Beginning gardeners are told to rotate vegetable crops each year for insect and disease prevention. That tells them the what, when, where, and why but sometimes leaves them fuzzy about which crop should follow another and which crop shouldn't.
Rotation can be complicated. I know one farmer on a 27 year rotation. I won't plan that far ahead for vegetables.
An average home gardener needs to learn four major families. They are the mustard family, the bean family, the gourd family and the nightshade family. All a gardener has to do is move the crops so that members of the same family don't get planted in the same area year after year.
There are two ways to learn which vegetables belong to which family. You can keep the list below or you can learn how the flower looks.
Learning how to look at a flower is not simple but just counting the petals is a start. The only common garden plants with four petals are in the mustard family which are better know as the brassicas. This includes a bunch of different types of mustard, cress, cressy, kale, kohlrabi, cabbage, broccoli, cauliflower, turnips, radish (including diakon), spinach, and rutabaga. Some uncommon members are Pak-choi, wallrocket, rocket salad, maca, and wasabi.
The lily family is not a major concern when planning a rotation. I will separate them here because they are the only major vegetable family with 6 flower petals. Asparagus is a member of the lily family but it is a permanent plant that isn't moved every year. Daylilies are seldom grown for food but they wouldn't get moved either. Some people put onions in the lily family. Other people separate them into their own family. Even if we didn't know what the flower looked like, we can still recognize onions by smell or leaf shape. Chives, shallots, onions, and garlic are all in this family. I don't personally know kurrat and rakkyo but they are in this family also. In my garden I don't worry about rotating this family. The major problem we have on these crops is thrips which are not controlled by rotation. In the wild you see garlic living several years in the same spot. I have garlic and chives in fairly permanent locations. I don't anticipate problems on either of these.
Most of the other common garden vegetables have five petals. Still the bean family, also know as the legume family, is easy to recognize. I can't describe a bean flower in a news column but will mention one aspect. Most garden vegetables have a flower that can produce a mirror image if cut several different ways. Beans flowers are not like this. Bean flowers have to be cut just right to produce a mirror image. I don't know a simple word for this. Zygomorphic is the correct term. Bi-lateral symmetry is longer but may be easier to understand. The only other vegetables I know with bilateral symmetry are in the violet family or the mint family. None of these are common in our local vegetable gardens.
If you are not comfortable recognizing the bean flower, you can go by name. This is a huge family, but most members are called either beans or peas. Peanuts are also in this family. Groundnuts, jimica and fenugreek are beans.
There is enough difference in insect and disease pressure between the peanuts, the Southern peas and the other beans that a gardener could be divide them into three separate groups. If the gardener separated the beans, they could plant either peas or beans as a quick summer cover crop without messing up the rotation. The southern pea group would contain moth bean, adzuki bean , black gram, mung bean, Madasgascar groundnut, rice bean, catjang, and yardlong bean or asparagus bean. Peanuts would be a separate group. Everything else in the bean family would be lumped together in another group.
In part 2 I will cover the final two major vegetable groups.
In my vegetable garden I have a minimum three year rotation. That isn't quite enough for melons and cantaloupes but more than enough for some crops. With a good fertility program a person could grow sweet corn several years in the same spot, but since the other crops must move, the sweet corn gets pushed around too.
Commercial farmers also consider the fertility program when planning a rotation. Some rotation programs try to put a heavy feeder behind a legume crop to take advantage of residual nitrogen the beans fix out of the air. If you were going to manure every third year you would place sweet potatoes the third year. This puts it as far as possible time wise away from the manure application. Irish potatoes should be the third year after liming if possible. Varying early and late crops will also help break weed populations.
Basically, I wouldn't worry about those criteria. I simply group plants into 4 major families and make sure members of the same family don't follow year after year.
Last week I mentioned the brassicas with 4 petal flowers, the lilies with 6 petal flowers and the beans with zygomorphic (a mirror image in one plane) flowers. That leaves the cucurbit family also known as the gourd family and the nightshade family. At first glance, there is surprisingly little difference between the flowers of these two groups. Both have five petals. Other parts of the flower are similar. The major difference is that nightshade plants always have the ovary above the petals while ovaries in the gourd family are always below the petals. Again if you fill uncomfortable remembering this just keep a list.
Solanaceae is commonly called the nightshade family. Peppers, Potatoes, Tomatoes, and eggplants are common members of this family. Tamarillo, tomatillo, rocoto, box thorn, golden apple (solanum aethiopicum), American black nightshade, Chinese lantern, cape gooseberry, gilo, and sweet pepino are uncommon members of this family. Tobacco and petunias are in this same family but we generally don't grow them in the vegetable garden. Some personal experience indicates hot peppers may not need as much rotation but if everything else it moving, it is simple enough to move the hot peppers also.
The cucurbitaceae family contains watermelons, cantaloupes, citron, gourds, gherkins, cucumbers, muskmelon, pumpkin, butternut squash, squash, pepino, loofah or luffa, chayote, casabanana, and oyster nut. The watermelons are least tolerant of short rotations.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.