"Hay" can make great winter mulch—but you must know just what you're getting. Here's the lowdown on straw, hay and salt hay.
Hay is an assortment of forage grasses. It usually includes seeds of grasses and broadleaf weeds, so it is better suited for feeding livestock than for mulching plants.
Straw is a collection of the stems of field crops such as wheat and oats. It is popular as a mulch because it rarely contains weed seeds and is readily available. It makes a particularly good winter mulch because the hollow stems hold air and act as insulation for the plants below.
Salt hay, or salt marsh hay, consists of grasses harvested from salt marshes. Their wiry stems do not mat down or rot as quickly as straw, and any seeds that are present will not germinate because they require wet, saline soil. Where it is available, salt hay is the best choice for mulching.