Can I plan interplanting in Heirloom?

Yes, and every bed that carries a second crop shows in your plan without being counted twice. The companion crop, basil, radishes or lettuce, is its own planting marked Interplanted in the Game Plan, and it shares the bed of a main crop without taking its space.

On the Crop Map, drop it on the main crop’s bed: it lays over it with a dashed border and stripes and never pushes other plantings aside. Its seeds, tasks and harvests work like any planting’s, and it is left out of the weekly used space of your Game Plan.

See How interplanting works.