(Sonchus species) Sow thistles are annual and perennial weeds that grow just about everywhere in the garden. They produce lots of seeds and quickly spread to become a problem.
What are sow thistles?
Several species of sow thistles – sometimes written as sow-thistle or sowthistle – are common garden weeds. They grow just about everywhere in the garden, making it unsightly.
There are both annual and perennial species of sow thistles. The commonest ones are the annual prickly sow thistle (Sonchus asper) and smooth sow thistle (Sonchus oleraceus) and the perennial, perennial sow thistle (Sonchus arvensis).
They all produce lots of wind-blown seeds that easily spread them all over the garden, making them a nuisance and a problem to control. These seeds can remain dormant in the ground for several years. And as “One year’s seeding means seven years weeding”, allowing plants to flower and produce seeds, means several years of trying to control and remove it.
Perennial sow thistle can also spread via its creeping roots.
Sow thistles are also secondary hosts to some aphid species – such as the currant aphid that attacks currant plants causing leaf distortion – helping to complete its lifecycle and forming a bridging host so that the aphids can then re-infect currants.
Where do they grow?
- Gravel paths
- Between paving slabs
- Waste or uncultivated ground
Sow thistles are in the same plant family as dandelions, and they produce yellow flowers that resemble those of dandelions.
Sow thistle flower stems can reach anything from 30cm (1ft) up to 2.1m (7ft) high, depending upon species and the growing conditions. They are characterised by soft, irregularly lobed leaves that clasp the stem and form a basal rosette, certainly when the plants are young and before they start to flower. The colour ranges from shades of green to bluish and purplish, especially on older plants. Sow thistles produce a milky latex sap when any part of the plant is cut or damaged.
The leaves of prickly sow thistle, as its name suggest, are covered in small spines on their margins.
How to control sow thistles
As with most weeds, never allow sow thistles to become established. This will make it more difficult to fully control them. Early identification and eradication is very important.
Start by digging out as much of the sow thistles as possible, including the creeping roots of perennial sow thistle.
Regularly hoeing seedlings and young plants as soon as you see them is the quickest and easiest method of control. The aim of hoeing is to sever the weed stems at or just below ground level, cutting the top growth from its roots. A sharp hoe blade will make this even quicker and easier, so always sharpen the hoe blade before using it. Hoeing on a warm and/or windy day will mean plants quickly dehydrate and die.
Digging out the plants when they are more established is more time consuming and more difficult – especially where they’re growing among wanted plants.
Flame guns and weeders that use an electric current are also effective in some locations.
Covering bare soil with weed-control membrane (landscape fabric) or even thick black polythene will exclude light and prevent seeds germinating. As will mulching the soil with organic matter, such as a bark mulch. For mulches to work properly, they need to be a minimum of 5cm (2in) thick, but 7.5cm (3in) deep works better.
In lawns, regular mowing throughout the year and the use of lawn fertilisers and lawn weedkillers should weaken and eventually kill them.
You may unknowingly introduce sow thistles into your garden when buying topsoil or manure. So be vigilant when buying these.
There are a number of weed control options available to treat sow thistles. In addition to traditional weedkillers there are now also a range of more natural alternatives.
Any weedkiller can be used to control and kill annual sow thistles in beds, borders, waste ground and on paths. Those marketed as “fast acting” are contact weedkillers – killing or damaging the plant tissue they are sprayed onto or make contact with. These tend to be based on “naturally-occurring” active ingredients, such as acetic acid and natural fatty acids. These will burn off the foliage of perennial sow thistle, but may not completely kill it.
Systemic weedkillers that also kill the roots can also be used, and are a better choice for perennial sow thistle.
To ensure the weedkiller works effectively:
- Spray the leaves when the sow thistles are growing actively; this is mainly from March/April to September/October. Plants are at their weakest just before flowering.
- The larger the leaf area present, the greater the amount of weedkiller that can be absorbed and move down to the roots. So don’t bother spraying until the leaves have enlarged.
- Use a fine spray to thoroughly coat the leaves in small droplets.
- During the summer, spray in the evening to prevent the spray evaporating and to give maximum time for the spray to be absorbed. In spring or if overnight dew is forecast, spray earlier in the day to allow the spray to dry before dew falls.
Most contact weedkillers are total weedkillers – that is they will damage or kill any plants whose leaves they are sprayed on. Make sure you keep the spray off wanted plants – including lawns – and, if necessary protect plants by covering with polythene or similar when spraying.
Use weedkillers safely. Always read the label and product information before use.
Never allow plants to flower and set seed. Although this is pretty easy in your garden, it’s more difficult to stop the seeds blowing in from a neighbour’s garden, any surrounding fields and waste ground and even further afield.
- Hand fork
- Hand trowel
- Weed-control membrane
- Lawn weedkillers