|How is it determined what plants can grow in a certain area of the country? Is it by weather systems or just general area of the country? Specifically, I'd like to know this with respect to April through September.|
|You have asked a very broad question, because plants respond to many factors including not only weather (such as heat, humidity and cold) but also variables such as soil moisture, soil type and even in some cases daylength. To make things even more interesting, plants seem to adapt to their local growing conditions and self-select over time for those best suited to that particular location. For this reason, plants of the same type but originating in different areas of the country might respond differently to say, a given level of winter cold.
The USDA has published winter hardiness zone maps (and more recently summer heat zone maps) which can be used in conjunction with local average frost dates and degree-day data to determine some approximations of which plants are likely to do well in which areas of the country. (Most commonly grown plants have been rated in accordance with the cold hardiness zones, however work on the heat zone ratings is just beginning.)
Good general gardening guides such as Gardening for Dummies by Michael MacCaskey (ISBN 1-56884-644-4) can also begin to point a gardener in the right direction in terms of selecting plants suitable for their own microclimate and growing conditions in their particular garden at home as well as provide an overview of some of the reasons why plants survive better in different areas.