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Isodon effusus

Isodon effusus

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A wonderful perennial for partial shade. In October, this delicate plant bears an abundance of small bright blue-violet tubular flowers, carried in light panicles, on long light green leaves. Very hardy, it is a deciduous plant that is easily grown away from the scorching sun, in any garden soil enriched with compost that is moist but not too dry. A sheltered position will protect the flowering from early frosts.
Flower size
1 cm
Height at maturity
70 cm
Spread at maturity
50 cm
Exposure
Partial shade
Hardiness
Hardy down to -18°C
Soil moisture
Moist soil
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Best planting time March to April
Recommended planting time March to June
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Flowering time October to December
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Description

Isodon effusus, or Plectranthus effusus, is part of a group of perennials from Asia that thrive in partial shade and are appreciated for their delicate late flowering. This effusa species, quite rare in cultivation, is a favourite among connoisseurs. It is a delicate plant with a beautiful fountain-like habit, which produces light panicles adorned with tiny bright blue-violet flowers in October. Resistant to cold, this rather accommodating plant prefers slightly sheltered exposures as well as humus-rich soils that remain moist even in summer, where it will fully thrive.

 

Isodon effusus is a deciduous perennial belonging to the Lamiaceae family, just like salvias. It is native to mountainous areas of Japan, where it is found on forest edges. Resembling a plectranthus, it forms a tuft with a flexible, spreading habit, composed of outwardly arching leafy stems. Its growth is quite slow. When mature, it will reach approximately 70cm (28in) in height and 50cm (20in) in width at the base. Emerging from the ground in spring, its sturdy stems bear opposite, narrowly triangular and elongated leaves with a long pointed tip, toothed edges, and a fairly light green colour. The small tubular flowers appear in October and continue to bloom on the plant as long as it doesn't freeze. They are carried in sparse panicles. The stems die back each year in winter and new growth starts rather late, from April onwards.

 

It is perfectly hardy and able to withstand temperatures of around -16°C (3.2°F). It is undemanding in terms of soil composition. Isodon effusus thrives in partially shaded positions, protected from the sun during the hottest hours of the day. These conditions can be met on the edge of a grove, in front of deep-rooted deciduous trees or shrubs that won't use up all the available nutrients at the soil surface but will protect the flowering from cold winds. Isodon also enjoys being near a north or east-facing wall, in the company of hydrangeas, Japanese anemones, Fuchsia magellanica 'Alba', corydalis, strobilanthes, aconites, and foxgloves. Bulbs of Naples cyclamen can be planted at its base. Pairing it with purple foliage plants in autumn, such as physocarpus, hydrangea Endless Summer® 'Twist and Shout', or the dentate viburnum is simply stunning. Ferns make perfect companions.

Isodon effusus in pictures

Isodon effusus (Flowering) Flowering
Isodon effusus (Foliage) Foliage

Flowering

Flower colour violet
Flowering time October to December
Inflorescence Cluster
Flower size 1 cm

Foliage

Foliage persistence Deciduous
Foliage colour light green

Plant habit

Height at maturity 70 cm
Spread at maturity 50 cm
Growth rate normal

Botanical data

Genus

Isodon

Species

effusus

Family

Lamiaceae

Origin

East Asia

Product reference158751

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Planting and care

Isodon effusus is hardy beyond -15°C (5°F). It appreciates rich and humus-rich, slightly acidic, neutral or even slightly calcareous soils, which remain moist during summer. Cold winds will burn its late flowers so plant it in a sheltered location (under the cover of trees or protected by a hedge, for example). This plant prefers partial shade, but tolerates dense shade and also grows in the (non-burning) sun if the soil remains consistently moist. Plant it in spring, so that it has time to acclimatise well. It seems to withstand moderate drought, but performs best in soil that retains some moisture. A layer of compost spread around the base of the plant will be beneficial. The stems can sometimes collapse: they can be left to naturally fall or be supported by a stake, or even leaned against neighbouring plants. The stems die late. We advise you to leave them in place during winter, and cut back the clump in spring before the start of vegetation. This way, you may be able to obtain seeds that will germinate a little further away. It can be multiplied by dividing the stump in spring or by rooting the base of the stems in September, or by sowing in spring under a cold frame.

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Planting period

Best planting time March to April
Recommended planting time March to June

Intended location

Suitable for Meadow, Woodland edge, Undergrowth
Type of use Border
Hardiness Hardy down to -18°C (USDA zone 7a) Show map
Ease of cultivation Amateur
Planting density 5 per m2
Exposure Partial shade
Soil pH Neutral
Soil type Silty-loamy (rich and light)
Soil moisture Moist soil, light, rich in humus

Care

Pruning instructions Prune the clump in late winter, before the start of vegetation.
Pruning Pruning recommended once a year
Pruning time March
Disease resistance Very good
Overwinter Can be left in the ground

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