I am developing undeveloped land and the weeds are popping up like crazy. I spray them every week and when I go out to collect the weeds that have died, there is always a new batch. I've got the yard under control, but I spend so much time battling weeds that I am not accomplishing much. There is a combination of careless weeds and another weed that grows like a tree. My dad calls it a iron weed, but I don't think that is what it is. I can't mow the weeds because of all of the rocks on the parts of the land that we haven't started to develop yet. I am open to suggestions!
Many weeds thrive on poor soil and will only be gotten rid of by improving the nutrients in the soil. And some are immune to weed sprays. It sounds like you have a spray that only kills the leaves above the ground and does nothing to the roots, if so you will never get rid of them. There is a variety of Vernonia that is called Iron Weed or iron weed may be a local common name for a totally different plant. Not all plants that grow wild are weeds, you might want to get some of them identified so you will know what you have, some you might want to keep. Will the undeveloped land eventually become a garden or lawn? If so it would probably be worth the time and money to start spreading compost on it instead. The compost will build up the soil, killing off some of the true weeds and in the long run make that area healthier when you are ready to develop it.
Some of it will be developed into a yard or gardens, but other parts of the land won't be. It is mostly rocky. We had 12 loads of top soil brought in for the backyard and it is all okay. We will be bringing more dirt for the vegetable garden and maybe the front yard, but the rest of the land we want to leave pretty much natural, maybe adding some mulch and trees, but that will take some time and in the meantime I need to get the weeds under control or the city will go ballistic. I am using Roundup Grass and Weed Killer. Does anyone know of something better that I can use to kill the weeds "almost" permanently? BTW Toni, the weed that we call iron weeds is nothing like the picture that you posted a link to. Honestly, they are like trees. I'll post a picture of one later. When we first moved in, we had a lot of those weeds that were about six feet tall. We sprayed them and then burned them to get rid of them, but they keep popping up and creating quite a chore.
About 5 years ago I was in the same situation. In areas where no lawn or flower beds were to be established I use a combination of vinegar and salt to get rid of the weeds, once they were knocked back we covered the area with black heavy plastic for a full year- this pretty much killed everything under the plastic and the heat from the sun and black plastic baked the weed seeds. In garden areas we only used the black plastic for a full growing season then topped up with compost in some area and triple mix in others. Hope this can give you a few ideas. Good Luck. Cindi