Controlling thistles

by Wendy
(Ottawa, Ontario)

There is a wall at the back of my property on the other side of which is a ditch and open land. I have recently planted shrubs and cedars in front of the wall but I have a problem with thistles which are spreading from under the wall and invading the bed and growing in the middle of the shrubs (weigela, ninebark). I have been pulling them out but they keep sprouting. Last summer I was removing 50-70 a week. I started using Roundup reluctantly in the areas away from the shrubs, but obviously can't do this around the shrubs. Any suggestions for getting rid of this noxious weed which won't damage my trees and bushes.
Many thanks,
Wendy, Ottawa

Doug says they propagate from seed so once you get most of the seedlings, you're going to have them under control.

Mulch works well as does a herbicidal soap spray to whack thistles. This burns the thistle and yes, it will burn the shrub if you hit it. Use a spray shield over your sprayer and it should just direct the spray to the thistle and not the shrub.

Mulch is also a good way to stop seedlings from germinating.

Check the label on corn gluten to see if it is registered for controlling emerging weed seeds such as this in ornamentals. I don't know what the current labelling is on this product but it works on a wide range of seeds so do check...