To be honest, I was always rather careless about mulch when gardening in the Midwest. My use of it was less than scientific. Now in a region defined by low rainfall and intense heat, I am looking much more closely at the stuff. In fact, I am getting a little creative...
One of the first things I noticed about gardening materials here in the Phoenix, AZ, area was the near-universal use of rock mulch. Crushed rock in various sizes and colours, much of it quite attractive, is the signature local product for landscaping. One can see why. It's much easier to think of rock than of forest products here. But it was confusing to a newcomer. I actually spent a fair amount of time wondering what, if any, were the reasons rock was preferable to organic mulches. Did wood mulch harbour scorpions and other hazardous creatures? Was it somehow bad for plants in an overheated climate? Yes, I have a fairly good imagination! But instinct still told me my gravelly clay soil would be much the better for something that would eventually break down and supply nutrients and conditioning. So I waited for more information.
With the aid of a book or two, I was finally able to verify that organic mulches would be an excellent choice. Also, I finally located the bags tucked back into corners and along far walls at the local GCs. So they did actually sell bark mulch! I bought some handsome red mulch and some less expensive dark brown ditto and began testing them in the garden. This was last spring. Both were acceptable. Here is the dark mulch in need of topping off under a rose bush.
I decided to test an idea I had been playing with for weeks. This was to repurpose our favourite horse bedding pellets for a walking surface. It worked quite well at keeping the ground surface cool enough to walk on. Soon thereafter, I decided it was time to try them out as garden mulch. It might work; it just might! I tried it. Then I tried it some more. It seems to work. It seems to work well.
As this is not a product that will be familiar to most gardeners, I am going to give a complete description of how I prepare and use it. I'll discuss the successes, pros and cons afterward. Here is where it starts.
It begins as compressed pellets.
Once the area is watered one more time, the mulch will expand just a little more as the remaining intact pellets dissolve. In the meantime, it already serves to protect the soil around the bushes.
First, it is an organic mulch, capable of actually improving the nature of the soil as it breaks down over time.
Second, it is incredibly absorbant. This is what it was designed to do - absorb moisture! I believe that for some plants it will allow me to nearly double my between-watering times. I can put more water on an area to begin with, and the mulch allows it to be released slowly in situ at the plant's roots. In a dry-soil garden this is a great advantage over standard bark mulches, which simply allow the water to filter down through a loose, non-absorbant heap of material.
Thirdly, it keeps the ground surface much cooler. If I place my hand on exposed soil, then on the mulch, the difference is tremendous. I believe this is due to the fine texture of the product. Unlike rock or even bark mulch, there are no broad, flat areas to receive and store sun heat. Instead, there are zillions of grains with air settling between them, forming a good, insulating cover to keep plant roots cooler.
Additionally, it does spread further, making it cost-effective at least with bagged mulch; I don't know how it compares with bulk.
As for disadvantages, most do not apply in this climate. There is an aesthetic question about the very light colouring; I would prefer a darker mulch. But that is a minor disadvantage.
My one ongoing concern is that the material may form a crust if allowed to soak and dry too many times. Only continued use will tell, and I am not sure that it will be very problematic even if this does occur.
Like any wood mulch, it will likely acidify the soil. Desert soil is proverbially alkaline and salty, so this is actually an advantage here, but it might be an issue in an area with higher rainfall or naturally acid soils.
Its absorbancy may damage sensitive plant crowns. As with any mulch, be cautious of smothering roots and crowns. This mulch when applied is nicely aerated, but it will surely settle over time and will need careful usage accordingly. Remember that it holds more water than standard mulches.
These things said, I have not had any negative responses from any of the plants I have been using it with. I began lightly, of course; and I have tried to be careful about the smothering effect as so many of my plants require "well-drained soil" and all the rest of it! But as I have had to learn that instructions "never water" are to be followed at peril of losing the plant, I am getting braver with mulching those "never mulch" plants here. It's just a totally different gardening equation.
In fact, the decisive plant has been an Agastache. A. "Ava", a dry-climate, well-drained soil, rugged western native plant has been nigh unto death time after time since planting. She was planted late because the order arrived late because Salvia reptans (sitting next to her and smiling its way through 110 F) was in the same order and wasn't ready to ship on time. It couldn't be helped. Ava has looked tolerably good only when kept watered twice daily, which is too much for this gardener! Once daily, and I would go out and find her at death's door every morning. It didn't help that I failed to make a large enough watering basin when I planted her, which makes it hard to give a thorough soaking.
So I put my chin up and applied a deep pile of the mulch around Ava. Right up to the stems, which I shall have to keep a close eye on! And she perked up! I actually let her go 48 hours between waterings the other day as the heat was not so bad. This turned out to be a little long, but with water once a day, she is looking quite healthy now. If only she can settle in, she should be her rugged self by next summer.