We planted several black locusts that were dug up at about 3 feet tall. We did it in the first week of April before the leaves had started emerging and they all took. Some had lousy scrawny looking
roots, so I cut most off at one foot tall, but the ones I didn't cut off all survived as well. Black locust leaves emerge much later than other
trees around here so we have to plant other trees in March but the Robinia is fine to do in April.
(I remember the date because the holes had been dug on the 1st of April by our students. On March 31, we told them we'd received a parcel of seeds of special mountain coconuts that might just grow in our cold-winter climate, but as everybody knows that coconuts seeds can't be touched by sunlight or they'll die, we had a choice to plant them that night or before sunrise the next morning. They chose to dig early the next morning. And for pre-breakfast, ended up having the candy that was in the parcel. The holes didn't go to waste, as we planted black locusts in them a few days later.)