Good Evening Carol, 

Thank you for contacting the U of M Extension Service. 

Yes, leaves can be used as mulch around your trees and garden. Please click on this link from the U of M Extension Yard and Garden website: https://extension.umn.edu/managing-soil-and-nutrients/composting-home-gardens

This information states: 

Particle size also affects efficiency. The smaller the plant pieces, the more rapidly they will break down. Use a shredderor power mower to chop up leaves and small twigs before adding them to the pile.

Although the article discusses composting, you can use your leaves and grass clippings directly as mulch around your trees, shrubs and garden. It might be a good idea to shred the leavese with a mower first so they are not as apt to fly away in the wind. Mulch helps to protect the tree roots, retain moisture, and keep the soil cool in the summer. Keep the mulch about 2 inches away from the trunk of your trees or stems of your plants. 

Good Luck!