I think that suggesting the use of toxic substances on plants that you have zero personal experience with is incredibly irresponsible.
In this day and age of hermaphrodite 5 legged frogs, and monster jury awards from cancer causing toxins that the defoliant-producing company still insists "safe enough to put on your sandwich"... We really want to examine ALL other options!
Chamberbitter starts flowering and producing seed as a two week old seedling. Any poison used may kill the plant, but you still have a thousand viable seeds on each plant...
It's also important to understand where this aggressive annual came from.
Chamberbitter and mulberry weed (another aggressive annual) are both considered "nursery weeds".
You get them when you purchase container plants.
So... If you ever get rid of the current infestation, you would have to be very careful about bringing in new plants....
Personally, I wonder if you might simply consider covering the entire yard in several inches of topsoil, and start over.
Woodchip mulch is also effective. The trick is... To bury the seeds deeply enough to prevent germination.
Incidentally, I deal with chamberbitter and mulberry weed by hand-pulling and placing in bucket... Never pull without a bucket in your other hand! When I get finished, the bucket of plants goes in the grill, or the burn pile... Never in the compost.
I get these plants when "friends" offer to share their flowers... Any soil from their yard is full of seed from these incredibly aggressive plants.
