Mulch is a material spread on top of the soil to control weeds, conserve moisture, moderate soil temperature and to keep soil from compacting after prolonged rain.
Organic mulches are ideal for perennial gardens because they break down over time and improve the soil structure. Materials such as compost, well-rotted manure and shredded leaves are good choices for perennial beds. Pine needles are frequently used in the Southeast because they are so plentiful. Though pine bark is commonly sold as mulch, it is better used around shrubs and trees than in perennial beds.
Spread a 2-inch layer of mulch over weed-free ground in the spring. If you live in an area where it is hot and humid, apply a 3- or 4- inch layer because theses conditions cause organic material to break down quickly. Organic mulches can be layered on the garden every spring. If you use soaker hoses, put the hoses in place then cover them with mulch. Don't mulch around the crown of the plant (where the plant touches the soil) because mulch holds moisture and plants tend to rot if the crown stays wet.
Late summer- and fall-planted perennials should get a 3- or 4-inch layer of mulch after the ground freezes. Once the soil has frozen, you want it to stay frozen because newly planted perennials don't have established root systems to keep them anchored in the ground as the soil freezes and thaws. This is especially common in spring. If you do find a few plants that have been "heaved" out of the ground by alternating freezing and thawing replant and mulch again as soon as you can.
If you matched the right plant to the right place and selected perennials that are winter hardy in your area you shouldn't have to apply winter mulch after they are established. The exception to this is shallow rooted perennials such as iris and coral bells planted in colder climates. Mulch these plants 3 to 4 inches deep after the ground has frozen.