We planted three dwarf butterfly bushes this spring and they were doing very well until about five days ago when one began to wilt. It is now completely dead. This is not from lack of water or overwatering, we are sure. What could have caused this? Are the others in danger?
Butterfly bushes are usually tough as nails. They can get a root rot (phytophthora), but its not common. If your bush just died, you may be able to see something if you pull it up and examine the roots. Look for white/pale roots (healthy) and dark or mushy roots (diseased), especially if the outside sheath of the root easily slips off the inner root. This disease is associated with saturated soils, especially heavier, clay-based soils experiencing prolonged, heavy rain events.
The roots were not wet and rotted and the ground was damp but not soggy wet. Is there anything else we should look for? One of the other bushes has a few yellow leaves on the lower branches.
Sounds like you are not dealing with a root rot.
Container plants grown in the nursery for several years can become tightly pot bound and when planted in the ground in this condition the root system may not establish itself. Container plants establish faster if you disturb the “around the pot” growth direction of the roots.
If the plants were root bound and the roots were not disturbed before planting, then water may not be able to penetrate the root ball. You may need a hose to water deeply to penetrate the root ball. Probe with a screwdriver and check. You can check soil moisture of newly planted trees and shrubs at least once a week. Soil that is moist or damp to the touch is fine. If the soil begins to dry out, water the plant thoroughly. Do not overwater; however, you can easily drown newly planted trees and shrubs through too much tender loving care with the hose.
See our website for planting techniques http://extension.umd.edu/hgic/trees-and-shrubs/concept-planting-maintenance