sow thistle

Q: Can you tell me what these tall weeds with yellow flowers are?

Q:  Last year I had mulch delivered and now there are dozens of these tall weeds with yellow flowers.   My gardener has been pulling them up by the handfuls.  Can you tell me what they are?

A:  I believe the weeds you brought to the office are sow thistles.  They are very tall with yellow flowers.  The perennial sow thistle is less spiny with a larger taproot than its annual cousin.  These are common lawn and landscape weeds found throughout most of the United States and Canada.  After the flower is spend it produces white, fluffy seeds that can easily be carried on the wind to other areas of your yard or even your neighbors’ yard.

Pulling them up now to get rid of the flower before it produces seed is a good strategy.  You might consider having your gardener put out a pre-emergent herbicide (providing he has the appropriate pesticide license) in the fall.  This will help prevent any seeds from germinating during the cooler winter and early spring seasons.