what do you make of this advice?

Mulch applied too thickly can cause problems. A wood-derived mulch may undergo high temperature decomposition causing it to dry out. The mulch may then be colonized by fungi that create water repellent conditions throughout the mulch. Water is unable to penetrate the mulch and reach the soil and plants fail to receive adequate moisture. Mulching too deeply can also cause the soil to remain continuously wet contributing to root and stem rot problems in addition to depriving plants of needed oxygen. Apply a mulch layer no more than 1 to 3 inches thick.

from: http://www.ladybug.uconn.edu/factsheets/tp_05_mulchbasics.html

Is mulching thicker than 3 inches really not advisable?

I'd have to say that it depends......on your location, the type of soil, the soil drainage, the temperature, the general weather (sun, rain, wind, etc), and the crop. On my main farm, most crops do well with a 3 inch mulch that is either coarse compost or grass clippings ( no access to straw or spoiled hay here). But the potatoes do better with deep mulch. As the potato plants grow, i add more mulch, usually in 3 stages. The mulch ends up being around 8 inches deep. The bananas do better with a very coarse 10-12 inch mulch. The orchard trees do better with mulch no deeper than two inches and which is a mixture of cinder and shredded compost.

On my seed farm, mulch needs to be much deeper. I try to maintain a 3-6 inch mulch. Less than 3 inches and things dry out too fast and the plants are stunted. Plants are doing much better where the mulch is closer to 6 inches. I suspect I could go deeper with the mulch but I don't have enough material yet. I have some 3x3x3 grow boxes that are filled with compost lasagna style. I grow sweet potatoes in them. Straight mulch, so to speak. The plants do super, sending roots all the way to the bottom of the boxes. Sweet potatoes is one crop that can be grown smothered in mulch.

But what works here for me may not work somewhere else. My two farms are only 5 miles apart, but I have to use different techniques on them....even for mulching.

...Su Ba
www.kaufarmer.blogspot.com

The paragraph quoted above may be true where the study was done, but is not universal information.

First, I have never seen woody mulch heat up very much...it needs nitrogen mixed in to do that.
Next, it will dry out (anywhere that doesn't get much rain).
And, drying out will not cause a fungal outbreak. Fungi need moisture.

In semi arid regions (like your Orange County), it is often recommended to rake the compost back before anticipated rains. This allows the rains to get to and into the soils. (A 1/4 inch rain will not penetrate several inches of mulch.) Once the rain has passed, push the mulch back over the soil to prevent the water from evaporating out of the soil.

Mulching techniques must be designed for each local. Advice from 3,000 miles away will probably not fit your circumstances.

Thanks all. I'm just a bit confused because from everything that I've read from permaculture types, I've never heard that more than three inches is a bad idea or in any way risky.

As for my situation: my climate is Mediterranean and my soil is heavy clay. I was under the impression that layering more and more organic material on top of my bare, hard, dirt is one of the best things I can do for it. For the time being, I've only accumulated enough mulch to put around some of my fruit trees. Truth be told, I probably haven't gone higher than three inches, but I'm somewhat concerned now that doing so might do more harm than good.

In other words, I was hoping y'all would say "whoever wrote that is off his rocker!"

Oh, my mulch is basically all my yard waste run through a chipper/shredder. Leaves, branches, a lot of pine cones and needles (that drop my my neighbor's two large Ponderosa pines). I also throw coffee grounds on there and whatever other organic material I come across.

Definitely depends on your climate. Here, mulch keeps the soil too cool in the summer and too wet the rest of the year. It also houses crazy levels of mice.

A little goes a long way and we keep it back from the base of plants.

I do know that wood chips will heat... We had about 30 truckloads delivered by the contract tree trimmers, and even though the carbon to nitrogen ratio had to be huge, the suckers heated up nicely. I don't imagine that even a 6" layer would have the "critical mass" needed to heat, however. In fact, I spread some of the heating/heated chips for sweet potato mulch, and it cooled off fine. The sweet potatoes were IMMENSE!

Update: I've been reading on hydrophobic mulch pretty much all day. Now I have noticed that when I dig through my mulch it does seem pretty dry and "powdery" but I didn't think much of it. So when I got home, I sprayed the mulch around some of my trees for close to five minutes. I made sure I really soaked it well.

So I pull back the soaked, top inch or so of mulch and guess what? Bone dry underneath! I couldn't believe it! And everything seems to be covered with a blueish-white powder, which I'm assuming is some type of fungus.

I'm a tad bit worried now! Any ideas?

The topic of mulch is really broad and large, so it's impossible to cover it all in short, simple responses. But I'll try to give you more info on what seems to be happening to you.

Mulch becoming hydrophobic is an extremely common problem. In my experience it happens when mulches are allowed to dry out. It is a worse problem when the mulch is fine rather than coarse. Lets look at fine vs coarse.

Coarse mulches dry out because of the large air spaces. When water is added it tends to run down between the pieces rather than being absorbed by the mulch. In reality, this tends to be a benefit. Whatever water you rapidly apply actually reaches the soil. Then the mulch keeps the wind and sun from evaporating the water away. I prefer coarse mulch in permanent situations because it allows water penetration, allows oxygen to reach the soil, it tends not to mat or lodge, nor does it blow away. I will rapidly apply water at first to wet the soil, then follow with a short period of sprinkling in order to moisten the mulch itself.

Fine mulch causes me the most headaches. I try to avoid fine particle mulches. If they are allowed to dry out, they are very difficult to get wet again. Wetting them requires lots of water plus physical turning or mixing of the mulch. Using a garden rake while spraying water helps. But using a hose means it can take hours of labor in a garden or small orchard situation. The thicker the mulch layer, the more water and time it takes. As you discovered, just hosing water on without stirring the mulch means you get some parts over soaked while other layers or spots stay bone dry. The only water that gets through to the soil are spots where the water created a channel through the mulch layer. Getting water to the soil when there is fine particle mulch is possible using a high water flow, that means a hose without a sprinkle or nozzle head. The idea is the punch a hole through the mulch layer so that the water bypasses the mulch.

Fine mulches have other problems associated with them. Fine mulches when kept moist tend to mat. Thus water, be it from natural rain or irrigation, does not evenly enter the soil. There are large areas where the water gets blocked. Wet fine mulch also blocks oxygen. Soil, surface roots, and microbes all need oxygen. Without it, plants suffer or die. I've seen these fine, wet mulches become slimy and slippery. The soil below such mulch is often bone dry.

The one way to use fine mulches to your benefit is to treat it as a dust mulch. That means having a thin layer, perhaps only an inch, and raking it lightly after each water application so that it stays dusty rather than matting up.

How a mulch performs depends upon what it consists of, the particle size, and it's stage of decomposition. For example, whole oak leaves will give different results compared to coarse ground or fine ground leaves. Oak leaves vs. pine needles v.s straw vs. grass clippings vs. pebbles are all vastly different. Every mulch material may have different characteristics and need to be handled a bit differently in order to be useful. And of course, it depends upon climate, location, soil type, etc.

Example....In a small area where I had planted a dozen fruit trees, I made the mistake of using finely shredded compost topped with coarse chopped weeds for mulching. In the drought, it all dried out. The trees wilted even after we had rains. When I discovered the problem, I knew I needed to remove the fine dusty stuff. Using a hay fork with tines about 2 inches apart, I raked the coarse stuff aside, leaving the dusty mulch behind. I then rototilled the dusty stuff into the top inch or inch 1/2 of soil. I did not deeply till because not only of shallow tree roots, but that deep tilled mulch decomposes differently than shallow tilled. I repeated the light tilling every two weeks, adding a light dressing of chicken and rabbit manure to encourage decomposition. I don't recall exactly how many times I lightly tilled, but I'd guess 5-6 times. Once I was satisfied that the hydrophobic problem was eliminated, I then added a light layer of coarse material. Since then, I haven't had to repeat the tilling. I now maintain a layer no more than two inches of mulch which consists of cinder and shredded coarse compost. This type mulch stays "light" allowing oxygen to enter while retaining moisture. Another orchard mulch used successfully here is macadamia harvest waste, which is a mixture of nut husks, shells, and the soil and cinder picked up by the machines. It's a bit dusty so it is applied lightly, less than one inch.

Hope this gives you some ideas.

...Su Ba

Su Ba - that was a truly amazing post - thank you for that information.

Ronnie - I live in Phoenix - so both hotter and dryer than you in OC. Here's my experience with mulch in this climate.

--I do use thick, coarse mulch - usually stuff I get from a local tree trimmer who knows a bunch of us wacky permaculture types and lets us know when he has a load of the "good stuff" (no oleanders, no palm debris). In various parts of my yard, the mulch can be anywhere from 3-8" deep. The depth pretty much depends on what's growing there (trees v. low-growing veg).
--John is right when he says that our scanty rain events cannot penetrate deep mulch - I, too, have found this to be true. So I stack the deck. All my planting beds are sunken to retain both irrigation water and rainwater. Say I'm planting out a veggie bed. I rake back any existing mulch, plant seeds/seedlings, once the plants are 4-6" tall I will flood the bed with a hose to get it nice and wet. I also have heavy clay soils that I've been amending over the years with compost. Once the bed is wet, I actually put the woodchips in a wheelbarrow and hose those down (not spray but HOSE them down - so the woodchips are like full sponges). Then I lay the wet woodchips over the wet bed. Dry woodchips have a natural "wicking" tendency and will soak up soil moisture. When you live in a very arid climate with low humidity, high temps and drying winds - that wicking by the woodchips can cause problems.
--even with thick mulch, I can't get away without supplemental irrigation here. My beds have the following layers - soil of the sunken bed, on top of that is t-tape irrigation tubing (holes facing down toward the dirt), then I usually lay a nitrogen source down thinly, such as a sprinkling of alfalfa meal, coffee grounds, etc. Don't let this form a "mat" though - just sprinkle on the soil, then the layer of saturated woodchips. Because the irrigation is between the woodchips and the soil (not on TOP of the woodchips) and the holes of the tubing are facing the soil - I'm watering down into the soil instead of just watering on top of the woodchips.

Note that this method is what works for me in a "dryland" situation. Obviously all that soaking and covering would be toxic to someone in a wet area with high rainfall - they would rot out their plants for sure.

Basically, you're going to have to try several "experiments" (my favorite part of permaculture!!) and keep notes and then let us know what worked for you in YOUR situation. I know that I for one, would be really interested to hear your results!

Southern California versus Connecticut. What works in one is almost certainly not going to work in the other

Fungal decomposition of woodchip mulch is a desirable thing, especially around your fruit trees, so it makes me wonder what the author of that advice was talking about with their recommendations. It sounds as thought they are trying to avoid decomposition of the mulch - and again, makes me wonder what their application is where that would be a goal (no, I have not followed the link and read the rest of the article )

In your situation, one thought that comes to mind is can you make a bit of a barrier around your trees to hold water, so that you can flood irrigate? I would think that letting the water sit there and soak in would deal with the mulch being hydrophobic, get it saturated and working "the right way" again.

The problem of wet roots is a real concern in the northeast where temperate rainforest is the natural condition, and probably a hard condition to produce in SoCal.