I have these prickles everywhere in my yard. By the time I pull them up in one spot, they have grown again by the time I've started another. Is there a way that I can kill them and stop they from growing again? My dogs are constantly in the backyard. Is there a way that won't affect them?
3 Corner Jacks
Certified GKH Gardening Expert
Those are some nasty weeds. And tough...
It appears that there has been some success using dicamba based weed killer. Also a weed killer called Broadstrike is suppose to be effective.
Most sources I found said to treat the soil with a pre-emergent (like Preen) in the early spring, which will help keep it from sprouting in the first place and then using the weed killers on it shortly after it emerged. Weed killer has to be applied as soon as possible as the older it gets, the less effective the weed killer is on it.
Unfortunately, this is a weed that has been a problem for Australian and US agriculture for several decades and they are still working on how best to deal with it.
If you are looking for a non chemical way to get rid of three Cornered Jacks, you may be interested in our experience. On two farms where we have lived there were large patches of these weeds when we first moved in. We got rid of them both times by first raking the area and collecting and burning the jacks, before disturbing the soil by pulling up (and burning) the plants. After that we went out for a while each day with a bucket, and walked backwards and forwards across the area while wearing rubber thongs (flipflops). Without leaving the area we'd take off the thongs and pull out and bucket all the jacks that had stuck into the bottoms, then repeat the process a few more times until we tired of the game - my children thought it was fun helping, except if a jack happened to prick their feet through the bottom of their thongs. With hindsight, if I needed to do this again we'd take an extra pair of thongs each and keep swapping them over, instead of balancing on one foot while de-jacking the other thong! Every day there were less jacks until finally I'd just check every few weeks to see if any more had worked their way to the surface of the soil, or if any new plants had appeared after rain. We soon got rid of a big problem this way and never had any further jacks, without using any poisonous chemicals on our soil.