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Good mulching boosts soil and adds beauty to your garden

  • Dot Zanotti Ingels
  • An ongoing mantra of the UC Marin Master Gardeners is to compost, compost, compost and mulch, mulch, mulch. Compost is decayed material used as a plant fertilizer and soil conditioner. Soil is the most important part of a garden so we always encourage that an abundance of compost be used.

    Mulch is any material that a gardener places on the soil to protect it. Good mulching conserves soil moisture by reducing evaporation, modifies the temperature of the soil and reduces weed growth by limiting the light weeds need to get established. It helps keep water where you want it by decreasing erosion. It keeps plant parts from touching the soil which can help reduce diseases and plant decay. Mulch can also enhance the visual appeal of your garden.

    There are so many choices when it comes to mulching materials. Deciding what to use as mulch depends on many factors including availability, cost, the effect it will have on the soil, durability, combustibility and rate of decomposition. Generally, mulches are classified as organic or inorganic. Organic mulch such as wood chips, seedless straw or chemical-free clippings will decompose and will need to be replaced as it degrades, but the decomposition improves the soil by adding organic matter and nutrients. They are usually placed directly on the soil, but can be placed over a landscape fabric. Although these are more expensive, inorganic mulches such as sand, pebbles or gravels are more stable over time and last longer. These inorganic mulches are best placed on landscape fabric or plastic sheeting to prevent them being moved about by the soil or sinking into the soil.

    Once you know what you want to use as a mulch it is always a challenge to know how much to buy. For organic mulches, a depth of 1 inch is the minimum but 2 to 3 inches are better. Mulches are sold by the cubic foot in bags or, if you have a bigger space to cover, by the cubic yard in bulk. It is much cheaper to buy in bulk if you need quite a bit.

    To calculate cubic feet:

    • Measure the area of the garden space you need to cover (length by width).

    • Then multiply the area by the depth of the layer you have chosen to apply.

    For example, my garden space was 12 feet by 10 feet. This equals 120 square feet. I chose to apply 3 inches or 0.25 feet so I multiply the 120 square feet by 0.25 feet. Now I know I need 30 cubic feet of product for this space.

    If I had multiple areas to cover I would follow the same procedure for each area and then add them all together for the grand total.

    To calculate cubic yards:

    • Measure the length and width of the area in feet. When you multiply these figures you find the area in square feet.

    • Multiply the total area in feet by how deep you want your mulch in inches.

    • Divide your answer by 324 (1 cubic yard of mulch will cover 324 square feet 1 inch deep). Round out the results to the nearest whole number to find out how many cubic yards of mulch you need to purchase.

    For example, let’s say that my space is 648 square feet and I want to put down 2 inches of mulch. I would multiply the 648 by 2 to get 1,296. I would divide my answer by 324 and round the result to the nearest whole number. I now know I need 4 cubic yards of mulch.

    Obviously, rarely are our spaces perfect rectangles. It is OK to take rough measurements and estimate. It is best to guestimate more than less.