How to Kill Green Alkanet

Green alkanet (Pentaglottis sempervirens) divides gardeners. Some treasure its vivid blue forget-me-not flowers that appear in spring and early summer. Others curse its ability to colonise shady borders and resist all attempts at removal. Both views are justified. It’s one of those weeds commonly found in UK gardens that sparks debate.

This bristly perennial was introduced to British gardens over 300 years ago as an ornamental plant. It escaped cultivation long ago and is now naturalised throughout much of the UK, particularly in the south of England. You’ll find it in hedgerows, woodland edges, and shady garden corners where it thrives in conditions many other plants struggle with.

Why Green Alkanet is Hard to Remove

The problem lies underground. Green alkanet develops a deep, thick, black taproot that anchors the plant firmly and stores enough energy to regenerate from any fragment left in the soil. The root is also brittle, meaning it snaps easily when you try to pull the plant, almost guaranteeing that pieces remain behind.

Unlike ground elder which spreads horizontally through shallow rhizomes, green alkanet’s taproot grows straight down. This makes it harder to dig out completely. You need to excavate deeply rather than widely, but the brittle root makes this difficult without leaving fragments.

The plant also spreads prolifically by seed. Each flower produces four small nutlets, and a single plant can scatter hundreds of seeds around your garden. Seeds germinate readily, and young seedlings quickly develop their own taproots. If you let green alkanet flower and set seed, you’re creating next year’s problem.

Identifying Green Alkanet

Green alkanet forms rosettes of large, bristly, oval leaves up to 40cm long. The leaves have a rough, hairy texture that can irritate skin in some people. The species name ‘sempervirens’ means evergreen, and in mild areas the leaves persist through winter. Even where leaves die back, new growth appears very early in spring.

From April to July, upright bristly stems bear clusters of small bright blue flowers with white centres. The flowers strongly resemble forget-me-nots, which is no coincidence since both belong to the borage family (Boraginaceae). The plant typically grows 60-90cm tall when in flower.

Green alkanet is sometimes confused with comfrey, which has similar bristly leaves but drooping tubular flowers in cream, pink or purple. Brunnera (Siberian bugloss) has similar blue flowers but heart-shaped leaves without the coarse bristles.

The Wildlife Value Dilemma

Before reaching for the weedkiller, consider whether green alkanet is actually causing problems. The RHS notes that its nectar-rich flowers are visited by bees, hoverflies and butterflies. In spring, when few other flowers are available, green alkanet provides valuable forage for emerging pollinators.

The leaves are food for larvae of the scarlet tiger moth, one of Britain’s most striking day-flying moths. The dense foliage also provides ground cover and shelter for various invertebrates.

In wildlife gardens or informal areas, you might decide to tolerate green alkanet, or at least limit its spread rather than attempting complete eradication. The same calculation applies to stinging nettles, another weed with significant wildlife value.

Digging Out Green Alkanet

For small infestations, digging remains the most reliable control method. The key is removing as much of the taproot as possible. Use a border fork or spade to loosen the soil deeply around each plant before attempting to lift it.

A narrow-bladed dandelion weeder or similar tool designed for deep-rooted weeds helps target the taproot with minimal soil disturbance. Work when soil is moist, as roots come out more completely in damp conditions.

Expect to need follow-up weeding as fragments you missed regenerate. Check the area regularly and dig out any regrowth while plants are still small and roots are shallow. Docks present a similar challenge, requiring removal of the top 15cm of taproot to prevent regrowth.

Don’t add roots to your home compost heap. They’re likely to survive and regrow. The leaves are safe to compost, or you can make a nutrient-rich liquid feed by soaking them in water for several weeks, similar to comfrey tea.

Using Weedkiller on Green Alkanet

Glyphosate can kill green alkanet, though you may need repeated applications. The systemic herbicide is absorbed through the leaves and transported down to the roots, killing the entire plant including the taproot. For stubborn plants, the strongest weed killer available gives the best results.

Apply in spring when plants are growing actively and have plenty of leaf area to absorb the chemical. Spray on a dry, still day and avoid application before rain. The larger the leaf area, the more effectively the plant absorbs the herbicide.

One application often isn’t enough. Surviving root fragments may send up new growth, which needs treating again. Plan for follow-up applications over the growing season if regrowth appears.

For green alkanet growing among wanted plants, careful spot treatment with a gel formulation allows precise application without drift to surrounding foliage. Alternatively, use the wipe-on method with a glove, applying glyphosate directly to alkanet leaves while avoiding contact with other plants. Triclopyr is another effective option for broadleaf weeds like this.

Preventing Green Alkanet

The most important prevention measure is stopping seed production. Never let green alkanet flower and set seed if you’re trying to control it. Cut plants down before flowers open, or at least before seeds develop.

Pull or fork out seedlings as soon as you spot them. Young plants have shallow roots that come out easily. Once the taproot develops, removal becomes much harder.

Dense planting helps suppress green alkanet. Like many weeds, it establishes most readily in gaps and open ground. Filling borders with vigorous ground cover plants, much like the approach for controlling ivy, reduces opportunities for green alkanet to gain a foothold.

Mulching helps suppress seedlings but won’t kill established plants. A thick layer of bark or wood chips prevents seeds germinating and makes it easier to spot and remove any plants that do emerge.

Frequently Asked Questions

Is green alkanet native to the UK?

No. It’s native to southwestern Europe and was introduced to British gardens before 1700. It escaped cultivation and is now widely naturalised, particularly in southern England. It’s classed as a neophyte, a plant introduced after 1500.

Can green alkanet cause skin irritation?

Yes, some people react to the bristly hairs on the leaves and stems. The hairs contain calcium carbonate crystite structures that can cause contact dermatitis in sensitive individuals. Wear gloves when handling the plant.

Will cutting green alkanet down kill it?

No. The deep taproot stores enough energy to regrow repeatedly. Cutting weakens the plant over time if done persistently, but won’t kill it without removing or poisoning the root system.

Should I remove green alkanet from my garden?

It depends on where it’s growing and your priorities. In wildlife-friendly areas, the flowers provide valuable nectar. In vegetable beds, formal borders, or areas where it’s crowding out other plants, control is usually worthwhile. Dandelions present a similar choice between tolerance and control.

How deep do green alkanet roots go?

The black taproot can extend 30cm or more into the soil. This depth, combined with the root’s brittle nature, makes complete removal challenging without leaving fragments that regrow.

Green alkanet is one of those weeds where you need to pick your battles. In the right place, its blue flowers and wildlife value earn it a spot in the garden. In the wrong place, persistent digging or repeated glyphosate treatment will eventually bring it under control. Whatever approach you choose, don’t let it set seed. For other deep-rooted problem weeds, see our guides to lesser celandine and bindweed.