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.