Mulch does many things but the most important is it insulates, keeping the temperature more steady and protecting roots from cold snaps and hot spells. It keeps soil moisture from evaporating so you don’t have to water as often. It shades out weed seeds so they don’t get enough light to sprout. If it’s organic mulch, anything that once was a plant, it breaks down to feed microorganisms that in turn nurture plants and make soil a better home for roots. Here are some things to think about when you are shopping for mulch.
Consider the source. Most commercial organic mulches, apart from straw, are wood byproducts from the lumber industry. Bagged mulches from good garden centers or home-improvement stores or bulk mulches from reputable landscape supply companies are likely clean and safe. But cheap bags of mulch, such as those often sold at gas stations, may contain shredded construction debris or other waste with toxic chemicals, metals or lead paint.
Choose mulch for its purpose. Consider the plant when you choose. Medium-textured mulch such as shredded hardwood will work in most places. But in a permanent layer around trees, big chunks, such as pine bark nuggets, will last longer. You wouldn’t want to dig through them in perennial beds, though, so use something finer. In vegetable beds, use something fluffy and easily decomposed, such as straw. An underlying layer of landscape fabric underneath mulch will help deter weeds, if you will never want to dig there. But replenish and tidy the mulch periodically to keep the fabric hidden. Mulch color is strictly a matter of taste; un-dyed dark brown mulch is the most classic look.
Bag or bulk? Mulch comes in bags, usually containing 2 or 3 cubic feet, or by the truckload, measured in cubic yards. (A cubic yard is 27 cubic feet.) Bagged mulch is much more expensive but easier to handle, especially if you have no place for a pile of bulk mulch. If you buy in bags, you can buy different kinds for different purposes. Bulk mulch can be ordered from garden centers or landscape supply companies. Prices vary; when you shop around, tell the seller what you want to use the mulch for and price the material they recommend for that purpose.
Plan ahead. To figure out the area to be mulched, break it down into smaller shapes, such as rectangles or circles, measure them and figure the area of each. Then decide how deep you want it: *1 to 2 inches for perennial beds and *3 to 4 inches over tree roots.
One cubic foot will cover about 4 square feet 3 inches deep; 1 cubic yard will cover about 110 square feet at that depth.
Here’s a useful calculator. Buying too much mulch may tempt you to spread it too deep, which can obstruct water and air to roots. There is no need to remove old mulch; spread a new layer on top so that the old and new together reach the right depth for plants.