Where does Heirloom get the history of what grew in each bed?

From the plantings you placed on your Crop Map in past seasons: Heirloom keeps that record for you, so you don’t have to.

To read it, click Block informations on the right of the Crop Map and pick the block in Select a block. Under Crop families history, each season lists the families that grew in each bed; click Show beds to open a season.

The history is only as good as your past plans: place each season’s plantings on the beds they really went into, even late in the year, and next winter’s rotation starts from what happened in the field.

The steps are in Check your crop rotation.