Rotating crops is standard practice, and is relatively easy in the average garden. But crop rotation in a greenhouse or polytunnel is a little trickier, for two reasons.
The first reason is obvious: space, or the lack of it. My greenhouse for instance contains only two small beds, which means a full crop rotation is impossible.
The second factor is the limited range of crops that are usually grown under cover. Most years I grow tomatoes, capsicum and chillies in my greenhouse, which are all related and so cannot follow each other in a crop rotation schedule without the risk of transferring crop family-specific pests and diseases.
Over the years I’ve hit upon several options for coping with limited crop rotation:
Short-term Crop Rotation
A rotation gap of at least three years is the goal, but in practice any length of rotation is better than none at all. Even a single year’s rotation can interrupt the life cycle of pests and diseases, and helps spread out the demand on specific soil nutrients that are particularly liked by some crops but less used by others.