do the old 'walk around the neighbourhood looking over fences routine' that can be a good way, if it grows for someone else there is a good chance it will grow for you. or just stick to indemic natives your local environment office should be able to help you with that one.

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I have the best luck with stuff purchased at the local farmer's market. It's grown locally, so I know it works in my zone, and the people are usually full of advice as to where the plant will do best. Usually some great deals can be made; the more you buy, the more the price per plant goes down.