction of Iberis sempervirens 'Whiteout' from Kieft Seeds offers improvements which may be appealing to commercial growers and allow them to obtain the consistency they've been looking for.

'Whiteout' is a seed-propagated cultivar which has improved plant characteristics and more uniform flowering than previous cultivars on the market. With its free branching habit, it does not require pinching and the center of the plant appears full, resulting in a higher quality finished plant. The compact mounded foliage grows eight to ten inches high by eight to 12 inches across. The height is incredibly uniform and as its name implies, the plants are absolutely covered with masses of bright white blooms in the early spring.

Iberis is an evergreen perennial with shiny dark green foliage. The plants are covered with pretty, bright white flowers in the early spring. It can be grown across most of the country and is hardy in USDA Zones 3 to 9. Candytuft prefers sunny locations and is often used as a non-invasive groundcover. It also makes a great border or rock garden plant. Once established in the landscape, iberis is drought tolerant and is resistant to deer and rabbit feeding.

With all of its desirable attributes, iberis 'Whiteout' is a Fleuroselect Approved Novelty award winner and makes an excellent candidate for small containers being marketed in the early spring or utilized as a reliable landscape performer.

Propagation

Iberis 'Whiteout' is propagated from seed. Light is optional for germination. Most propagators sow three to four seeds per cell and cover the seeds lightly with vermiculite after sowing. The covering will help to maintain sufficient moisture levels around the seed during the germination process. Moisten the seed flats and move them into a warm environment, where the temperatures can be maintained at 65 to 68¡ F until the seeds have germinated. Using a germination chamber during this stage will improve the uniformity of emergence and the germination rate. It is recommended to keep the growing mix wet, but not saturated and to maintain high relative humidity (95 percent) until the radicals have emerged.

After the seeds have germinated (four to seven days), reduce the humidity and soil moisture slightly to allow the roots to grow through the growing mix. The temperatures can also be reduced to 60 to 65¡ F during this stage of propagation. Fertilizers can be applied once the true leaves are present, applying 75- to 100-ppm nitrogen (less than 0.7 mmhos/cm EC) from a balanced nitrate-form fertilizer with low phosphorous. The amount of fertilizer applied can be increased to 100 to 175 ppm (0.7 to 1.2 mmhos/cm EC) after two to three sets of true leaves have developed. At these
temperatures, small plug sizes (such as 288-cell trays) of iberis 'Whiteout' will be ready for transplanting or shipping in seven to eight weeks.

Production

As mentioned above, iberis 'Whiteout' is suitable for production in small container sizes; it is most commonly produced in one-quart to one-gallon sized containers. Candytuft performs best when it is grown in a moist, well-drained medium with a slightly acidic pH: 5.5 to 6.2. When potting, the plugs should be planted so the original soil line of the plug is even with the surface of the growing medium of the new container. Plant one plug per pot when transplanting into one-quart containers or two to three plugs per container when transplanting into one-gallon sized pots.

Although this cultivar branches freely, some growers still elect to pinch the plugs prior to transplanting or after the plants have become established in the final containers. Pinching is optional as well-branched plants can be produced without the pinch. Be sure to allow at least four to six weeks between the last frost and the beginning of the cold treatment when providing the pinch.

They should be grown under average irrigation regimes; growers should note that they do not tolerate wet or overly dry growing conditions. When irrigation is necessary, water iberis thoroughly then allow the soil to dry slightly between waterings. Severe drought in containers often leads to yellowing and loss of the lower leaves.

Iberis can be grown using light to moderate fertility levels. Growers using water soluble fertilizers either apply 150 to 200 ppm of nitrogen as needed or feed with a constant liquid fertilization program using rates of 75- to 125-ppm nitrogen with every irrigation. Controlled-release fertilizers applied as a topdress onto the media surface using the medium labeled rate or incorporated into the growing medium prior to planting at a rate equivalent to 1.0 pounds of elemental nitrogen per yard of growing medium is also an effective method of delivering nutrition to candytuft.

'Whiteout' has a naturally compact growth habit and will usually not require height control strategies. If height management is required, the plants can be toned with spray
applications of paclobutrazol (Bonzi, Paczol or Piccolo) at 15 ppm or uniconazole (Concise or Sumagic) at 2.5 ppm.

Insects and Diseases

Iberis can generally be grown without the occurrence of diseases or significant insect feeding injury. The most common pathogens which may be observed on occasion include Alternaria leaf spot, Botrytis, downy mildew, Pythium and Xanthomonas leaf spot. Spider mites and slugs are the most problematic pests of candytuft. None of these insect pests or plant pathogens require preventative control strategies. Growers can detect the presence of these problems through routine scouting programs and determine if and when control strategies are necessary.

Forcing

With its early season flowering, iberis 'Whiteout' is usually marketed in the early spring. To obtain the fullest and highest quality containers, it is best to provide a bulking phase in the late summer or early fall prior to overwintering them; allow eight to ten weeks for bulking in the fall. Iberis requires eight to ten weeks of vernalization for flowering. After the cold treatment, 'Whiteout' is a day-neutral plant and can be grown under natural day lengths. Iberis blooms quickly and should be grown with cool temperatures 50 to 60¡ F. At these temperatures, 'Whiteout' will bloom in four to six weeks. Although it flowers early, the blooms last up to six weeks giving it an extended shelf life and marketing window.

Availability

Iberis sempervirens 'Whiteout' is brought to the market by Kieft Seed (www.kieftseed.com). To obtain seed, contact your Ball sales representative (www.ballhort.com). Plug flats can also be obtained through your Ball sales representative or various reputable perennial plant brokers and propagators.

Perennial Solutions: Iberis sempervirens 'Whiteout'

Paul Pilon

Paul Pilon is a horticultural consultant, owner of Perennial Solutions Consulting (www.perennialsolutions.com), and author of Perennial Solutions: A Grower's Guide to Perennial Production. He can be reached at 616.366.8588 or paul@perennialsolutions.com.