Answer: Carpinus betulus, orEuropean hornbeam, has bright green foliage which turns a clear yellow in Autumn. Hornbeams prefer long growing seasons with hot summers however their leaves burn if not given some protection from the sun at the height of summer.
I think your trees are coping with some transplant shock and perhaps too little water. The easiest way to assure your trees get enough water to sustain them and promote healthy root and foliage growth is to create watering basins beneath each by mounding up a few inches of soil in a circle about 12" from the trunk. Fill the basin with water, allow to drain, then fill it a second time. This will concentrate the moisture over the root mass and allow it to trickle down, wetting the entire root mass. You should only need to water once each week; twice if weather is really hot. By next spring the roots should be well established and your trees should require only weekly watering.
Best wishes with your hornbeams!
Q&A Library Searching Tips