Mulch is any material, organic or inorganic, put in a layer on the soil surface. Mulching is a wonderful practice; two of the greatest benefits it provides are holding moisture in the ground by reducing evaporation (we certainly need that now) and reducing weeds-as long as the mulch material is weed-free and applied in a deep enough layer to prevent seed germination.
So, how thick should it be? On average, three inches deep, but it does depend on the material used. The coarser the material, the deeper it can be because the larger chunks allow for better air flow to the roots. Something like bark can be about four inches deep, where ground leaves should only be two inches deep. If you cannot put mulch on that deep, it is still helpful in retaining water, so put on what you can. More than 5inches deep would be too much mulch.
Figuring out how much is needed requires only some simple ‘ciphering.’ Figure out the square footage (width x length) of the area, then multiply by depth in inches that you want the mulch to be, and then divide by 324. That will give you the number of cubic yards needed. As an example, if you are going to mulch under your trees in an area 8 feet wide by 10 feet long, and you want a 3-inch layer of mulch, multiply 8 x 10 x 3, which is 230. Divide by 324 and you get 0.75 cubic yards. There are 27 cubic feet in a cubic yard, so multiply the desired 0.75 cu yds by 27, and you get about 20 cu ft. Bagged mulch is usually sold in 2 cubic feet bags so 10 bags are needed. Again, the formula is square feet times mulch depth in inches, divided by 324 to get cubic yards. If the space you plan to mulch is not a rectangle, just estimate as best you can. It does not have to be exact. If you have a large area to mulch, you may want to buy it in bulk rather than in bags.
Using mulch also reduces soil compaction and soil erosion, keeps soil-borne disease organisms from splashing up onto the plants, and protects plant stems and trunks from the dreaded “weed-eater disease.” Organic materials also increase the population and activity of beneficial soil microorganisms – a particularly good thing since they break materials down into a form that plants can take up and inhibit the “bad guys” that cause plant diseases. Plants with organic mulch will put out additional roots in the mulch, ending up with more roots than plants without mulch.
When talking mulch, ‘organic’ refers to materials that once were alive. Common organic mulch materials include such things as shredded bark, chipped or shredded hardwood, compost, wood chips, leaves (including oak leaves), and grass clippings. Paper, either shredded or sheets of newspaper, also makes a good mulch, but generally needs a layer of another material such as bark, on top of it, to hold it down.
Rocks and woven “weed” or “landscape cloth” are some inorganic mulch materials. They have just one advantage – they do not need replacing very often; however, both absorb heat, a feature we are not looking for most of the year. Organic mulches, on the other hand, have several advantages over inorganic materials; as they break down, they improve the soil by adding nutrients, and increase the ability of the soil to hold both nutrients and water. Organic material improves the structure of the soil and can even improve drainage in clay soil.
Woody mulch should be watered thoroughly after it is spread. It needs rehydrating to inhibit the growth of nuisance fungi that can form a water-repellent barrier. Add some nitrogen (‘N’ on the fertilizer bag) to the soil before applying wood mulch. Lightly work in some bloodmeal, urea or a high nitrogen lawn fertilizer. Soil microorganisms will use nitrogen in the soil as they decompose the mulch, which can result in yellowing of the leaves in many annuals and perennials.
If you are using compost, grass clippings or leaves, which all have some nitrogen in them, as your mulch, it is not necessary to apply additional nitrogen. If you have already put the mulch down and plants are showing some yellowing, just spread some nitrogen on top of the mulch and water it in well.
An especially important point is that mulch must always be kept away from the trunk, stem or crown of the plants to prevent rotting of the bark or stem. Mulching helps tremendously in reducing water loss from containers also so use it in them also. Mulch everything and reapply often!
