|I think I heard somewhere that certain vegetables should not be planted close to each other, e.g., tomatoes, eggplant, cucumbers and other squash. Is this true and why?|
|It sounds like what you are thinking of is vegetable families: tomatoes and eggplants are in one family (Solanum -- the nightshade family), cucumbers and other squash are in another family (these are Cucurbits). Plants related to each other are generally subject to the same insects and diseases, so most gardeners practice crop rotation to avoid planting members of the same family in the same place too often.
You might also be thinking of the possibilities of cross pollination, where seed savers would separate plants to avoid crossing.