There are three forms of the tall, dandelionlike noxious weed called the sow thistle: Sonchus arvensis, or perennial sow thistle; Sonchus oleraceus, or annual sow thistle; and Sonchus asper, the spiny annual sow thistle. All three sport spiky serrated leaves in a dense basal rosette in spring which rises up to a bushy mass midsummer, topped by stems 1 to 5 feet tall bearing yellow composite flowers. Sow thistle eradication is a challenge requiring persistence in mechanical and chemical methods.
Determine whether you have perennial or annual sow thistle in your yard. Examine the leaves of the plant: perennial sow thistle leaves are alternate and have toothed margins, while annual sow thistle leaves fully clasp the main stem, with either rounded or flat spiny lobes.
Dig around one of the thistles to determine its root structure: Annual sow thistles have a single primary taproot, but perennial sow thistles spread by horizontal creeping root networks. Bring an exemplar to your local extension service office if you cannot determine whether you have an annual or perennial variety.
Dig out annual thistle with a shovel in the spring before it flowers, as annual sow thistle spreads from seeds. Till the soil regularly to control annual sow thistle wherever possible to further prevent seeds and to discourage the seeds from germinating, as they require light to do so.
Discourage perennial sow thistle expansion by pouring diluted white vinegar around the plant root area; perennial sow thistle thrives in an alkaline environment. Pour a teakettle full of boiling water on the plant each day before it flowers. Dig out each plant after it dies from the boiling water, and pour more diluted vinegar into the hole.
Apply 2,4D or glyphosate-based herbicide at the maximum recommended rate to each plant before it flowers. Dig out the plant after it dies back, and pour another application of maximum strength herbicide into the hole to discourage the roots from resprouting. Repeat the process as necessary until the sow thistle is eradicated.