I get asked this question on nearly a daily basis. And frankly, there is not one easy answer to give. There are just so many variables involved.
You need to realize that nearly all gardening models and methods are an unnatural act! They are an attempt by us humans to control and modify the very design of the natural world. That is, we are trying to lord over nature. Don't believe me? Stop doing any gardening or yard work and see what its natural state reverts to in one year, five years, or longer.
I suppose that the logical starting place for answering this question is to determine the specific germination and cultural requirements for the plant variety you are interested in growing. Learning where various plants originated from is a useful bit of knowledge for a gardener to possess. For example, if you know that watermelons originated in Africa, you can make a very good guess that they require heat to thrive. More information on vegetable origins can be found by clicking here.