That was a trick question. You can use compost as a mulch all by itself. But you shouldn't. Finished compost or "humus" is a cornerstone of organic gardening. In the photo above you can see Tricia spreading it in a raised bed. Compost is decomposed organic matter full of microorganisms that will populate the soil and expand the soil food web in your garden. The compost will also improve your soil quality and structure—in clay soil it increases drainage, and in sandy soil it helps retain water. Compost feeds your soil and mulch protects your soil. You can use compost alone as a mulch, but it will sink into the soil pretty rapidly. What we think of as a "best practice" is to spread a one inch layer of compost and then cover it with a two inch layer of natural mulch like straw (not hay that has seeds), paper, bark, wood chips, dry grass clippings -- or with a layer of plastic mulch. Some gardeners worry that those layers of compost and natural mulch will keep water from reaching the roots of their plants. Not true. Even with our preferred drip irrigation, the water will trickle in and, importantly, will evaporate much more slowly because of the mulch layer.
When to use plastic mulch
Be sure to put the plastic mulch on top of the irrigation (or it will block water flow to the soil). Plastic mulch is -- plastic -- and therefore an effective weed barrier. That's a good thing in the vegetable garden. From the strawberry plants that are hard to weed between, to the big vegetable leaves that conceal weeds, you can see how it would be handy. Plus, certain plants like certain colors of plastic mulch. Really.
How to choose from the rainbow of colors in plastic mulch
Who's rockin' the red plastic mulch? The strawberries and the tomatoes, that's who. Eggplants too. Tomatoes and eggplants do 12% better with red mulch. If you don't want to go the whole red plastic film route with your tomatoes you can take a shortcut with a red plastic Tomato Crater for each plant. Silver mulch film reflects well on certain vegetablesAphids don't like silver mulch film. Which means the rest of us do like it. Use silver mulch film with your peppers and, according to Pennsylvania State University Extension, you can expect a 20% increase in size and yield. Black plastic mulch to heat things up Potatoes respond to all mulch colors, but they produce at their highest quality with black plastic mulch. Are you growing onions? They're broad-minded and respond to all of these colored plastic mulches.
Two big no-nos in mulching
1. For our first item on the DON'T list, let's step outside the vegetable garden and get a vivid example of "volcano" mulching around a tree. See that mound of mulch? Not unlike a volcano? It's a bad idea, and for some reason has become popular around the U.S. One of the worst of the many bad consequences of volcano mulching is excess moisture around the tree trunk, which can lead to fungal canker diseases. Friends don't let friends volcano mulch a tree. Keep mulch at least six inches away from the tree trunk, and don't pile it up deeper than two inches. 2. Now we're back in the strawberry bed and it's easy to remember, in this smaller venue, not to volcano mulch the strawberries or vegetables either. Keep all mulch one to two inches away from the stems of vegetables and soft fruits. Those are your pointers on mulching. Now get out there and mulch! You'll save yourself from endless weeding and watering.