Winter is a great time for garden planning. I love looking through catalogs and books and browsing online, mulling over various planting possibilities. I can while away an entire snowy afternoon drawing and redrawing different schemes, some based in fantasy (what I'd do with an unlimited budget), others more grounded in reality.
One of the things I always keep in mind when sketching out ideas for the upcoming season's vegetable garden is where in my plot the crops I grew last year were located so I can be sure to put them in a new spot in the coming year. Even if your garden is small, it's a good idea to rotate the placement of your vegetables within your garden plot for several reasons.
Some pests and diseases can carry over in the soil; switching the spots where particular crops are grown can help foil their attacks. For example, all members of the cabbage family are susceptible to many of the same fungal diseases. The spores that spread these diseases can survive in the soil, often for several years. So making sure to plant your broccoli, cabbage, kale and other cole crops in a new spot for three years reduces the likelihood that your plants will become infected. The spores that cause the fungal disease early blight on tomatoes can survive in the soil for at least a year. Setting out tomatoes in a new location will help you grow a healthier crop.
Different plants take different amounts of nutrients from the soil. Leafy greens like spinach need lots of nitrogen, while legumes like peas and beans actually add nitrogen. Members of the squash clan, including cucumbers, melons and pumpkins, have a higher phosphorus requirement than tomatoes, eggplants and peppers.
Rotating crops can also help to keep soil in good tilth. If shallow-rooted crops like cabbage are followed by deeper rooted crops like tomatoes, soil compaction can be lessened. The repeated hilling of the soil needed for corn and potatoes helps to reduce the number of annual weeds in that section of the garden.
So how do you put all this together into a plan? First of all, it may not be possible to have an "ideal" rotation plan in a small backyard garden -- just do the best you can. If you can only site your tomatoes in a new location for two years rather than three, that's better than setting them in the same spot year after year.
The first step is to group the plants you plan to grow into families with similar pest problems. Here are some common family relations to help you plan your moves.
Carrot family: carrots, celery, chervil, cilantro, dill, fennel, parsley, parsnip
Goosefoot family: beets, chard, spinach
Gourd family: cucumbers, melons, squash, pumpkins
Grass family: corn
Mint family: basil, mint
Mustard family: broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, kohlrabi, radishes, rutabagas, turnips
Onion family: garlic, leek, onion, shallot
Nightshade family: eggplant, peppers, potatoes, tomatoes
Pea family: beans, pea
Sunflower family: artichoke, cardoon, chicory, endive, lettuce
Then try to come up with a plan that will move these groups through the garden according to their nutrient demands. One plan might be to start with legumes like peas and beans to build up the soil. Follow this the next year with leafy greens and cole crops to take advantage of the increased nitrogen in the soil. Next come fruiting crops like the squash and tomato families that will thrive with a little less nitrogen. The fourth season, finish up with root crops and onion family members with their relatively low fertility needs. Then it's back again to peas and beans.
So get out the graph paper and have some fun sketching out possible plans now. Then, when spring finally comes, you'll be ready to move your best plan from paper to garden plot.