Everything You Ever Wanted to Know About Mulch

A mulched garden is a happy garden. Why? Because mulch does some very important things:

  1. Mulching prevents weeds from germinating. If sunlight can't get to the weed seeds, they won't germinate. It also helps smother any small existing weeds. Weeds compete with your garden plants for water, food, and space. Get rid of weeds, and your plants will have more of what they need to grow.
  2. Mulching helps keep the soil moist and cool. Moist and cool are the conditions that help facilitate root growth, leading to healthier plants. When you mulch, water is retained longer in the soil, and you don't have to water as often.
  3. As it breaks down, mulch adds important organic matter to your soil.

But these benefits only come if you add the right amount of mulch. Three inches deep is a good, general guideline. To figure out how much mulch you need for a given area, here is an easy formula:

  1. Find the square footage of your bed (the width times the length (roughly, if your bed is curvy, just measure from the farthest-out curve). Example: bed is 10 feet long by 8 feet wide, so 8 times 10 = 80
  2. Multiply this number by three, which is the depth in inches that we want our mulch to be. So, 80 times 3 = 240
  3. Divide this number by 324 (if you really want to know why, see the sidebar). So, 240 divided by 324 = .75 cubic yards

So, if you were ordering bulk mulch, you'd tell them you need .75 cubic yards of mulch.

An easier way:

Go to This Old House's Materials Calculator, plug in your bed size and depth for the mulch, and it will give you your answer.

An even easier way, especially if you buy your mulch in bags:

Figure out the square footage of your bed (like in step one, above.) One bag of mulch will cover about 10 square feet to a depth of three inches. So, divide your number by 10. So, 80 square feet (from above example) divided by 10 gives you 8. So, buy 8 bags of mulch.

Types of Mulch

Shredded Bark

Around here this is typically sold as cedar or cypress mulch. It is an attractive, natural color, and, when newly applied, has a wonderful woodsy aroma. It is very effective at weed control and water retention, because the pieces knit together, but at the same time, it resists compression. It is also inexpensive.

Straw

Straw is a wonderful mulch. It is not the most attractive, but it is great for vegetable gardens. To get the most weed control and water retention, however, you will need to layer it deeper than the other mulches. For straw, five to six inches is ideal, because it is so light and fluffy. One caveat: be sure you are getting "straw" and not "hay." Hay contains weed seeds, which will inevitably be sprouting up all over your garden.

Grass Clippings

Free and plentiful, grass clippings make great mulch. They decompose quickly, giving your soil a quick nitrogen boost. If you don't find them especially attractive, you can always use them to mulch the veggie garden.

Pine Needles

This is great mulch because it doesn't mat down, is easy to find (it is also sometimes called "Pine Straw") inexpensive, and gives the landscape a very natural look. If you have pine trees, you have a free source.

Leaves

Another plentiful, free option, especially if you have lots of trees. A mulch of leaves lends a natural look to your beds, reminiscent of the forest floor. The best way to use them is to shred them with your lawnmower or a shredder before you apply them to the beds. If you apply them whole, they tend to mat down and can prevent water from getting through to the soil.

Cocoa Bean Hulls

These are attractive, lightweight, and, if you're a chocolate lover, you will love the aroma they give off when newly applied. They are not my favorite for a few reasons. First of all, they are not always easy to find. Secondly, they are the most expensive of the mulches listed. And thirdly, and most importantly, they are toxic to dogs. According to an ASPCA study, dogs that ate cocoa bean mulch became ill, vomiting and experiencing muscle tremors.

Bark Nuggets

These take the longest of all of the mulches listed to decompose, which means you don't have to apply them as often. However, they tend to wash away in heavy rain or watering, and may not be the right look for every landscape.

Other mulches, such as rocks, gravel, shredded rubber from tires, or black plastic may keep the weeds down and help conserve water, but they are inorganic, which means that they don't break down into anything that the soil can use. They don't contribute to overall soil health the way the organic mulches listed above do. And, since healthy soil is the basis for a healthy garden, it is better to go with an organic mulch. Besides, other than the stones, none of these look particularly natural in the landscape.

A Few More Things...

When you mulch, be sure not to put it right up against plant stems, tree trunks, or wood siding. It will make it too moist, which will cause rot over time. Pull the mulch back from plants and wood siding two to three inches.

Once a month or so, you may want to go out and "fluff" your mulch. This is just an aesthetic thing to make everything look a little fresher. Simply use a rake, hand cultivator, or a hoe to fluff it up a bit.

You want the mulch to be three inches deep, and not much more. Over time, the mulch will decompose, and you will need to top it off. Don't buy another three inches of mulch. Adjust your formula so that you're only adding an inch or so on top of your old mulch. Get it much deeper than three inches and you've created a haven for pests, including rodents, who may make a buffet of your plantings.

So, there you have it: everything you could possibly need to know about mulch. Now, go mulch something, eh?