Crop rotation on a 5 year cycle...
Many thanks for your recent message regarding a five bed crop rotation.
In my experience, a three year rotation is adequate with four being utilised if you have the space; however, if you can fit all the plants you wish to grow into a 5 bed system, then why not?!
In answer to your first question, onions are traditionally grouped with root crops in a three year system. This would place them in with potatoes, and of course roots, from the crops listed in your message. If sowing or planting onions in spring, it’s best to dig in either manure or well-rotted compost the previous autumn; don’t plant onions into freshly manured ground as they tend to be more fleshy and suffer from disease.
Onto asparagus; these perennial plants can occupy a bed for up to 20 years as you say, and I’ve never included them – or other long term crops such as artichokes and rhubarb – in any crop rotation, simply because the length of time they occupy the bed makes it impractical.
Hopefully this information is of help and answers your questions; however, if anything else springs to mind, please don’t hesitate to contact me and many thanks once again for your enquiry.