Answer: Mulch serves several purposes. It can help conserve water, suppress weeds, protect plants over the winter and give a “finished” look to the garden. Organic mulches are derived from something that was once living. They will decompose over time, and some types can add nutrients back into the soil as they do. Non-organic, or inorganic, mulches are made of materials that were never alive. They will not break down over time.
Organic mulches include bark nuggets, grass clippings, straw or salt marsh hay, compost and chopped leaves. Of these, bark nuggets decompose slowest. Compost provides the most nutrients as it degrades.
Inorganic mulches include gravel, black plastic and landscape fabric.
As to which is better—organic or inorganic—it depends on how you feel about costs, garden maintenance and aesthetics.
Organic mulches need to be replaced as they decompose, so they represent a recurring cost. However, you may be able to avoid this cost if you use your own materials, such as compost you make yourself or shredded leaves from your own deciduous trees.
Gravel does not break down, so it is a one-time cost, but light-colored gravel can increase heat and dryness around plants—which may be good or bad, depending on the likes and dislikes of your particular plants.
Landscape fabric can be expensive, but it does a good job of blocking weeds when installed properly. It looks better with an organic mulch, such as bark nuggets, laid on top; however, as the bark nuggets break down they will form a layer of soil in which weeds can grow, defeating the purpose of the fabric. Topping the fabric with another inorganic material, such as gravel, avoids this problem.
Avoid black plastic around permanent plantings of trees, shrubs and perennials. It does not let air and water reach the soil. Plastic is better applied around annual food crops, such as strawberries or tomatoes. It should be removed at the end of the growing season and replaced the following year. You’ll have to also install an irrigation system under the plastic, or water the plants carefully by hand to be sure they get adequate moisture.
Learn about all methods of making and using compost in The Complete Compost Gardening Guide.
Get 5 illustrated techniques for improving your soil for $3.99 in Smart Gardening Techniques: Soil.
Easily clean up gravel-mulch areas, gently rake the garden and even collect pine needles or maple seeds with the Wizard rubber rake.