My daughter has thistle growing in her flowerbeds. She has pulled it out by the roots but it continues to come back. Is there a plant killer she can use in her garden without harming the desirable plants?
Howard County Maryland
A targeted application of a systemic herbicide such as glyphosate (e.g., Roundup® or Rodeo®), which carries plant toxins to the roots, can be effective to control Canada thistle. This is a very difficult weed to control. The plant’s root system holds huge energy reserves. Taproots send out side roots as deep as three feet and can regenerate from a one-inch-piece left in the ground. Hand pulling is only possible with the youngest seedlings in moist pliable soil. If you use a systemic herbicide, be sure to apply it only to the weed and avoid any contact with your desired ornamental plants.