The pretty blue forget-me-not flowers of green alkanet (Pentaglottis sempervirens) are attractive and are pollinated by bees and bumblebees. However it can soon spread in damp shady gardens where it may become a nuisance.
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 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. This page looks at options for the gardener when green alkanet is becoming a problem.
First, consider whether this can be done using non-chemical means such as digging out or suppressing with mulch. Where these methods are not feasible, chemical controls may need to be used.
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
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.
- Hoeing weed seedlings is time consuming and needs doing promptly - before plants flower and set seed - to be effective. Use gloves if grasping stems directly. As seeds can lie dormant in the soil for long periods of time, this task will be on-going.
- It is also important to avoid adding any mature weeds, which have set seed, to a home compost bin.
- To prevent germination of weed seedlings, apply an opaque mulching film or layer of bulky organic mulch, such as woodchips, to the soil at a depth of at least 8cm (3in).
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 Ultra, SBM Job done General Purpose Weedkiller or Doff Advanced Weedkiller).
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 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.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see section 4)
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.