A field production nursery grows trees in open ground over several seasons, then digs them for sale — most often as balled-and-burlapped stock, where a tree spade lifts the root ball and it is wrapped in burlap and wire. This format suits larger deciduous shade and ornamental trees, which are graded by caliper: the trunk diameter measured at a standard height above the soil line.
Digging is safest while trees are dormant in spring or fall, but many species tolerate summer digging when handled with larger root balls, speed, and careful watering — the subject the Akron Tree Farms site addressed on its "Summer Dig" page. Wholesale growers in this trade sell to landscapers, municipalities, rewholesalers, and garden centers rather than to retail customers.