Crop Rotation 1: Why bother?

This particular post is going to get quite detailed. I am not claiming to be an expert on the subject of crop rotation, but it is very important and I want to summarize here what I have learnt.

There are many good reasons for not growing vegetables from the same family on the same piece of ground year after year. 

a) Avoiding Pests   The first is to do with pests.  If a particular grub likes to eat your sweetcorn, say, and overwinters in the soil, then it makes sense for you to have moved your sweetcorn somewhere else when the next generation of the grub hatches out in the spring.  The grub finds that its sweetcorn cafeteria has moved away, and has to look somewhere else to find lunch.

It’s a moot point whether moving crops around a plot as small as an allotment is going to be that effective. Many pests (such as the dreaded carrot fly) can scent their desired meal from great distances, and will just follow their noses and fly to your new bed.  But moving the crop will at least make the pest go look for their favourite meal, rather than having breakfast in bed when they wake up.

One example: failure to rotate your potato crops can lead to damage by a microscopic pest called potato eelworm. So they say. As far as I know I have never had eelworm in my potatoes and wouldn’t recognize it if I did. By moving my potatoes to a fresh bed every year I hope to never have to find out.

b)   Avoiding Diseases    Similarly with plant diseases. If crops are moved round each year, there is less likelihood of a build-up of a particular disease in the soil.  Going back to potatoes again, the Irish potato famines of the 19^th century were the result of growing potatoes on the same land year after year. Blight built up in the soil, and the results were devastating.

Another example is with brassicas, which are susceptible to an unpleasant fungal disease called club root. One of my neighbouring allotmenteers used to grow his brassicas on the same bit of his land year after year, presumably because that is where his anti-pigeon cage was situated. Now he has club root.

c)   Use of Nutrients in the Soil     Another reason to rotate crops is to do with plant nutrients. It is good policy to follow one crop with another that takes different nutrients from the soil. At the simplest level, this would mean following an above ground vegetable, such as cabbage, with a below ground vegetable, such as parsnip.

Annuals 
It makes sense, then, to organize your allotment so that you follow one annual crop with another from a different family.  The various families are:

1.     Legumes:  peas, broad beans, runner beans, french beans

2.     Brassicas: cabbages, brussel sprouts, cauliflowers, calabrese, sprouting broccoli, kale, swedes, turnips, kohl-rabi, radishes, cress, rocket, mizuna

3.     Potato Family: potatoes, tomatoes, chilli peppers, sweet peppers, aubergines

4.     Carrot Family: carrots, parsnips, celery, celeriac, Florence fennel,

5.     Alliums: onions, leeks, shallots, garlic

6.     Beet Family: beetroot, spinach, swiss chard, spinach beet,
      Gourds: cucumbers, courgettes, marrows, pumpkins, squashes, melons,

8.     Salads: lettuce, chicory, endive,

9.     Other: sweetcorn

Perennials
Some vegetables however, have to stay in the same place, either permanently, or at least for several years, for practical reasons. These we can consider perennials:

The important perennials are: rhubarb, asparagus, globe artichokes, Jerusalem artichokes, seakale.

I grow my annuals at The Allotment, and my perennials, together with the fruit, at The Orchard.

Exceptions to the Rule  
Rotation is most critical for brassicas and potatoes, less so for some other vegetables. Here are some examples of liberties that can be taken:

Brassicas  I don’t worry about growing fast-growing brassicas such as radish, rocket and mizuna in the brassica beds. I reckon that they are in the soil for such a short period of time, that there is little chance of damage.

Lettuce   Similarly, lettuce , and other salad veg, are fast growing, and can be just fitted it in wherever there is space.

Tomatoes  Many gardeners group their tomatoes, aubergines etc with other fruiting crops, such as sweetcorn and courgettes, rather than with potatoes, which is of course a root crop.  For me this isn’t an issue; the British climate is rarely dependable enough to get a good crop of tomatoes outside anyway. I did try to grow tomatoes outside twice. In both cases I lost the crop because of blight. So I grow them in the greenhouse instead. (Rotation inside a greenhouse is more difficult. Best is to rotate the soil, rather than the crops. I will talk more about that another time.)

Runner Beans  Many allotmenteers have a permanent framework for growing their runner beans, and grow them in the same site year after year. They seem to be able to do this without ill-effect. It makes sense to refresh the soil each year if you are going to do this, however.