Answer: These trees (Laburnum) are in my experience very finicky to grow. They grow best in cool summer climates with high humidity and mild winters -- they are rated hardy only in zones 5 to 7. They are sensitive to excessive heat, excessive cold, overly dry conditions, overly wet conditions, and overly windy conditions; they would prefer some noontime shade, too. They naturally grow very slowly -- although they should average about a foot a year at the start -- so yours is certainly lagging. It is possible that the transplanting slowed it down somewhat, the raised bed is possibly overly well drained for a tree that likes evenly moist soil (not soggy, but moist), or that for some reason the roots are failing to grow, thus stunting the top. It is also possible that late freak frosts have damaged the growing tips, or any number of other possibilities such as twig blight. According to Michael Dirr's Manual of Woody Landscape Plants, this tree is quite difficult to grow in the Midwest and "probably east" and I know from my own tree (it is surviving but also refusing to grow) it is not easy to grow on the east coast, either.
At this point I would not suggest any pruning or removing the lower branches because that foliage is valuable in providing energy to the plant. (Eventually the plant is somewhat self pruning, and will drop any unneeded lower branches naturally.)
You might try shading it with shade cloth (a fabric netting typically used over greenhouses) on a frame to see if that helps keep it cooler during the summer. Make sure it has a good wind break in the winter time. Water so that the soil stays evenly moist (not soggy but damp) all season up until it freezes, and use several inches of organic mulch over the root zone (do not allow it to touch the trunk.)
Finally, I would strongly suggest you run some soil tests to find out what type of nutrients are plentiful and which might be actually lacking. You would also want to check for any extreme in the pH. Although you are using a foliar feed, trees generally respond well to a granular application to the soil in very early spring along with applications of compost in fall and spring. I would recommend using a good quality compost to try to ensure that as many of the minors are supplied as possible and to also add organic matter on a regular basis. (The mulch will also help provide organic matter since it rots down over time.) This is really important in a raised bed situation. Often, a raised bed leaches quickly and is not very fertile after a time, the organic matter becomes depeleted, and sometimes too the mixed ingredients used to fill the bed can be unbalanced in an unexpected way. Testing is the only way to tell for sure about the soil. Your county extension should be able to help you with the tests and interpreting the results.
There are several botanic and public gardens in your area, perhaps one of them might have a tree and/or could offer some suggestions as well. Best of luck with your tree, I only wish I could be more encouraging.
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