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Lithodora is a plant genus that is part of the Boraginaceae family. It contains about nine species of shrubs that are native to the Mediterranean region. These plants feature oval leaves and trumpet-shaped flowers. Landscapers use these plants as ground cover, because of their compact form, or as showy flowers in flower beds and rock gardens. Most of the plants in the genus Lithodora are propagated through semi-hardwood cuttings.
The name of this genus is derived from the Greek words lithos and dorea, which translate to "stone" and "gift," respectively. The common name for each species varies. Lithodora oleifolia is called olive-leaved gromwell, while the heavenly blue variety of Lithodora diffusa is called purple gromwell.
A majority of the plants in this genus are located in the southern countries of Europe that border the Mediterranean Sea. Lithodora nitida is endemic to the southern regions of Spain, while Lithodora zahnii populates the rocky peninsulas of Greece. Lithodora rosmarinifolia is distributed throughout southern Italy, including Capri and western Sicily.
Olive-leaved gromwell is a commonly cultivated plant. It generally grows 6 inches (15 cm) in height and spreads about 20 inches (50 cm). The foliage is dense, consisting of smooth leaves that are olive-green in color. Each branch grows close to the ground, creating a mat-like form.
The flowers of the olive-leaved gromwell feature five blue petals that spread like a star, but converge into a narrow neck, resembling a trumpet. This species lacks the vertical white stripe that is present in the white star variety of L. diffusa. Generally, the flowers bloom from late spring to fall.
To successfully grow the olive-leaved gromwell, it is recommended to plant it in well-draining soil that is slightly alkaline. Loamy soil is considered a better medium for this plant than clay or sandy soil. It is also beneficial to place the plant in an area that is exposed to direct sunlight.
The white star variety of L. diffusa does not tolerate alkaline soil. It should be planted in slightly acidic to neutral soil that is well-draining. The natural habitat of this plant isn't that fertile, so it is recommended to limit the use of fertilizers.
Usually, these plants are propagated through semi-hardwood cuttings between late summer and early winter. This involves cutting the current season's growth that has partially matured. Typically, a 4-6 inch (10-15 cm) branch is cut, just below a leaf node. The leaves on the lower half of the cutting are removed. The base of the cutting is treated with rooting hormones and placed into a rooting medium to establish roots.