Late summer is the main time for controlling weed seeds in the garden, because nasty weeds like thistle or wormwood and so on develop most of their seeds in August. Obviously, there are weeds that are much quicker than this, so it’s important to weed throughout the growing season, but late summer is prime time.
A noxious weed expert assured me that 95% of Canadian thistle seeds are not viable. However, when a plant can produce many thousands of seeds, there will still be plenty to wreak havoc on a garden.
An English gardener who happens to be a neighbor suggested a new trick when removing the seed heads. Whether it is thistle or an ornamental plant that tends to throw more seeds than you want, it is important to physically remove the seed heads and discard them in the trash not the compost pile (lest you plant them unintentionally with the compost). The gardener’s idea was ingenious….buy a cheap hairspray and spray the seed head beforehand so the seeds are held in place while you cut and bag them. In practice this works best when you don’t have a field full of weeds, but if you’re out controlling weed seeds it may just be the trick. Certainly if you are working on some seeds and you see them disperse when you cut them, this may be your perfect remedy.
So put the seeds into a bag, discard them and then spray the plant with a weed killer such as glyphosate (pulling a thistle just does not get the root…it comes back). If you kill the parent and you get rid of all or most of the seeds, your garden will be safer. In the area where the thistle (or other weed) was removed, you might consider a pre-emergent application for fall and another for spring to reduce whatever seeds did survive being able to germinate. Of course, you would skip the glyphosate if you are simply taking seeds off a desirable ornamental plant. We only wish to kill the weeds and reduce the babies on the ornamentals.
Which chemical to use for control really depends greatly on the location. Different chemicals are labeled for different weeds and applications and it is important to follow label directions. If you have a BIG problem in a field you will likely do best hiring a professional applicator (in Spokane I often use Chris from Rock Creek Spraying). Glyphosate would not be used in a field typically, since it would kill both the weeds and the pasture grass. That won’t do.
Good luck controlling weed seeds! It’s our fate as gardeners.