David L. Ryan/Globe Staff

Courtney Hennessey, co-founder of the Higher Ground Farm atop the Boston Design Center, tended to tomato plants.

David L. Ryan/Globe Staff

Hennessey and co-founder John Stoddard (pictured) are using 1,400 milk crates as planters.

David L. Ryan/Globe Staff

The urban farmers are sprouting tomatoes, basil, and arugula.

Essdras M. Suarez/Globe Staff

In April, before 25 tons of dirt were hauled in, the roof stood empty.

Essdras M. Suarez/Globe Staff

A view of Boston from Higher Ground Farm in April. At full build-out, the farm will span 55,000 square feet.

Essdras M. Suarez/Globe Staff

Hennessey posed on the empty roof in late April.