Lithodora is a small genus of flowering plants but the genus name is also used as the common name for the most popular of the species, Lithodora diffusa. It is a low-growing flowering groundcover that will work well near pathways and around the edges of flower borders as well as in rock gardens. It also does well in window boxes or containers.
This hardy plant. which is best planted in the spring, produces abundant tiny, vividly blue, star-shaped flowers from late spring to mid- or late summer though the bloom is more sporadic later in the season. It grows densely, and its dark-green leaves remain evergreen year-round in more southern climates with warm winters. Reaching modest heights of six to ten inches, a single plant can slowly spread 24 to 36 inches wide.
|Botanical Names||Lithodora diffusa, formerly Lithospermum diffusum|
|Plant Type||Perennial, groundcover, herbaceous|
|Mature Size||6-10 in. tall, 24-36 in. wide|
|Sun Exposure||Full sun to partial shade|
|Soil Type||Sandy, well-drained|
|Bloom Time||Spring, summer|
|Hardiness Zones||6-8 (USDA)|
|Native Area||Europe, Mediterranean|
Lithodora is easy to grow, quite hardy, and largely resistant to disease. With little maintenance, this groundcover will put on a prolific floral show. The flowers attracts butterflies so it is a good addition to a pollinator garden.
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, acidic soil, preferably sand. The plant does not tolerate heavy clay soil.
Water newly planted Lithodora thoroughly at least once a week to encourage new roots to grow down deeply. 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. One inch of water per week should suffice.
Generally, Lithodora does not require fertilizer unless the leaves turn pale green-yellow, which indicates nutrient deficiency or if the soil pH is too alkaline. Use a fertilizer for acid-loving plants like rhododendron and azalea and avoid using granular fertilizers near the plant crown and foliage, as these may cause fertilizer burn. Instead, use a diluted slow-release fertilizer.
Temperature and Humidity
While some gardeners have managed to overwinter certain cultivars of this Mediterranean native in USDA Hardiness Zone 5 or make it last in zones 9 and 10, this herbaceous perennial is generally hardy in only zones 6 through 8. It does not like extremely hot and humid conditions.
Most Lithodora are cultivars of Lithodora diffusa. Popular ones include:
- 'Grace Ward', a sprawling groundcover with deep blue flowers reaching 12 inches in height
- 'White Star', a patented cultivar with a mounded growth habit and vibrant white and blue flowers
- 'Blue Star', a low-growing cultivar whose deep blue flowers have white margins
- 'Heavenly Blue', a low-growing groundcover with hairy grey-green leaves and royal-blue flowers
Consider several things when carefully pruning Lithodora. The plant can become straggly, and the stems or leaves can 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 cause disease.
The plant can also be cut back after the flowering period ends in mid to late summer to refresh them and maintain the desired size. Remove any unwanted tall or leggy growth.
Unless propagation is prohibited because the plant is patented, Lithodora can be propagated by division in the early spring or fall:
- Lift the entire clump out of the ground with a shovel.
- Divide it in half or more sections using sharp pruners or a spade.
- Dig a hole for each plant large enough to amply accommodate the division, and plant it at the same depth as the original plant.
- Thoroughly water and apply one to two inches of mulch around the plant to keep the soil moist and reduce weeds. Keep it well-watered until you see new growth in a few weeks.
You can also propagate the plant from cuttings of established plants in mid- to late summer just as flowering comes to an end.
How to Grow Lithodora from Seed
Lithodora is mostly propagated vegetatively by commercial growers and sold as transplants; seeds are not widely available. As many varieties are cultivars, the seeds from a Lithodora won't produce a plant that is true to the parent and growing it from seed is not recommended.
Potting and Repotting Lithodora
Lithodora grows well in a containers. Use a pot that is large enough to accommodate the root system and has large drainage holes because the plant does not like wet feet. Unglazed clay is ideal because it allows excess moisture evaporate.
Fill the pot with a quality potting mix. Water it slowly and thoroughly until the soil is evenly moist. Like all container plants, potted Lithodora needs regular watering to keep the soil moist.
When roots grow out of the drainage holes or the plant becomes root-bound, it’s time to transplant it to a larger pot. Or, you can divide it and replant a section of it in a pot of the same size with fresh potting soil. Do this in the spring before the new growth starts.
When grown in the lower range of its temperature range (USDA zone 6), apply a thick layer of mulch to insulate the plant roots against the winter cold. Stems or leaves might still become damaged by cold winter temperatures but the plant should bounce back in the spring.
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 can be used on mites and whitefly, too.
In the event of a root-knot nematode invasion, plants can 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 can 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 soil and debris can mingle with the moisture and lead to further disease.
How to Get Lithodora to Bloom
In hot and humid weather, Lithodora tends to bloom less or stop blooming. If you can rule out other causes, such as lack of sunlight or overwatering, it might be that the plant has finished blooming for the year but will bloom again next spring,
Is Lithodora invasive?
The plant spreads and might self-sow, but it is not considered invasive.
Is Lithodora native to North America?
Lithodora diffusa is native to southern and western Europe and the Mediterranean. It is not native to North America.
Is Lithodora the same as lobelia?
They are two different plants although the both have blue flowers. Lobelia is a tender perennial in the bellflower family.
Where does the name Lithodora come from?
The genus name, Lithodora, comes from the Greek word lythos for stone and dorea for gift.
“Lithodora Diffusa - Plant Finder.” Missouribotanicalgarden.org, https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=277981&isprofile=0&