Lithodora is a low-growing flowering ground cover, that will work well near pathways and around the edges of flower borders. It flows well in window boxes or containers too. The genus name, Lithodora, comes from the Greek word lythos for stone and dorea for gift. This "stone gift" is also a welcome addition in rock gardens.
Typically classified as Lithodora diffusa, this hardy plant produces tiny, vividly blue, star-shaped flowers profusely in spring and less frequently through much of summer. The blue flowers have reddish-purple stripes. Reaching modest heights of six to 10 inches, a single plant can stretch 24 to 36 inches wide.
While some gardeners have managed to overwinter certain cultivars of this Mediterranean native in USDA Zone 5 or make it last in Zones 9 and 10, this herbaceous perennial is generally hardy in only Zones 6 through 8. In more southern locations of North America, the groundcover grows densely and its dark-green leaves remain evergreen year-round.
|Botanical Names||Lithodora diffusa, Lithodora spp. (formerly Lithospermum diffusum)|
|Common Names||Lithodora (previously lithospermum)|
|Plant Type||Herbaceous flowering perennial groundcover|
|Mature Size||6 to 10 in. tall, 24 to 36 inches wide|
|Sun Exposure||Full sun to part shade|
|Soil Type||Well-drained sand|
|Bloom Time||Early spring through mid to late summer|
|Flower Color||Blue with reddish-purple stripes|
|Hardiness Zones||6-8, USDA|
|Native Area||Europe (The Mediterranean, northwestern France, southern Greece, Algeria and Turkey)|
Overall, Lithodora is easy to grow, quite hardy, and largely resistant to disease. With little maintenance, this groundcover will put on a prolific, floral show.
To prepare the bed, turn the soil under to a depth of six to 12 inches. Remove any debris. Rake lightly and level the site. Plant on a cloudy day or in the late afternoon to limit transplant shock.
Dig a hole to fit the root ball, which will likely be quite large. Gently loosen the root ball as you unpot the plant. Put the top of the roots even with the level of the surrounding soil. Fill the hole with soil to the top of the root ball. Press the soil down firmly by hand.
While Lithodora thrives in full sun, it can also grow in partial shade. Give the plant afternoon shade in hotter climates. Protect from direct, hot sunlight and extreme winds until plants become established.
Provide moderately rich, well-drained soil, preferably sand, as the plant will not tolerate heavy clay soil. Acidic soil is good, though Lithodora oleifolia "Olive-leaved gromwell" prefers a pH that is alkaline to neutral.
Water thoroughly at least once a week to encourage new roots to grow down deeply. One inch of rain per week would suffice and is needed for most perennial plants.
Add a light layer of mulch on top to reduce weed germination and hold in moisture and warmth. Organic aged bark or shredded leaves will contribute a natural aesthetic and improve the soil as it breaks down. Always keep the mulch off the stems of plants to prevent rot.
Stick your finger in the soil to check that it remains damp about one inch below the surface. Do not let the soil become bone dry or soggy; overwatering could also cause root rot.
As new growth appears, a light fertilizer may be helpful but it is not necessary. Avoid using granular fertilizers near the plant crown and foliage, as these may cause burn injury. Instead, use a low rate of slow-release fertilizer.
Generally, though, Lithodora does not require fertilizer unless the leaves turn pale green-yellow. If this happens, use a fertilizer that is good for acid-loving plants like rhododendron and azalea in late winter or very early spring.
Adding organic matter such as compost or well-rotted manure may be beneficial, especially if the garden is at a new home in a recently constructed neighborhood.
Temperature and Humidity
Lithodora tends to bloom less or stop blooming in the hot and humid summers of the Deep South of the United States. Cut plants back to refresh them. Reapply mulch in autumn if planting in a cooler region such as USDA Zone 5 or 6.
Lithodora zahnii is pictured above. Popular cultivars also include Lithodora diffusa “Grace Ward," Lithodora diffusa "White Star," Lithodora diffusa "Heavenly Blue," and Lithospermum purpurocaeruleum "Purple gromwell."
There are several things to consider when carefully pruning Lithodora. The plant may become straggly, and the stems or leaves may become damaged by cold winter temperatures. As you would do for any evergreen, wait until early spring to prune it. Remove any blackened leaves that could prevent new growth and possible disease.
The plant can also be cut back after the flowering period ends in mid to late summer to maintain the desired size. Remove any unwanted tall or leggy growth.
When pruning, use sharp garden shears. Clean them before every use. Do not remove more than one-third of the plant at any given time as this could cause severe damage or death. Cautiously cut and dispose of any damaged, diseased or dead leaves.
Lithodora plants can be found in many garden centers. Plants can also be started from seed. Sow seeds directly or start them indoors a few weeks before the last frost. Propagate cuttings from established plants in mid to late summer just as flowering comes to an end.
Common Pests and Plant Diseases
Usually, pests are not a problem. However, if aphids, spider mites, or whitefly appear on the plant, spray with insecticidal soap. Hot pepper wax may be used on mites and whitefly, too.
In the event of a root-knot nematode invasion, plants may wilt or look stunted. Remove and discard the infected plants and reach out to your local extension service for advice.
First signs of bacterial leaf spot are small translucent spots with a broad yellowish edge, which gradually enlarge and develop a reddish center. This disease thrives in cooler temperatures, and if the plant is in bloom, it may also cause disfigured flower heads. If this happens, remove any infected plants. To prevent future leaf spot, avoid overhead watering and refrain from working around wet plants where dirt and debris may mingle with the moisture and lead to further disease.