Experts and BHG readers answer.
White foam stuff on top of stems
The foamy material that you see on various perennials is from an insect called the spittlebug. As they suck on plant juices they secrete this frothy foam as a protection. Because they are protected inside the foamy, insecticides are of little use in getting rid of them. You can blast them off the plants with a forceful spray of water. They do little harm to the plant, so there's no need to actually kill them.