Crop rotation, put as simply as possible, is the practice of planting crops in a different place each growing season. There are compelling reasons to do this (adapted from Rodale’s Illustrated Encyclopedia of Organic Gardening, the best $25.00 I ever spent on a gardening book):
Different crops use different nutrients. Growing the same thing two years in a row might not even work if you did not amend the soil, because the first crop may have depleted the soil of one or more nutrients that the second crop can’t survive without.
Some plants, like pumpkins, require little weeding because they shade out most competing plants with their large leaves. This creates a break in the weed cycle that you can use to your advantage by following the weed-suppressing plants with the difficult-to-weed plants like onions or carrots (and vice versa).
Different plants also send their roots down to different levels. Alternating deep-rooted and shallow-rooted crops can help your soil structure.
The biggest reason that crop rotation is used is to avoid buildup of pests and diseases in the soil that prefer one vegetable or vegetable family over another.
These factors can work together to really boost (or ruin) the crop that follows. You can follow almost anything with a root crop, because they tend to be lighter feeders and will use up what the previous crop left behind. This may mean that you can grow two crops in one area in one year (as long as they are both fairly quick-maturing) on one application of fertilizer. On the other hand, alliums like onions and garlic can be set back as much as 60% when planted after a member of the cabbage family (Four-Season Harvest by Eliot Coleman).
The easiest way to group plants for crop rotation is by family. Families aren’t always readily apparent – it seems obvious that carrots and parsnips are in the same family, but you may not have guessed that beets and spinach are kissing cousins. Any gardening guide worth its salt should list which veggies are in which families.
Above, I have the rough draft of my 2011 main garden layout. I have noted on the left what was planted where in 2010 using the family names of the crops. I have a list of what was grown in 2009, but no chart (ARGH!) so I wasn’t able to include that in my considerations.
This year I had more crop rotation criteria than usual. In addition to keeping plant families out of the areas they were in last year and keeping carrots out of heavily-manured areas (because they’ll fork), I also have pigs and new crops to deal with. In 2010, we kept the pigs on the half of the garden that happened to have been cleared first (the end closer to the house) and in 2011 we’ll keep them in the other half. The pumpkins and dry beans were still ripening in the other half of the garden when the piggies arrived last year. In 2011, I’m probably not doing pumpkins, but I did make sure to put the beans in the area that will not have pigs on it in September, when the bean pods are still drying.
I am trying out several crops this year that I have not grown before: parsnips, leeks, beets, cress, and spinach among them. I had originally planned to plant all new crops in the front yard in the (as-yet non-existent) raised beds. With no pumpkins taking up 195 square feet in the main garden, I had some more room to work with, so I moved some new crops into the main garden. I left the cress and spinach in the front yard because they will be successively sown (a light sowing once every few weeks so that I can cut and come again), and therefore need much less than 22 feet of row space. The leeks and parsnips also stayed in the front yard because they will be overwintered – that is, left in place with a mulch of straw and pulled as needed over the winter. We can’t overwinter crops in the main garden because we plan to plant a cover crop and also because if the pigs mature quicker than in 2010 or we have more pigs than in 2010 we may need to expand their pen to take up the whole garden area.<
The general consensus on the time frame for rotation is four years. I’ve seen books that advocate more or less, but most recommend four years. That means that after four years it is OK to plant a veggie from the same family in the same place. In my case, that means that in 2014 I could put an Apiaceae family member in the row closest to the house, where I had carrots in 2010.