The principle of crop rotation is simple: don’t grow the same or a similar crop on the same piece of ground in consecutive years and, if possible, don’t grow the same crop on the same piece of ground for another three or four years.
The practice is simple too: draw up a plan and stick to it.
Of course, you can add something new into the scheme at any time, as long as it rotates in the right family group along with all the cousins.
Why rotate crops?
• To reduce disease and pest problems. Vegetables that belong to the same family (see the lists below) tend to suffer from the same pests and diseases. Crop rotation won’t solve all problems, especially in the small space of a polytunnel, but it is a good starting point in the battle. Obviously pests can hop from one part of the polytunnel to another, and soil-borne diseases can be trampled around on the bottom of boots, but crops that are rotated still fare better than ones that are grown on the same soil every year.
• To provide the appropriate food for each crop. Some crops fare best in soil that was manured for the previous crop. Some, such as sweetcorn, are heavy feeders; others such as peas and beans don’t need much nitrogen, since they make their own and leave it in the soil for the next crop to use. Carrots tend to fork if there is too much manure in the bed and brassicas do best on a firm soil not too rich in nitrogen. Each year’s planting and feeding should improve the soil and leave it in good heart for what is to grow there next.
• To ensure acidity requirements are met. Brassicas like an alkaline soil, but tomatoes like an acid one. Hence it makes sense to lime the brassica patch, having planned that the tomatoes won’t grow there for another three years, by which time a lot of the lime will have washed away.
• To get a balance between deep-rooted and shallow-rooted crops. Deep-rooting crops such as sweetcorn draw minerals up from lower layers of the soil. Lettuce feeds happily nearer the surface. Alternating deep and shallow feeders will ensure deep cultivation and improve the structure of the soil.
• To maximize weed control. Some crops, such as courgettes and pumpkins, are good weed suppressors and will effectively reduce the number of weeds in following years.
How many years in a rotation?
A four-year rotation is ideal – simply view the polytunnel as four quarters and move everything around clockwise, into the neighbouring quarter, each year. If the tunnel is divided into three long beds, a three-year rotation might make sense. In a small polytunnel it might only be possible to do a two-year rotation, unless you introduce containers and growbags to effectively include a third area into the scheme. Basically, anything is better than same crop, same soil, for year upon year.
Family ties
All vegetables fall into a family grouping and it is best to grow all of one family in the same bed and to rotate these crops together.
• Cabbage (brassica) family: Cabbage, broccoli, kohl rabi, cauliflower, Brussels sprouts, turnips, kale, swede, radish, mizuna, rocket, mustard greens, etc.
• Onion (allium) family: Onion, leek, garlic, shallot, chive
• Carrot (umbellifer) family: Carrot, parsley, Florence fennel
• Cucumber (cucurbit) family: Cucumber, marrow, courgette, pumpkin, squash, melon, gourd
• Pea and bean (legume) family: Pea, broad bean, French bean, runner bean, soya bean
• Potato (solanum) family: Potato, tomato, pepper, aubergine
• Goosefoot (Chenopodiaceae) family: Swiss chard, spinach, spinach beet, beetroot (these often get mistaken for brassicas)
• Asteraceae family: Lettuce, endive
• Corn (Gramineae) family: Sweetcorn
A few tips
• Some crops, such as lettuce, spinach and sweetcorn, don’t belong to the major families and can be fitted in almost anywhere where space allows.
• Keep planting on a year-round basis to fill gaps, but try to make sure the new planting is part of the planned rotation scheme.
• Remember, nothing is written in stone. It is often better to fill a gap than to leave it empty, and it is also better to tweak the plan as the years go by. The aim is to use the polytunnel to its full potential.
• Pots, growbags and containers can be moved around as desired, particularly if these don’t allow plants to root down into the border soil.
A sample plan
This plan will rotate one-quarter clockwise in the following year.