Answer: You can alternate them in any order you wish each year, but it is easier to keep plants with similar water needs near each other when it comes time to haul out the hose. Keeping them in sections can also help with planning your crop rotation from year to year. On the other hand, separating them can help slow the spread of an insect or disease problem that might affect related plants.
You might also be thinking of rotation. Rotation means not growing a plant (or its relatives) in the same spot for at least three years. This is done to reduce the chances for insects and disease to build up. When you rotate crops, all you really need to do is not plant things in the same place. For example, you can grow your tomatoes in a place that held carrots or lettuce last year. If you plant the same veggies year after year, just try to put root crops in a place where you had leafy crops, leafy crops where you had above-ground crops, and above-ground crops where you had root crops (the exception here is not to follow potatoes with tomatoes, peppers or eggplant and viceversa). That way insects and diseases that might over-winter in the soil will not be able to infect your new plants in the spring.
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