Wednesday, October 14, 2009


Mulch is amazing stuff, and probably my biggest area for improvement in the garden would be to be more judicious about using it. Mulch has so many uses in the garden and landscape. It cools the soil. It holds moisture long enough for more of it to penetrate than bare soil. It allows moisture to penetrate deeper into the soil and keeps it from evaporating as fast. It smothers weeds thus reducing weed problems in your landscape. Most importantly, it feeds the soil by providing a slow, constant source of organic matter to feed the micro- and macro-organisms in the soil.

It is worth noting that I am almost exclusively talking about organic mulches. There are also inorganic mulches, such as plastic liner, gravel, decomposed granite, river rock, and landscape fabric. Plastic barriers do a great job of keeping out weeds and keeping in warmth, but they smother the soil, preventing oxygen and moisture from reaching the soil. If you remember that soil is a living organism, you might see how smothering the soil is a bad thing. I also don't particularly like rock mulches since leaves and weeds have to be removed regularly or they look bad. It is a lot more work maintaining rock mulches in an attractive fashion than it is to maintain organic mulches. I will say, though, that mulches made of large rocks can create a useful micro-climate where needed. The rocks absorb heat to help heat-loving plants in cool climates and can act as a thermal mass, protecting tender plants from some of the harshness of cold nights.

As far as organic mulches go, there are a number of different kinds, each with their own benefits and disadvantages:

Yes, compost can be considered a mulch. It shades the soil and helps water penetrate. In addition, it provides a phenomenal benefit of nutrients and beneficial bacteria and fungus to the soil. As for disadvantages, it may not prevent weeds as well as other materials and it doesn't give that neat, clean, landscaped look that other mulches provide. It just looks like black dirt. Also, if you are making your own, it is really difficult to produce enough to provide enough that it actually counts as mulch.

This one is really nice for the veggie garden. You usually want to apply it a little thicker than other mulches to get the same weed protection. It does break down rapidly, though, and it makes a great soil amendment.

Wood Chips
This is one of the easier mulches to come by. Wood chips make your landscaping look nice and neat. They also take a long time to break down, giving them lasting power in your landscape. Conventional wisdom is that woodchips also bind nitrogen in the soil, holding on to it until they are broken down. This can be a good thing and a bad thing. On the bad side, they can rob your plants of nitrogen and stunt their growth. On the good side, they do the same thing to weeds.

Fallen Leaves
When the leaves fall off your trees in the autumn, they usually get thrown in the compost bin (or, heaven forbid, go out with the trash). But they can also be used as a mulch. One of the big advantages is that if you have enough moisture, the two dimensional nature of the leaves comes into play and they stick together to make a dense mat. It can be so dense that if left on a lawn, just a few inches can kill your lawn. The grass can't penetrate it, and neither will weed seedlings. It is also great food for the fungus and worms in your soil.

Cocoa Bean Hulls
I am still looking for this mythical mulch. A neighbor had some a few years ago and it made her whole garden smell like brownies. It also looked pretty nice. The only problem is that some dogs find it irresistible. The chemicals in chocolate, including the hulls, can kill a dog. So use this one with care.

Grass Clippings
Fresh grass clippings are very high in nitrogen and moisture. If you put a big pile of them on your landscaping, they will get smelly. However, if you spread them out to dry or just put a thin layer on your landscaping or garden, they will dry out pretty quickly and add a little nitrogen to your soil.

Wood Pellet Fuel
Wood pellets are made specifically for pellet stoves. They are sawdust that is dried and compressed to make pellets. The nice thing about them is that when you add water, they swell to several times their original size. Plus, a 40 pound bag sells for about $4.50 in my area. The resulting mulch is a bit finer than I usually like, but it really is pretty nice. I will offer a caution, though. I haven't found much information about what is in them, so I always look for one that says "100% Organic" or "100% Wood" or something of the sort on it. I don't think they put additives in it to help it burn, but it'd be good to know for sure. Also, it is generally only available in winter.

Cardboard and Newspaper
Cardboard and newspaper work in much the same way as landscape fabric or plastic sheeting with the added benefits that they are cheap or free and will biodegrade over time. Few organic mulches are better for weed control than a nice layer of cardboard or newspaper. In my experience, these usually work best with wood chips or some other material on top. You need something to hold them down or the wind will pick them up and put them somewhere else.


  1. I'm using a shredder for my leaf mulch to get rid of the oak leaves which cannot compost in one winter. So far it's a good investment to avoid having to bag the leaves for the curb. However, I think I read somewhere, that too much leaf mulch can make the soil acidic to some plants and upset the neutral loving plants. Should I put a few drops of left over high ph or phosphorous at this time to promote root growth. I did buy lots of the stuff - because plants tended to bolt - I have lots of shade.

  2. Anna, I have heard that as well for oak leaves, though not necessarily for other kinds of leaves. Adding the fertilizer is certainly worth a try. I'd let it break down a bit first and test the pH. Then add a little of your stuff and test again. Wood ash will also raise the pH, but don't use anything from charcoal briquettes, just logs or natural charcoal.