Monday, July 26, 2010

Which Mulch is Best for You? 2010 Update

Adding a three to four inch layer of mulch now beneath shrubs and trees, out to the drip line, can help stop weeds, keeps the ground evenly moist and feeds the plants as well. But which mulch is best?
Each type of mulch has its Pros and Cons:

Bark: lasts a long time, looks nice, feeds the soil slowly, suppresses weeds. Beware of finely shredded redwood bark: the hairs can easily ignite if someone tosses in a cigarette butt.
  Medium to large bark stays in place better than the mini-bark. Buy bark bulk from a sand and gravel yard to save money. It may seem like a lot of bark, but you'll find a place to use it!

Cocoa shells: look nice, last a long time, but some varieties may be toxic to dogs, due to the potential of theobromine poisoning.

Chipped/Shredded tree trimmings: inexpensive, feeds the soil, supresses weeds, but needs to be replenished yearly.  In the good old days, you could bribe a tree trimming company doing work on your street to drop off their load in your yard; after all, they were probably just going to take it to the dump. Not any more. Due to the demand of this as a mulch, many arbor companies are keeping or reselling chipped/shredded tree trimmings. Still, it can't hurt to ask! However, consider this: you may be importing someone else's plant pests or diseases into your yard. One way to mitigate this: keep those trimmings in a pile for several months. That can help to destroy pest eggs and certain pathogens, if the temperature in the pile gets up to 140 degrees or so. The downside of keeping tree trimming in a large pile: ant colonies might move in. Be careful when shoveling!

Pine needles: best spread about three inches deep around acid loving plants.
Don't let it get too thick though. It might block the transfer of air and water into the soil, creating an anaerobic environment.

Compost: Inexpensive (if home made), provides soil nutrients and improves soil structure. But weed seeds can germinate in it.
And, it needs to be replenished every year.

Grass Clippings: inexpensive, but should be applied when thoroughly dried. Also, clippings from weed-like grasses, such as bermudagrass, may get established in other areas. And, don't use grass clippings from a lawn that has been treated with weed killers; in particular, postemergent selective herbicides. This could harm the roots of desirable plants.

Straw: Not to be confused with alfalfa, which can germinate if used as a mulch. Keeps weeds down, retains soil moisture, adds nutrients as it breaks down. Can be bulky to transport in bales (get bale hooks!). A good mulch for walkways, too. Ruth Stout wrote entire books on the subject of using straw mulch in the garden. They're collector's items now.

Leaves: Why do you think God named them "leaves"? "Leave" 'em be, where they fall. Better yet, shred the leaves with a bagging lawn mower to create a quicker nutrient source for your flower beds and borders. They break down quickly, so leaves must be constantly added to an area.

Worm Castings: The Master Composters at Texas A&M University do not recommend using worm castings as a mulch. Worm castings, they report,  are too nutrient-rich to use as mulch on outdoor plants; they tend to dry out and the nutrients are wasted

Permeable Weed Cloth: Not recommended by Washington State University. It is inorganic, suppresses weeds while allowing air and water to pass through. Needs a layer of bark on top to keep it from disintegrating from sunlight. May keep soil too moist. May lead to surface rooting of plants. Pulling up weeds that are growing in weed cloth may pull up portions of the cloth as well.

Newspapers: inexpensive, suppresses weeds. Must be replaced often, needs to be secured in place.

Plastic: Suppresses weeds, but does not allow air or water to pass through. In the summer, it may raise soil temperatures too high. Needs to be secured to the soil.

Rock: Looks good, but provides no nutrients, may raise soil temperatures, damaging shallow plant roots.

Benefits of organic mulch:
• retains moisture
• keeps soil temperature constant, reducing plant stress
• suppresses weeds
• gradually increases soil organic matter
• attracts beneficial organisms that improve soil fertility and porosity.
• mulch encourages healthier plants, reducing the needs for pesticides and fertilizers.
• protects roots and plants from mechanical injury.
• on hillsides and around homes, it suppresses the spread of brush fires.

Some cautionary notes: 

• Don't pile up organic mulch around the trunks of trees and shrubs. 

• Keep mulch a few inches away from trunks to lessen the chances of rots and other diseases.
• Don't import someone else's problems. Avoid using as mulch any diseased plant material, including suffering tree limbs, diseased leaves, herbicide-treated lawn.

To cover an area with three inches of mulch:
apply about 1 cubic yard for every 100 square feet of area.

One of the downsides of mulch: native bees will be dissuaded from nesting in mulched areas. To attract ground-dwelling native bees, keep a portion of your yard unmulched.

For a great publication about mulch, download this pdf, "The Landscapers' Guide to Mulch" from the California Integrated Waste Management Board.

And, the video, Mulch Madness!


  1. I've got a Ruth Tout book...I love it, just started mulching in earnest this year.....

  2. Man, I just discovered your blog today via Facebook. This is AMAZING! I've forwarded a link to all of my employees.

    Jen Molica, Assistant Manager
    Worm's Way Mail Order
    Bloomington, IN

  3. I'll have to look into straw mulch, though it sounds like it might be pretty fire-hazardous, not good for my location. BTW, re your next post about horses: we composted our horse manure and it made actually a nice mulch, breaks down pretty quickly and when composted is entirely inoffensive. Now the horse himself is compost - RIP!