Over 80% of invasive terrestrial plants started out as garden flowers, shrubs, and trees. Be a responsible gardener and purchase plants that are native, or known to be safe and not spread. Encourage fellow gardeners, garden clubs, and your local nursery to use only non-invasives.
Many of today's invasive plants were brought to the United States from Europe or Asia, propagated and sold through nurseries. Purple loosestrife got its start this way. Unfortunately, there are no regulations that require a plant to be ‘tested’ for its invasive tendencies. Nurseries may not know that a plant is invasive, or they may continue to sell a known invasive plant until a law makes it illegal to sell. Vermont has a quarantine rule that currently lists over a dozen species. More will be added in the future. It takes time, however, for the Rule to accurately reflect what is happening on the landscape. Japanese barberry and Norway maple, for example, are quite a problem in much of the state, but it will be a few more years before they are illegal to sell.