Acanthus sennii

Acanthus sennii is a spectacular acanthus that will delight fans of unusual plants. Bushy and evergreen in its native lands, this botanical species generally behaves as a perennial in our gardens, regrowing from the base each spring. It bears on dark stems thistle-like foliage, highly dissected, spiny, glossy, veined with silver, and is adorned in late summer or autumn with spikes of bright red flowers. It is highly architectural, surprising, and ideal for a large exotic or Mediterranean bed in milder areas. Collectors of rare plants could keep it in a large pot, allowing for winter storage. With its unique features and elegant appearance, this lesser-known plant is quickly becoming popular among gardeners.

The Acanthus sennii is an African cousin of the Mediterranean Acanthus mollis, belonging to the same family, Acanthaceae. It originates from the high plateaus of Ethiopia, where it grows between 1700 and 3200m (5577 and 10499ft) in altitude. This thorny plant, which can form a bushy shrub exceeding 2m (7ft) in height, is often used as a hedge to protect vegetable gardens. Some gardeners suggest that in our climate, whilst frost can damage the foliage, the base may survive winter in zone 8b if it is well-protected and established in well-drained soil. This means the base can withstand brief freezing temperatures around -6°C/-8°C (17.6°F).

It is an herbaceous plant that can become woody in mild climates. This acanthus develops strong, branched stems of a black-violet colour from the perennial base. The foliage forms a bush that can reach, depending on the mildness of the climate, from 1 to 2.50m (3 to 8ft) in height and 60cm (24in) to 1.20m (4ft) in spread. The stems bear large leaves that resemble those of holly or certain thistles. Thick, leathery, and glossy, they are deeply lobed, with each tooth extended by a fierce spine. Their colour is more or less greyish green, with silver veins. The drier the soil in which the plant grows, the more silver the foliage will be. The young shoots are less spiny and display a soft pink-grey colour bordered by violet; the mature leaves often close around these young shoots to protect them from the sun. Flowering usually occurs in warmer areas in September-October, sometimes as early as August, and sometimes in late autumn in cooler climates. Plants at least 1.2m in height develop typical acanthus flowers in spikes at the tip of their stems. These spikes bear large flowers measuring 8-10cm (3-4in) in length, bilabiate, with a well-developed lower lobe, in a scarlet red colour. A spiny bract tops each flower. Acanthus attracts large pollinating insects, such as bumblebees or carpenter bees. The fruits are shiny capsules containing large seeds. The plant should be pruned and its base protected when the temperature drops below -5°C. Vegetation will regrow in spring. This very sober and undemanding plant needs some water during the flowering period.

The Ethiopian acanthus is not lacking in character or charm; it is a truly magnificent plant that cannot go unnoticed. When planted in the ground in mild climates, it can form a beautiful shrub that can be displayed as a standalone specimen or tucked into a defensive hedge or exotic bed. It can also be grown in a pot but requires caution when handled due to its spiny vegetation. In a dry garden, it is a robust perennial that pairs well with the late blue blooms of Caryopteris, Ceratostigma griffithii, or Perowskia 'Blue Spire'. It also pairs well with tall euphorbias (Euphorbia mellifera or dendroides), Centaurea pulcherrima, and Lobelia tupa.