The purple gromwell really lives up to its name. Read here how to properly plant and care for the interesting flowering ground cover.
Profile of purple gromwell:
Scientific name: Lithospermum purpurocaeruleum
Plant family: borage family (Boraginaceae)
Other names: –
Sowing time: November to March
Planting time: Spring to Autumn
Flowering period: April to June
Location: sunny to partially shaded, light spot under trees
Soil quality: stony to sandy, nutrient rich, humus rich
These information are for temperate climate!
Use in: flowerbeds, ground cover, group planting, planters, under wood, borders, cottage garden, natural garden, rock garden, pot garden, forest garden
Winter hardiness: hardy, USDA Plant Hardiness Zones: 6 (-23 °C / -5 °F)
Bee and insect friendly: yes, especially by carder bees
Plant characteristics and classification of purple gromwell
Plant order, origin and occurrence of purple gromwell
The purple gromwell (Lithospermum purpurocaeruleum) of the borage family (Boraginaceae) was formerly known under the botanical name Buglossoides purpurocaeruleum. The identifier is still very common today. The perennial wild herbaceous perennial is widespread across Europe. It is considered the most important horticultural species within its genus and is mainly used as a robust and decorative ground cover for the underplanting of woody plants.
Characteristics of purple gromwell
The purple gromwell is fast growing and spreads out by creeping rhizomes. The lying, partially arched overhanging leaf shoots are up to 60 centimeters (24 in) long, while the shorter flower shoots grow upright and only reach a height of 15 to 30 centimeters (6 to 12 in).
The approximately eight-centimeter (3.2 in) big leaves of the purple gromwell are narrowly elliptical to lanceolate and pointed at the front. They are dark or gray-green in color and ciliolate on the leaf margins. The leaf blade feels rough. Lithospermum purpurocaeruleum loses its leaves in autumn. The tips of the leaf shoots often root by layer because they run close to the ground.
Purple gromwell owes the first part of its name to its flowers, which undergo a striking color change: while the buds and early flowers appear in warm purple or red, they later change to bright blue / blue violet. The flowering period lasts from May to June, but depending on the weather, it opens its flowers already in April. The flowers are tubular and arranged in bunches that open in a bowl shape towards the front.
The name gromwell (in German stone seed) refers to the fruits of the perennial, which are very hard, round nuts that adhere to the plant for a long time.
Purple gromwell – cultivation and care
Lithospermum purpurocaeruleum develops adequately both in the sun and in the shade. The perennial grows best in the long term in partial shade or in light shade under trees.
To the delight of many hobby gardeners, the purple gromwell can be planted excellently in dry and calcareous soils. In addition, the soil should be rich in nutrients and permeable.
Since the plant grows more in width than in height, it is important to keep a distance of at least 35 centimeters (14 in) from other plants when planting. There are eight to ten plants per square meter of bed plot. The purple gromwell can basically be planted throughout the season and take root even in autumn.
Purple gromwell usually reproduces itself very well, for example by self-sowing or via runners, sometimes even more than desired. Despite everything, they can also be sown by hand or propagated using cuttings, runners or by division.
Sowing is usually somewhat more complex than, for example, the propagation by cuttings. The best time to do this is from November to March. The seeds are first sown in seeding compost in a seed tray. The substrate is moistened and placed in a warm spot with temperatures between 18 and 22 °C (64 and 71 °F) for the next 2-4 weeks.
After these four weeks, the seeds must be subjected to a cold treatment, because gromwell are so-called cold germs. Accordingly, the seed tray should now be placed in a much cooler place at -4 to +4 °C (25 to 39 °F) , for example in the refrigerator, for about 4-6 weeks. At temperatures above +4 °C (39 °F), the cold period must be extended accordingly.
It takes between 8 and 10 weeks to germinate. If the seedlings can be seen, the temperatures should not rise too quickly, but should ideally be between 5 and 12 °C (41 and 53 °F). Later the daytime temperatures can be raised to 15 and 17 °C (59 and 63 °F). From March to end of May, the sowing is at best placed in a cold house, cold box or outdoors. If the plants are large enough, they can be prick out.
Semi-ripe head or shoot cuttings from this year’s shoots can be cut for propagation. The best time for this is between May and June, at this point the cuttings should have a certain maturity. They are cut under a leaf knot and should have three pairs of leaves. When cutting the cuttings, squashing should be avoided and only sufficiently sharp cutting tools should be used.
To support root formation, the cuttings can be placed in willow water. This is a very effective rooting agent because it contains, among other things growth hormones, so-called auxins, which support rooting. You can produce willow water yourself, provided you find a willow in the vicinity.
Rooting in willow water
For the willow water you need young leafless shoots from a willow. These are then chopped into 1-2 cm (0.5 in) small pieces, put in water for 24 hours and placed in a cool and shady place. About 300 g (0.6 lbs) pieces of willow are expected per liter (2 pint) of water. After 24 hours, pour through a sieve. The cuttings are now placed in this water, also for 24 hours.
Then you take them out of the water, rinse them briefly and plant them in a growing substrate. In order to create an optimal microclimate for the cuttings, a translucent film bag or transparent film is put over the cuttings. Usually rooting takes place within 10-14 days. When the first shoots appear, the cuttings can be separated into small pots or planted out on the spot.
A division is possible in autumn but also in spring. To do this, the plant is dug out, carefully divided and the newly obtained sections are re-planted separately from each other at their final location. Then water well.
Propagation via layer is also not a problem. You bend a side shoot down, place it on the ground and cover it with earth. To prevent it from slipping out again, you can fix it to the ground with wire. At the points where the shoot touches the ground, new roots are formed, so that the layer can be separated from the mother plant and planted in its final location.
At suitable locations, the shrub does not require any care and does not need to be watered even during long periods of drought. If the shoots become too long, they can easily be cut back.
Lithodora diffusa is one of the plants that do not necessarily have to be cut. Pruning can be particularly useful if the plants are already bare from the inside. Evergreen perennials are best cut in spring or after flowering. In this way, the seed heads can also add a splash of color to the garden in winter.
In addition, dead plant parts offer additional protection against severe frosts in winter. It is best to cut out dead plant parts in spring and shorten them a little about every 2-3 years after flowering, so that they sprout more strongly and become denser. Otherwise, you should leave the plants alone so that they can develop into dense, colorful cushions.
Diseases and pests
Plant diseases or pests hardly cause problems for the purple gromwell. Even snails seem to avoid the rough leaves of the perennial.
There is not much to consider during the winter. The purple gromwell is usually hardy up to -20 °C / 4 °F, but should be provided with winter protection, for example made from brushwood, in particularly cold locations.
Use in the garden
As a wild perennial, the purple gromwell blends harmoniously into natural gardens, where it also serves as a foodplant for bees and other insects. Due to its location requirements, Lithospermum purpurocaeruleum is also perfect for the rock garden. Since it is very competitive and can cope with high root pressure, the purple gromwell is traditionally used for greening and planting on the edges of trees or under trees and bushes. You can also use it for planting in large containers.
Examples of use:
- Suitable for small and large areas as ground cover and green areas
- As a fast-growing, branch-forming perennial for fixing slopes
- As a flowering shrub in the foreground of partially shaded beds or borders
- As a drought tolerant perennial for planting trees and shrubs
- Dense bee pasture in the garden