Green alkanet

The pretty blue forget-me-not flowers of green alkanet (Pentaglottis sempervirens) are attractive but it soon becomes a weed in damp shady gardens. It is pollinated by bees and bumblebees and a brownish-red dye was extracted from its roots.

Green alkanet

Quick facts

Common name Green alkanet
Botanical name Pentaglottis sempervirens
Area affected Damp shady areas and along walls and buildings
Caused by Spreads by seed and regenerates from the roots
Timing Flowers April to June; treat spring to autumn

What is green alkanet?

Green alkanet is a perennial found growing wild in damp and shady places in southwest Europe. It has long been cultivated in gardens, especially on alkaline soils and occurs as a garden escapee naturalised in woods and grassy places. Its ability to self-seed and regenerate from the roots make it a troublesome weed.

Appearance

Green alkanet is a member of the forget-me-not family, Boraginaceae, and is related to borage and comfrey. Hairy stems to 80cm (32in), leaves broadly ovate and noticeably bristly. Bright blue forget-me-not type flowers with white centre, 8-10mm (½in), open from pink buds and are borne April to June.

The problem

Green alkanet spreads by seed and regenerates from the deep perennial tap root. Colonies arise close to the parent plant but the seed capsules can also be transported over some distance on animal fur or clothing.

Control

Non chemical

Cultural methods can effectively control this weed. Dig deeply to remove roots and hand weed or hoe off any seedlings as they appear. Note that the tap root can develop in even quite young plants.

Chemical control

This is a deep-rooted perennial weed and it should be possible to eliminate the problem with repeat applications of a glyphosate-based weedkiller (e.g. Roundup).
 
Glyphosate is a non-selective, systemic weedkiller applied to the foliage. It is inactivated on contact with the soil, so there is no risk of damage to the roots of nearby ornamentals.

As glyphosate is not selective in its action, it is essential to avoid spray or spray drift coming into contact with garden plants. If treating weeds in the immediate vicinity of garden plants, apply carefully using a ready-to-use spray formulation in cool, calm weather. Branches or shoots can be held back, using canes, or by covering or screening while spraying, but make sure that the weed foliage has dried before releasing branches or removing the covering

Glyphosate is most effective when weed growth is vigorous. This usually occurs at flowering stage, but before die-back begins.

This particular weed may take more than one application to eliminate; if more than one treatment is necessary wait until the re-growing weeds have sufficient leaf area to take up the chemical.

Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Download

Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see section 4)

Links

Chemicals: using spot and broad-scale weedkillers
Chemicals: using a sprayer
Chemicals: using safely and effectively
Weeds: non-chemical control

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