Strobilanthes - Persian Shield: Planting, Growing and Caring

Botany and description

Strobilanthes belong to the Acanthaceae family. Native to Asia and Madagascar, the Strobilanthes genus comprises nearly 250 species of perennials and undershrubs. Their natural habitat is forest edges or woodland undergrowth. The genus was first described by botanist Carl Ludwig Blume in 1826.

Most species aren’t hardy enough to survive our winters, like Strobilanthes dyerianus, and are grown in pots or as annuals.

However, around thirty species originate from the Himalayan mountains and are sufficiently hardy to remain in our gardens year-round. This includes, among others, Strobilanthes attenuata, Strobilanthes yunnanensis, Strobilanthes atropurpureus, Strobilanthes nutans, Strobilanthes walichii or Strobilanthes penstemonoides (formerly rankanensis).

Strobilanthes are cultivated for their beautiful foliage, sometimes highly colourful, and their summer flowering extending into autumn, often in striking blue hues. Hardy species tend to be undershrubs forming bushy growth, sometimes reaching one metre in all directions, as with Strobilanthes penstemonoides.

The foliage is deciduous. The ovate leaves are opposite and finely dentate. They are green in colour, occasionally purple (Strobilanthes brunetthy) or violet-mauve (Strobilanthes dyerianus).

The stems are characteristic: they feature more or less swollen nodes, from which new roots frequently emerge when touching the ground, thus propagating the plant which doesn’t produce many seeds.

The inflorescence consists of large tubular flowers in blue to blue-mauve, sometimes white as in Strobilanthes nutans. The flowers are most often paired at the end of a slender stalk. Flowering begins in summer and continues for months, until the first frosts.

Note that the plant resumes growth very late in spring, during May.

Some flowering species: Strobilanthes penstemonoides, Strobilanthes nutans and Strobilanthes attenuata

Our favourite varieties

[product sku=”8497″ blog_description=”Strobilanthes attenuata is a lovely hardy perennial groundcover plant that’s easy to grow. Perfectly hardy, it thrives even in poor soil under bush cover. It forms rather tall herbaceous clumps adorned from late summer until frost with dense spikes of beautiful tubular blue-violet flowers.” template=”listing1″ /]

[product sku=”810471″ blog_description=”Strobilanthes attenuata ‘Blue Carpet’ is a beautiful selection of the species Strobilanthes attenuata. The plant is lower-growing and the flowers are a more intense blue-violet than the original species.” template=”listing1″ /]

[product sku=”8498″ blog_description=”Strobilanthes nutans is a very hardy perennial in cold oceanic climates and easy to cultivate. Its large pure white tubular flowers bloom in summer, arranged in short, dense spikes. Strobilanthes nutans grows in shade or partial shade in any cool, acidic soil.” template=”listing1″ /]

[product sku=”8499″ blog_description=”Native to China, Strobilanthes rankanensis (penstemonoides) forms beautiful bushes (1m tall) with ovate, finely dentate leaves of a striking bright green. Late in the season, it becomes covered with spectacular blue-mauve flowers.” template=”listing1″ /]

Planting Strobilanthes

The genus Strobilanthes should be divided into two cultivation categories. Hardy varieties that can be grown in open ground, and non-hardy types that should be cultivated in pots or containers during the warmer months.

Where to plant?

Hardy Strobilanthes such as Strobilanthes atropurpureus, Strobilanthes penstemonoides, Strobilanthes attenuata or Strobilanthes nutans should be planted in open ground in partial shade or full shade beneath tree cover. In northern regions, most species tolerate sunnier positions. These perennials thrive in fresh, well-drained soil, rich in humus and free from lime. Once established, strobilanthes can tolerate dry conditions.

For non-hardy Strobilanthes like Strobilanthes dyerianus, ensure the container has good drainage. Use a pot or planter with drainage holes at the base! Place a layer of clay pebbles or gravel at the bottom, then fill with a mix of one-third garden soil, one-third universal compost and one-third well-rotted compost. Position the pot away from direct sunlight and cold draughts.

When to plant?

Planting of species hardy enough to survive year-round in the garden should be done in spring once the soil has warmed sufficiently, between March and June depending on your climate. Earlier in southern and southwestern regions, and around May-June in northern areas.

How to plant?

• Loosen the soil thoroughly to a depth of about twenty centimetres;
• Moisten the root ball before removing from its pot;
• Dig a hole approximately twice the size of the root ball;
• Strobilanthes require fertile, well-drained soil: don’t hesitate to mix some gravel or clay pebbles with the soil used to backfill the hole. A good handful of compost will also be beneficial;
• Position the root ball in the centre of the hole;
• Backfill with the excavated soil (mixed with gravel if the soil is too compact) and gently firm around the collar with your fingers;
• Water thoroughly to eliminate any air pockets between soil and roots;
• Apply mulch generously to reduce watering needs and maintain soil moisture.

Strobilanthes penstemonoides (syn. rankanensis) – Photo Peganum (Flickr)

Maintenance

Growing strobilanthes is easy and requires very little maintenance.

In the ground

The plant alerts the gardener when thirsty by drooping its foliage. If this happens, give it a good watering with non-calcareous water. A good mulch will help maintain sufficiently moist soil to prevent the plant from suffering. The mulch will also protect the stump from severe frost in winter.

You can pinch the terminal shoots in June (this will also provide cuttings!), to encourage the plant to become more ramified. The clump will be more compact, the plant will produce more flowers, but they will appear later.

At the end of winter, cut back the dry stems before new growth resumes: in March-April.

In pots

Place your Strobilanthes on a saucer filled with clay pebbles (or small gravel) kept moist. Mist the plant regularly to maintain good humidity. If needed, apply a balanced liquid fertiliser, with a good phosphorus content.

If your Strobilanthes becomes bare at the base, you can prune it in April before new growth starts. Cutting back about a third of its height will be sufficient.

Diseases and Pests

Outdoor Strobilanthes are rarely affected by diseases and pests.

Only Strobilanthes grown indoors may suffer from mealybugs. You can remove them with a single damp sponge or a cotton bud soaked in alcohol. Repeat this process as many times as needed until the mealybugs disappear.

Propagation

Seeds are available commercially, but sowing yields disappointing results. Unlike propagation by cuttings!

Taking cuttings from a strobilanthes is, in fact, child’s play.

• Take terminal cuttings in summer, around 10 cm, cutting just below a swollen node;
• Remove the lower leaves and keep only the two leaves at the top;
• Place your cuttings in a bucket with a special sowing and propagation compost;
• Cover with a cloche or half a plastic bottle: propagate under cover;
• And place your cuttings in warmth and light without direct sunlight;
• After 5 to 6 weeks, the cuttings should have rooted;
• Keep your young plants in buckets, then transplant them into a larger pot when the roots emerge from the drainage holes;
• For hardy species, transplant into open ground the following spring.

Even simpler, if your strobilanthes is thriving, you can take advantage of the plant’s natural propagation ability. At the swollen nodes, there is what’s called an “abscission point”, a natural breaking point. In short, if you shake the plant, pieces will detach at the nodes. These stem fragments will then fall to the ground and produce roots from the node. The plant thus propagates itself.

Association

Hardy Strobilanthes are woodland plants that thrive in partial shade and well-drained, moist soil. They will therefore be perfectly at home in the garden alongside other plants that enjoy these growing conditions.

Strobilanthes penstemonoides will make excellent companions with hostas, such as the Hosta ‘Blue Cadet’ with its lavender flowers, and barrenworts like the Epimedium grandiflorum ‘Purple Pixie’ with its bright purple-pink flowering.

A few Rodgersia henrici will add vertical interest with their flower spikes reaching over a metre high. The scene can be completed with meadow rues and perhaps a Kirengeshoma palmata, whose buttery yellow flowers emerging from highly graphic palmate foliage will perfectly complement the long flowering period of the strobilanthes.

A planting idea: Strobilanthes penstemonoides, Hosta ‘Blue Cadet’, Epimedium grandiflorum ‘Purple Pixie’, Rodgersia henrici and Kirengeshoma palmata