Andrew Chick Ecology

Green Alkanet (Pentaglottis sempervirens)

Green alkanet Pentaglottis sempervirens is one of those plants that splits opinion sharply. With its vivid blue, forget-me-not-like flowers appearing from early spring, it is often admired as a welcome splash of colour in hedgerows, verges and gardens. Yet for many ecologists and gardeners, it raises more complicated questions about non-native species, competition and biodiversity.

Originally native to south-western Europe, green alkanet was introduced into Britain several centuries ago as a garden ornamental and has since become widely naturalised across much of the country.  Its success lies in a combination of traits familiar to any field ecologist: tolerance of shade, a deep and persistent taproot, and prolific seed production. Once established, it tends to stay put but can steadily expand through self-seeding and regeneration from root fragments. 

Is it an invasive species in the UK?

Green alkanet sits in a slightly grey area. It is unquestionably non-native and can behave aggressively, but it is not classed as a legally controlled invasive species in the UK.

In practice, its status is best described as a vigorous naturalised plant with invasive tendencies rather than a high-impact invasive like Himalayan balsam or Japanese knotweed. It spreads readily in disturbed or urban habitats, road verges, garden edges, waste ground, and in places like London it is now one of the most common non-native plants. 

The key issue is scale and context. Individual plants do not typically spread rapidly across landscapes, but local populations can become dense and persistent. Its ability to self-seed and regenerate from deep roots means it can be extremely difficult to remove once established, and in favourable conditions it forms thick stands. 

Value for pollinators

From a pollinator perspective, green alkanet is undeniably valuable. Its flowers are rich in nectar and are visited by a wide range of insects including bees, hoverflies and butterflies. 

It flowers relatively early and over a long period, making it an important food source at a time when resources can still be limited. Bumblebees, honeybees and solitary bees all make use of it, often in large numbers. 

In addition, it supports other wildlife. Its foliage is used by the larvae of the scarlet tiger moth, and its dense growth can provide shelter for invertebrates and small animals. 

From a purely functional perspective, then, it behaves much like other members of the borage family, high nectar output, accessible flowers, and broad appeal to pollinating insects.

Does it compete with native species?

This is where the ecological debate becomes more nuanced. Green alkanet is capable of forming dense patches that shade out and suppress smaller or less competitive plants.  Its large leaves and clumping growth can reduce light availability at ground level, while its deep taproot gives it access to water and nutrients that shallower-rooted species cannot reach.

In gardens and disturbed habitats, this often results in reduced plant diversity, particularly where it dominates borders or woodland edges. In more sensitive habitats, there is potential for it to form near-monocultures, excluding other species locally. 

However, its ecological impact is generally considered moderate rather than severe. It tends to thrive most in already disturbed or urbanised environments rather than intact semi-natural habitats. Unlike some invasive species, there is little evidence that it is driving widespread declines of native flora at a landscape scale.

A balanced view

Green alkanet is a classic example of a plant that is both beneficial and problematic, depending on perspective. It provides a reliable and abundant nectar source for pollinators and can contribute to urban biodiversity, especially in otherwise species-poor environments. At the same time, its vigour means it can dominate locally and reduce plant diversity if left unchecked.

For ecologists, the key is context. In species-rich habitats or carefully managed sites, it is generally undesirable and worth controlling. In urban settings, brownfield land or less sensitive areas, it can be part of a functioning ecosystem, albeit a slightly unruly one.

In short, green alkanet is not among the UK’s most damaging invasive plants, but neither is it entirely benign. It sits somewhere in between: a resilient, adaptable species that offers clear benefits to pollinators, while still requiring a degree of management to prevent it from overwhelming native plant communities.