Answer: Both fertilizers and herbicides degrade in the presence of sunlight and oxygen. By covering the area with plastic you essentially cut off the sunshine and fresh air. The chemicals could not degrade naturally and ended up being concentrated on the soil surface, then slowly being absorbed by the soil. Concentrated chemicals can burn plant roots and, if the concentration is heavy enough, can render the soil sterile for a period of time.
I guess my question is - why would you cover the area with plastic? Plastic in itself would keep weeds from growing; chemical weed control will keep weeds from growing, so a double whammy isn't required. At this point I would water the area and keep it open to sunshine and fresh air. The combination of fresh air, water and sunlight will help both chemicals degrade. In the meantime, the plants growing in the immediate area may not survive. I wouldn't replace them until all the chemicals have leached out of the soil.
Best wishes with your garden!
Q&A Library Searching Tips