By Bill Kleiman, The Nature Conservancy at Nachusa Grasslands
Pastinaca sativa, wild parsnip lives two years, with flower and seed set on year two. Every June you will find us mowing it somewhere on the preserve, as this is when they are in full flower. We also use our weed spades to cut the root which looks like a carrot. Before the flower stalk bolts a broadleaf herbicide is effective.
Parsnip does not compete well in prairie plantings or prairie remnants, except where the vegetation has been disturbed by some past issue, like brush encroachment. Parsnip does well in low competition areas where past disturbance has left a simple plant community. We mowed a bush honeysuckle thicket several years previously and it filled with wild parsnip a few years later. I sprayed a pasture to reduce brush and a few years later there was the parsnip.
There are various weeds we put some effort into in case their small populations might increase if left alone. Weeds like parsnip, king devil, butter and eggs. You may have such a list. Then there are weeds that are everywhere, like the exotic cool season grasses, so we shrug our shoulders and don’t attempt to control them.
Our resources are limited so we manage what we need to, and not more.
Here is a link to some resources on invasive weeds:
https://www.nachusagrasslands.org/managing-invasive-plants.html
grassland restoration network 
Hi Bill,
I have found Clethodim to be a wonderful solution for nonnative upland grasses when applied at maximum rates in the spring, preferably after a burn. Of course, there is a seed bank, so repeated applications will be needed to reduce these species to minor components. I think it is well worth the effort. Removing nonnative grasses creates space for interstitial species, such as forbs, juncus, etc. while allowing for better air movement near the ground, increased soil warming, lower humidity and much greater ease of movement. Pity the turtle or snake moving through a poa choked community, let alone the impacts on grasshoppers, etc.
I have sprayed Clethodim in the spring with spectacular results on Bromus inermis, B. tectorum, B. japonicus, Poa compressa & P. pratensis. For anyone who has not tried this improved grass herbicide, I encourage you to experiment. It is now approved for use in nature preserves.
Thanks Floyd. That all sounds right. We have done what you describe a few times and we felt like it worked to open the ground up to the seed we were planting.
Your question seems to be whether wild parsnip infestations will fade and whether the area will become colonized by prairie species given seeding and time. I think this would likely occur, but not within a time frame that is acceptable to most people. To my recollection, where there has been wild parsnip in a disturbed area, and now quality prairie vegetation has been established, there has always been efforts to control the wild parsnip. Among the invasive species, it is such an easy one to control and so despised that I’ve never seen anyone leave it to see if it disappears without intervention. People really hate its phototoxicity.
To control it, I clip off the seed heads into a lawn bag when they are ripe. At Thelma Carpenter on July 13th, I was able to clip the seed heads off 400 plants during the three hours Dee and I worked together in the afternoon. By clipping seed head, and not cutting the root with a weed spade, I am able to remove seed heads from a little over twice the number of plants. Unfortunately, a lot of the wild parsnip that is mowed is only knock over or sends up auxiliary inflorescences. I also went over the mowed areas to clip off any seeds that had developed. Since the seed heads in the mowed areas are farther apart, this likely reduced the total I removed during the three hours of work. Also, the heat index that day was 105 to 110 degrees F which slow things down considerably.
I should also mention that clipping the seed heads is only what I do later in the season. I clip seed heads after the basal leaves start dying back. Earlier in the season, I have been digging them out by the root. I will have to try cutting the root below the crown, since I think this would work just as well and be a lot easier.
Hi Bill, does burning wild parsnip help? What about burning after a disturbance? Thanks.
I don’t think fire sets back wild parsnip. I guess that a diverse plant community uses up underground resources and that sets back parsnip.