The hollow stems are part of this plant's growth habit. Could be lack of water. In a drought newly planted shrubs and trees must be kept watered so the soil in the root-ball does not dry out. Temperatures that climb into the nineties further stress the plants. I have several shrubs to plant and have deliberately waited to do the job this fall. If you fertilized -that only added to the stress. Shrubs and trees should not be fertilized when planted.
Forsythia Bushes Keep Dying
Question From: OHIO
Q: My forsythia bushes keep dying. They turn brown one branch at a time and I have replanted twice. When I cut the stems they are hollow. What is causing this? The surrounding bushes of lilac, hydrangea, and butterfly bush haven't been affected.