<em>Lithodora</em>: planting, cultivation and care

Botany

Lithodora are bushes or shrubs, generally with a very spreading, prostrate habit and evergreen foliage. Lithodora diffusa (or Grémil diffus) is the most widespread in cultivation. In the wild it is found in scrubby habitats and woodland edges, sometimes in mountainous areas, from south-west Europe to Turkey and North Africa. Generally, lithodora (and related genera) grow mainly in dry, arid, rocky and relatively warm habitats.

Lithodora belongs to the family Boraginaceae (nearly 2,700 species). This family includes borage (Borago), comfrey (Symphytum), lungworts, forget-me-nots, brunneras, Omphalodes, bugloss (Anchusa)… It is interesting to note that most of these plants bear small five-petalled blue flowers, like lithodora. Most produce delicate, subtle blooms. Boraginaceae are generally pubescent plants that bear flowers arranged in cymes, with five fused petals, five sepals and five stamens.

Lithodora diffusa was previously named Lithospermum diffusum; today its name has changed again and it should now be called Glandora diffusa. It is very close to the genera Lithospermum and Buglossoides. Its name comes from the Greek lithos: stone, and dorea: gift, because it thrives among stones, in rocky places. The specific epithet diffusa refers to its “diffuse” habit: very spreading. In French, lithodora is called Grémil.

Lithodora diffusa and its cultivars (‘Star’, ‘Heavenly Blue’…) are the most commonly grown. However, other very decorative species deserve attention: Lithodora oleifolia, Lithodora zahnii or Lithodora fruticosa, for example. Don’t hesitate to try growing them!

Lithodora is a shrub with a compact, spreading habit, reaching up to 30 cm in height at most (often less than 20 cm), and spreading up to one metre. Its branches are very ramified and its foliage is dense, which makes it a good groundcover, preventing weed growth. Some cultivars, such as ‘Heavenly Blue’, are even lower than the typical species. Lithodora therefore prefer to be planted at the front of borders, or on rockeries, with other carpeting plants. They can also find a place at the foot of bushes.

Flowering of Lithodora diffusa (photo KENPEI), Lithodora prostrata (photo Pancrat), Lithodora diffusa ‘Star’, and Lithodora hispidula (photo Zeynel Cebeci)

Lithodora flowers in spring, usually in May–June; sometimes the flowering extends a little into summer. The ‘Peter’s Favourite’ cultivar has the advantage of flowering for a longer period than others, through spring and summer. Lithodora’s flowering is more generous when planted in full sun rather than in partial shade.

The plant then bears numerous star-shaped flowers, gathered in terminal cymes. These flowers are small, measuring between 0.5 and 2 cm in diameter, but are abundant and cover the foliage nicely. Their large number makes the flowering impressive and highly decorative. These flowers consist of five petals, fused into a tube that opens into five spreading lobes, giving a star-shaped form. The base of the corolla is surrounded by a calyx with five lobes (corresponding to five fused sepals). In the centre, five stamens are inserted into the corolla tube and bear the pollen. The flower also has a style (female organ that will receive the pollen).

The blossom colour is an intense, deep blue. The flowers, however, take on slight violet-pink shades around the corolla tube, and the flower buds also have a purplish tint. The flowers are bicoloured in the ‘Star’ cultivar: blue at the centre of the petals and white at the edges, which really emphasises their star shape. Flowers can also be white, as in the ‘Alba’ cultivar. Lithodora hispidula, on the other hand, bears pink–red flowers. In general, lithodora are appreciated for their soft, luminous hues that can brighten borders.

Lithodora leaves are single and alternate, placed one after the other along the stems. They are small, narrow, elliptical, and measure between 1 and 4 cm in length. The median vein is strongly marked, forming a fold at the centre of the leaf. Leaves are also villous, covered with hairs. Those of Lithodora rosmarinifolia are a little longer and narrower than in other cultivars. The names of Lithodora can also inform us about the shape of their leaves: Lithodora rosmarinifolia = lithodora with rosemary-like leaves; Lithodora oleifolia = with olive-like leaves, etc.

Leaves are dark green, which sets off the lighter bloom. They are paler on the underside.

Because the foliage is quite dense and covers the ground well, it limits weed growth by leaving them no room to develop.

Foliage of Lithodora diffusa ‘Heavenly Blue’ (photo Dinkum), Lithodora ‘Cambridge Blue’, and Lithodora oleifolia (photo Guérin Nicolas

Lithodora is evergreen, so it has the advantage of keeping its leaves in winter. It is therefore decorative all year round! Its roots penetrate deeply into the soil, which allows it to cope reasonably well with drought.

After flowering, lithodora produces achenes (dry fruits that do not open at ripeness), which contain the seeds.

Main varieties

Most popular varieties

Lithodora diffusa Star

Variety offering small star-shaped flowers, blue at centre of petals and white around edges, which beautifully emphasises their shape. Appreciated for originality of its flowering, very delicate.

• Flowering time June, July
• Height at maturity 15 cm

See the product sheet

Lithodora diffusa Heavenly Blue

'Heavenly Blue' is a must-have variety, covered in spring with a multitude of small flowers in a lovely soft yet deep blue. It has a more compact, lower habit than other varieties.

• Flowering time June, July
• Height at maturity 15 cm

See the product sheet

Lithodora diffusa Cambridge Blue

This variety produces many small, very pale blue flowers in spring, nicely set off by its dark green foliage. Its flowering is very delicate and luminous.

• Flowering time June, July
• Height at maturity 15 cm

See the product sheet

Planting

Where to plant?

Place lithodora in full sun: its flowering will be more generous than in partial shade. It will tolerate partial shade but will grow less well. In full sun, young plants will be healthier and more vigorous, with faster growth and a more balanced habit. Preferably it should receive at least six hours of direct sunlight a day.

Lithodora needs free-draining soil, and prefers stony or sandy ground. It does not like heavy, clay soils, and copes less well with winter cold if soil retains water. You can also plant on a mound or on sloping ground to aid drainage. Lithodora is perfect in a rockery or on an old stone wall, alongside other small plants that tolerate drought. Because it is spreading and low-maintenance, it can be used to cover a bank. You can also plant it in a pot or container, and place it on your balcony or terrace. It can also be planted at the front of a perennial border, in full sun.

Lithodora diffusa (notably variety ‘Heavenly Blue’) needs slightly acidic soil, while other Lithodora species prefer calcareous soils. It is also preferable that soil be humus-bearing, rich in humus.

Because lithodora is a spreading plant, allow enough space around it for it to develop properly. Avoid siting it in a confined spot, crowded between other plants. Leave at least one metre between lithodora and other plants.

When to plant?

Plant lithodora in spring, around May. Autumn planting is still possible if you live in an area with a mild climate. Most important is to avoid periods of frost.

How to plant?

• Dig a planting hole two to three times the size of the rootball. You can improve drainage by adding gravel or coarse sand. It may also be advisable to plant on a mound or on sloping ground to help water run off.
• Remove lithodora from its pot, tease the rootball slightly, then place it in the planting hole, with top of rootball level with soil surface.
• Replace soil around it, then firm down with the palm of your hand.
• Water thoroughly.

Continue to water in months following planting to allow Lithodora time to develop its root system, making it more resistant to drought.

See our video tips on planting perennials:

Care

Lithodora tolerates drought fairly well, because its roots seek water deep in the soil. However, if you grow Lithodora diffusa, it is preferable to water it during the year of planting, and during prolonged periods of drought, while avoiding excess moisture. Other lithodora species (L. oleifolia, zahnii, rosmarinifolia, etc.) prefer the soil to remain dry in summer.

We recommend pruning lithodora lightly once it has finished flowering (usually from the end of June). This helps keep a compact, dense habit. Shorten the shoots and take the opportunity to remove dead or damaged branches, as well as spent flowers.

You can also add some well-rotted compost in autumn to enrich the soil. If soil is too poor, Lithodora leaves may tend to yellow.

If you grow it in a pot, remember to repot from time to time into a pot slightly larger.

Lithodora is not susceptible to diseases; the main problem you may encounter is excess moisture, which can cause roots to rot. As for parasitic insects, Lithodora is sometimes attacked by aphids. These small insects settle on the plant and suck the sap, which weakens it. To get rid of them, we recommend using black soap diluted in water, sprayed onto the plant.

Lithodora diffusa ‘Heavenly Blue’ (photo Ghislain118)

Propagate Lithodora

Lithodora is easy to multiply using propagation by cuttings. Do this in mid-summer.

• Prepare a pot with a mixture of potting compost and coarse sand (for drainage). Water until well moist.
• Take a shoot, preferably healthy and well formed, without flowers, and cut it to about 5 cm in length.
• Insert the base of the shoot into the substrate, then firm well all around to ensure good contact between soil and stem.
• Place pot under cover, out of direct sun, and ensure substrate remains slightly moist.
• During first year, it is preferable to protect lithodora from winter cold. Keep them under cover and wait until spring to plant in open ground.

Companion plants for lithodora

As it enjoys full sun and well-drained, stony soils, Lithodora integrates very easily into a rockery. Plant with sedums, Phlox subulata or Phlox bifida, Alyssum montanum… Enjoy white flowering of Arenaria montana or Cerastium. You can also add small gentians, such as Gentiana acaulis, valued for trumpet-shaped flowers of a truly intense blue. Optionally incorporate clumps of grasses for added volume and movement. It is also possible to plant Lithodora on a low wall, between stones… It will add colour! It can be planted there with sedums, houseleeks, Cerastium tomentosum and Cymbalaria muralis.

Lithodoras fit easily into a rockery, with other plants adapted to stony, dry soils. Cerastium tomentosum (photo Krzysztof Golik), Gentiana acaulis, Lithodora diffusa ‘Heavenly Blue’ (photo Meneerke bloem), Saxifraga cotyledon ‘Southside Seedling’, and Phlox subulata (photo Meneerke bloem)

You can also plant Lithodora at the front of a perennial bed. Pair with taller plants that will provide volume, such as large grasses and some bushes. We also recommend combining it with other blue, mauve and white flowerings, to create a bed with soft, delicate tones. Opt for, for example, Anchusa azurea, Aquilegia vulgaris, centaureas… Also discover graceful pale-blue inflorescences of Veronica gentianoides! Not forgetting Iris germanica and elegant flowering (for example varieties ‘English Cottage’ or ‘Blue Crusader’).

However, you can also play with contrasts, knowing that complementary colour to blue is orange. These hues reinforce each other and will energise the bed. You can therefore associate Lithodora with Euphorbia griffithii ‘Dixter’, with Geum coccineum ‘Borisii’, with Iris germanica ‘Good Show’, or with Papaver orientale ‘Harvest Moon’…

You can also plant Lithodora as groundcover, for example at foot of your bushes. Place in front of roses, Deutzia, mock orange, Weigelia… As Lithodora effectively covers soil by forming a fairly dense tapetum, it will limit weed development. Similarly, it can be installed on a slope to green up a corner of garden you rarely tend.