We think these are the protective masses produced by an insect called spittlebugs. Spittlebugs feed on various types of herbaceous plants, trees, and shrubs, and they are active in the landscape right now. The immature nymphs of these insects are covered in a foamy mass as a form of protection.

Most species of spittlebugs have one generation per year and treatment is not usually necessary.

Here is additional information about them:
http://extension.umd.edu/hgic/spittlebugs-trees-and-shrubs

CKC