Hay is simply grass;
it contains the stalk and seed heads, and may have legumes such as alfalfa
and clover mixed in. Straw is made up of the stems left over when grain
crops are harvested. Because hay mulches often contain seed heads, you
should be aware that they can introduce weed seeds to the garden. Straw,
on the other hand, contains few or no seed heads, and makes a great
mulch.