Noxious Weeds and Illegal Plants
Some plants have the potential to become "noxious weeds," crowding out native plants species.  In Ohio, purple loosestrife is taking over the habitat of endangered wild plants.  As part of a beautification campaign, the state government planted wildflowers along the highways.  Unfortunately, seed mix containing only native Midwestern wildflowers was too expensive.  The government used a mix containing cheaper non-native species (including purple loosestrife), with disastrous results.  Non-native plants can also spread from gardens and damage the local ecology.
The  individual states publish lists of prohibited "noxious weeds" which should not be planted.    Check with the state wildlife bureau or agricultural extension agency.  Ohio's government currently has 14 plants on its noxious weed list.
Plants on Dr. Preston's list of English native species (some are included in the plant database) are wildflowers and are likely to propagate freely.  Before adding them to the garden, find out if they are considered noxious weeds in your state.  It is unlikely that an indivual species will be a problem, but it is better to do your research than create a biohazard.
Even if they are not prohibited, many wild or near-wild plants will spread aggressively.  If you want to prevent their proliferation, remove seed pods before the seeds are ripe.  If a plant spreads by roots or runners, sink a physical barrier around it or plant it in a container.
Under federal law, it is illegal to grow Papaver somniferum (Opium Poppy) in the US.  The poppyseeds used for cooking are heated to destroy their viability before they are imported. 
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