Hay is dried grass, usually from grasses such as timothy, clover or alpha. Because hay contains the seeds from these grasses, it can possibly germinate in your garden. Hay makes a great organic mulch for both flower and vegetable gardens because it can help retain water, maintain soil temperatures and add essential nutrients to the soil as it decomposes.
Spread hay around the entire garden in early spring. If you are only using hay to mulch around specific plants, then be sure to expand out to the widest point of the plant's branches or foliage, which is called the drip line.
Use enough hay so that it is initially 4 inches deep. It will settle to be about 2 inches deep, which is sufficient for water retention and to prevent most unwanted hay seeds from growing.
Replenish the hay as necessary. The hay will decompose over time. Again, when you add hay, add enough so it is 4 inches deep and expect it to settle to just a couple inches.