Cauliflower.

We grew cauliflowers last year (our first proper year of allotmenting).

We duly watered and weeded the brassica patch.

We planted them out with other brassicas and promptly forgot what they were.  A few months later we parted the leaves to find out what we were growing and stumbled across some lovely little cauliflowers.

Not long after that we had the biggest (and tastiest) cauliflowers I had ever seen.

Probably not much help in answer to your question but we didnt actually do anything to them.

But maybe we were just lucky!