Perennials are mulched both winter and summer. The summer mulch retards water loss and hold down weeds. The winter mulch prevents the alternate freezing and thawing that heaves or kills plants. Use a two inch layer of neutral colored, organic material such as buckwheat hulls, grass clippings, compost, or wood chips for a summer mulch. Peat moss can be used but it repels water if allowed to dry out. Apply the summer mulch before the plants have made much growth and water it into place.
Apply the winter mulch after the first several killing frosts or after the ground is frozen. The winter mulch should be some loose material such as straw. Remove it in spring as soon as growth starts. Do not use fallen leaves unless they are stiff leaves as from oak or beech. Soft flimsy leaves, such as maple leaves, pack together making it difficult for air and water to enter and leave the soil.