Hi David,

There may not be an easy solution to controlling the ragweed problem in your 1/2 acre wildflower plot but here are some suggestions.

I would not disturb the soil. That will just increase the likelihood of bringing more ragweed seeds to the surface and enhancing your ragweed problem.

Since ragweeds are annuals (do you know which ragweed species you are dealing with?) the key is to prevent seed production again this summer with the aim of depleting the existing ragweed seed bank. It may take another couple years of diligent ragweed treatment to accomplish this. 

Hand pulling or spot treating ragweed with herbicides such as  2,4-D, Chlorsulfuron, Clopyralid, Fluroxypyr, MCPA, Quinclorac, Sulfosulfuron, Triclopyr, Trifloxysulfuron is recommended by Penn State University Extension.  https://plantscience.psu.edu/outreach/plant-id/broadleaf/ragweed

Here is also a link to a very thorough publication dealing with integrating herbicides in management of prairies and grasslands.

Starting over is of course another option.  The herbicide isoxaben has shown to be an effective pre-emergent of ragweeds (Penn State Extension) and using a pre-emergent along with other techniques such as solarization  ("Prairie Up" by Benjamin Vogt).

Thanks for your response. To answer your question, we have been plagued with common ragweed.  

We are planning to do spot treatments with 2,4 D. However, in areas, where the infestation is widespread, I'm debating whether or not to use a pre-emergent like isoxaben.  I would like to try and save as many of the native wildflowers that have already been established, but don't know if either option makes this more likely.  Thoughts?  

Assuming that both the 2,4 D and isoxaben will kill all native wildflowers.  Is that correct?  Maybe first try the 2,4 D selectively and see which wildflowers persist, only using isoxaben as a last resort to start over? 

Finally, how long do I need to wait before reseeding after an herbicide application? 3 months? 6 months? Thinking of 2 issues---wildflower seed success and avoiding bioaccumulation as much as possible.  Appreciate the help, thx.

Hopefully, persistent spot spraying with the 2,4-D throughout the growing season will be effective and the use of isobaxen will be a last resort measure.

Because herbicide formulations vary, it is best to read the label on the isobaxen  formulation (usually sold as Gallery) your are going to use before planting your wildflower seed. 

Isobaxen does not breakdown quickly in the soil. According to NIH fact sheet: Isoxaben has reported half-lives in soil ranging from 2.07-6.6 months. Soils in which fertilizer such as green manure, cow manure, and pig slurry were added had the longest half-lives of 4.03, 4.75, and 6.6 months respectively. https://pubchem.ncbi.nlm.nih.gov/compound/Isoxaben