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

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

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

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

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

Hennessey posed on the empty roof in late April.