I. Appearance and Characteristics
Ceratostigma plumbaginoides (蓝雪花), the hardy blue-flowered leadwort, is a species of flowering plant in the plumbago family, native to Western China (Beijing Shi, Henan, Jiangsu, Shanxi, Zhejiang), where it is usually found in rocky foothills.
Ceratostigma is derived from Greek, meaning ‘horned stigma’. This is in reference to the ‘shape of the stigmatic surface’.
Growing to 50 cm (20 in) tall and broad, it is a mat-forming herbaceous perennial with small ovoid leaves and bright blue flowers in late summer and early autumn. The leaves may turn red or purple before falling.
The Latin specific epithet plumbaginoides signifies its resemblance to plants in the closely related genus Plumbago.
Ceratostigma plumbaginoides is grown as an ornamental plant in temperate climates, valued for its late season colour. It is hardy down to −10 °C (14 °F), but prefers a sunny, sheltered position in moist, well-drained soil. As it can become invasive, it is particularly suited to growing in a pot, or crevices in a dry stone wall. It has gained the Royal Horticultural Society’s Award of Garden Merit.
II. How to Grow and Care
Sunlight
Leadwort plants can grow either in full sun or partial shade. The best flowering will occur in full sun, though the plants do benefit from afternoon shade in hot climates, particularly during the summertime.
Temperature and Humidity
When choosing a leadwort variety, consider your hardiness zone. Some plants will not remain evergreen in cooler zones. They will wilt in very hot weather but typically bounce back once the temperature cools. They also benefit from a light layer of mulch for winter protection, especially in the colder parts of their growing zones where they might not be fully hardy. Be sure to remove the mulch come spring once new growth is beginning. Humidity typically is not an issue as long as soil moisture needs are being met.
Watering
Leadwort has moderate moisture needs—roughly 1/2 inch of water per week is usually sufficient. Water during prolonged periods of drought to prevent the soil from fully drying out. But make sure your plants don’t become waterlogged, as they can easily rot in such conditions.
Soil
These plants are tolerant to most soil types, including sandy, loamy, and clay soils, as long as the planting site has good drainage. The best performance will occur in slightly acidic soil, but leadwort readily grows in neutral and in slightly alkaline soils
Fertilizing
Leadwort likes somewhat fertile soil. Apply a balanced, slow-release fertilizer in the early spring as new growth is picking up. Fertilize again in the early summer to help the plant fill out. Do not fertilize in the late summer, as this will promote tender new growth that will be vulnerable and could weaken the plant as the fall temperatures cool.
Pruning
Because these plants are slow to leaf out in the spring, many gardeners opt to leave the old stems in place to mark where a plant is. That way, they don’t accidentally disturb it by trying to plant something else in the area. Cut back the old stems in the spring as soon as new growth starts to appear.
Propagation
While leadwort plants can self-seed and spread via rhizomes (underground stems) in the garden, they are easily propagated through stem cuttings. Here’s how to do it:
- Cut a 3- to 6-inch stem segment during the late spring or early summer, using sharp pruners.
- Remove the leaves on the lower half of the cutting. Apply a rooting hormone to the cut end.
- Plant the cutting in a small pot filled with ordinary potting soil, then place the pot in a loosely secured plastic bag to hold in moisture.
- Place the pot in a bright location, but out of direct sunlight. Keep the medium evenly moist but not soggy for several weeks. Once you feel resistance when you lightly tug the stem, you’ll know roots have developed. This normally can take as much as four weeks.
- Remove the plastic bag, and continue to grow the cutting for about two more months. When new leaves begin to emerge, the plant is ready for the garden.
Propagating by root cuttings is another easy method of creating new plants. In early spring, dig up a section of the root ball and find good, thick pieces at least 1/2 inch thick. Snip the root sections into 4- to 6-inch-long segments, and plant them about 1/2 inch deep in small pots filled with commercial potting soil. Keep the pots moist in a bright but shaded location until they sprout and begin producing new leaves. Grow for another few weeks in the pot, then transplant into the garden.
How to Grow Leadwort From Seed
Propagation from seed is not a common approach, as the seeds of leadwort are slow to germinate and require a lengthy cold stratification period to be viable. But if you want to try it, you can collect seeds from the dried seed heads in early fall. Mix the seeds with moist sand in a plastic bag, then place them in the refrigerator for five to six weeks. This cold stratification period is essential to break the seeds’ dormancy.
About three months before outdoor planting time, sow the seeds in pots filled with commercial potting mix, just barely covering them. Grow the seeds in a warm location with plenty of bright indirect light, but out of direct sunlight. Be patient, as the seeds will be slow to germinate.
The seedlings can be transplanted into the garden when outdoor temperatures reliably remain above 50 degrees Fahrenheit.
Potting and Repotting
Leadwort makes a good “spiller” plant for mixed outdoor container gardens. Ordinary commercial potting mix makes a suitable growing medium; make sure containers have good drainage. Container plants should be placed in a sheltered outdoor location for the winter; or, grow as annuals. This is not a plant suitable for overwintering indoors.
Regular division and repotting will be necessary if growing leadwort in a permanent container, as these plants will quickly fill their pots.
Overwintering
In zone 5, leadwort can be borderline hardy and will benefit by being protected with a thick layer of mulch over the winter. Remove the mulch promptly in the spring to prevent crown rot.
Pests and Diseases
There are only a small handful of common complaints about leadwort:
Plants are Slow to Leaf-Out in Spring
It is normal for leadwort to leaf out rather late in the spring. Many gardeners like to intersperse spring bulbs among leadwort plants since the bulb flowers and foliage will fade about the time that leadwort begins to actively grow.
Plants Spread Too Fast
Although leadwort is usually not regarded as an invasive plant, its rhizomatous roots may spread faster than you like. It’s generally an easy enough matter to pluck out the volunteer seedlings and to dig up and remove roots to keep the plant within your desired boundaries.
Plants Don’t Return in Spring
If your leadwort plants don’t return at all in the spring, unseasonable winter cold has killed the roots. This is most likely to occur in zone 5, where the plant is borderline hardy. It’s also possible that excessive winter moisture has suffocated the crowns and killed the plants. Leadwort likes well-draining soil, and soggy winter soil can sometimes kill the plants.
III. Uses and Benefits
Leadwort plants are most often used as a low-maintenance ground cover under large shrubs or small trees. Avoid combining leadwort plants with other perennials, as they can outcompete other plants under optimal conditions.
IV. Types of Leadwort
“Leadwort” is a common name used for several different species in two different genera of the Plumbaginaceae family of plants: Ceratostigma and Plumbago. The Ceratostigma genus includes about eight different species, while Plumbago contains nearly 20 species.
C. plumbaginoides is the most common type of leadwort offered in the nursery trade, and it has no widely available named cultivars. Although few consumer garden centers routinely carry the other types of leadworts, gardeners trade them as pass-along plants.
- Ceratostigma willmottanium: This 4-foot-tall species is known as Chinese leadwort. It has blue flowers with triangular-shaped petals and is hardy in zones 6 to 9.
- Plumbago auriculata: Also known as cape leadwort, this plant features deep or pale blue, cup-shaped flowers. It is a shrubby, upright plant that grows to 3 feet. This is a warm-weather plant, appropriate for zones 8 to 11.
- Plumbago europaea: Known as common leadwort, this plant sports dusty pink to lilac flowers. It is a native flower, rarely grown in cultivation. It is hardy to zone 6.
- Plumbago indica: Also called Indian leadwort or scarlet leadwort, this plant has bright red or deep pink flowers. It is appropriate for zones 8 to 11.
