For example, melons don't do well in the North, as cool August nights and early frosts seriously reduce the chances of a ripe crop. Stick to local plant species. Even if foreign varieties do well, they may end up doing too well. Plants with no natural enemies can thrive to the point where they begin displacing local species, creating all sorts of problems. If plants call for bright sun, don't expect that they'll do well in the shade.
Plants tend to become lanky, with few flowers.
It is very important to protect young seedlings. Shelter them by covering with paper bags, or for a see-through shelter, slice the bottoms off plastic milk jugs and place the jugs over tiny plants. Cut the bottoms into strips and use as markers. Recycle when done. Save old seeds.