Protecting Unplanted Asparagus Over Winter

I started asparagus in a planter box this spring. When can I plant it in my Zone 4 garden?

Consider overwintering it in the planter box. Sink the container into the soil in a sheltered location. This will insulate the roots from cold winter temperatures. For added protection lay evergreen boughs, straw or mash hay over the sunken planter and surrounding soil after the ground freezes. Plant the asparagus in its permanent location next spring starting as early as mid April or as soon as the soil can be worked.