Scotch Laburnum (Laburnum alpinum) is a cold hardy species of Golden Chain Tree, and it's renowned for its long yellow flowers and super dense wood. It also is a nitrogen fixing tree.
These are 2nd year bare root seedlings, 6 to 10 inches tall.
Photo Credit: Etienne, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons