If no plants are growing in an area a year after spraying an herbicide, what is most likely true about the herbicide?

The most plausible reason that no plants are growing in an area a year after spraying an herbicide is that the herbicide had residual, persistent effects. This means that the active ingredients in the herbicide remained in the soil or on the plant surfaces long enough to continue affecting plant growth for an extended period after application.

Herbicides with residual activity can inhibit seed germination and prevent seedlings from emerging, which would explain the lack of plant life a year later. Such residues can bind to soil particles, persist in the environment, and still be effective at suppressing plant growth during that time frame.

In contrast, a contact herbicide works only on the surfaces it touches and does not typically leave lasting effects, which is why plants might still regrow if they are in the vicinity. A postemergent herbicide targets actively growing plants; if it were solely postemergent without residual effects, one would expect some plants to establish afterward. Lastly, a selectively systemic herbicide affects particular plant types but does not imply lingering effects that would prevent any regrowth at all.