September 2011 Culture

Perennial Solutions: Lithodora diffusa ‘White Star’ By Paul Pilon

When in full bloom, lithodora is a show stopper in containers and garden settings. Lithodora 'White Star' is no exception as it produces an abundance of gleaming white star-shaped blossoms outlined in vibrant blue during the late spring and into the early summer. With its flower power and shelf appeal, 'White Star' can easily be marketed alongside annuals in the late spring.

Lithodora forms neat, round bright evergreen cushions that reach 6 to 9 inches tall by 12 to 18 inches wide in the landscape. When blooming, the entire plant is covered with the showy one-of-a-kind bicolor flowers. It performs best in sunny locations with good drainage throughout USDA Hardiness Zones 6 to 9. Many gardeners successfully over-winter it in Zone 5 when adequate protection is provided; however, it is usually short lived in these environments. Additionally, 'White Star' does not perform well with the heat and humidity of the Deep South.

'White Star' is commonly used as an accent plant in border plantings and makes a great addition to rockery or cascading down retaining walls. Growers looking for mid season flowering perennials should consider adding this variety to their perennial programs.

Propagation

Lithodora diffusa 'White Star' is vegetatively propagated from tip cuttings by licensed propagators. This is a patented cultivar; propagation without permission is illegal. Licensed propagators can easily root lithodora by using the following guidelines.

The best results are obtained when propagating lithodora using vegetative tip cuttings that are not woody or blooming when they are harvested. The cuttings should be 1 to 2 inches long. Stick the cuttings into a pre-moistened media. Spraying the cuttings with 2,000-ppm IBA Soluble Salts within 24 hours of sticking will improve rooting.

For the first few days of propagation, provide a low to medium misting frequency. Gradually decrease the mist throughout propagation. Too much misting leads to poor root initiation and rotting of the foliage. Conversely, do not allow the cuttings to dry out or they may not bounce back. The main key is to propagate them with the least amount of misting as possible.

At seven to 10 days after sticking, it is beneficial to apply water-soluble fertilizers using 50- to 75-ppm nitrogen at each irrigation. If chlorosis begins to appear during propagation, provide supplemental chelated iron treatments. The misting can gradually be reduced as the cuttings form callus and root primordia. Remove the cuttings from the mist once they are rooted. Lithodora usually takes three to four weeks with soil temperatures ranging from 68 to 72¡ F to root. Liners take approximately eight to 10 weeks from sticking to become fully rooted and ready for transplanting.

Production

'White Star' is suitable for production in 1-gallon or smaller sized containers with a single liner planted in the center of the pot. To promote branching, it is beneficial to pinch the liners a couple of weeks before transplanting. When planting, the growing medium should be even with the top of the plug. Lithodora performs best when grown in a porous, well-drained medium with a slightly acidic pH: 5.5 to 6.0. Production of lithodora with high pH levels often leads to chlorotic foliage caused by iron deficiency.

They prefer to be grown moderately dry; avoid extended wet periods. When irrigation is necessary, water them thoroughly then allow the soil to dry moderately between irrigations. They are light feeders. Nutrients are commonly delivered using water-soluble sources, providing 75 to 100 ppm using a constant liquid fertilizer program or 150 ppm as needed. Since they are sensitive to high salts during production, avoid using controlled-release fertilizers or use less than half the normal rate used on other containerized perennials. It is not usually necessary to apply growth regulators to control plant height.

Insects and Diseases

Although lithodora can be produced relatively free of diseases and insects, growers may observe aphids, spider mites, whiteflies and root rots during production. None of these insect pests or diseases requires preventative control strategies. Growers should utilize routine scouting programs to detect their presence early and to determine if and when control strategies are necessary.

Forcing

Lithodora 'White Star' is best produced for mid to late spring sales. One of the keys to producing full containers is to allow sufficient time for bulking. It is best to plant them in the late summer or early fall allowing adequate time for the plants to bulk up prior to overwintering. Although lithodora does produce some flowers without cold; they are cold-beneficial plants. Flowering will be sporadic unless the plants are vernalized; provide at least six to nine weeks of cold temperatures (35 to 44¡ F). Avoid vernalizing them as liners since they will not put on much growth before flowering once the temperatures increase in the spring. Following the cold treatment, they will flower under any photoperiod (day-neutral plants) and can be forced into bloom under natural day lengths. Plants will flower in seven to nine weeks when they are grown at 60 to 65¡ F.

Availability

This unique cultivar was discovered by Danny Takao at Takao Nurseries in California and was brought to the market by Blooms of Bressingham (www.bloomsofbressinghamplants.com). Liners of Lithodora diffusa 'White Star' can be acquired from Skagit Gardens (www.skagitgardens) and many reputable perennial propagators or plant brokers.