Why is Mulch Important?
Mulch has so many functions and these are more critical in our Colorado climate than in most. You may notice that appearance is not in this list though it generally has aesthetic value as well.
- Cooling the soil- soil organisms such as fungi, bacteria, and the insects or smaller organisms of the “detritusphere” cannot survive baking heat. They must be protected from heat and UV light to thrive. Mulch provides shade to the soil.
- Retaining water– mulch helps prevent evaporation but also it acts as kind of a sieve that pierces the soil at irregular angles and so funnels water down into the soil. It is something like the difference between pouring water onto a sponge or onto a smooth granite countertop.
- Creating top soil- wood mulch will eventually break down into a nice black layer of rich soil. Insects, bacteria, and fungi will break it down and add their “fertilizer” to make it so that it is teeming with life. Plants actually share their carbohydrates with these organisms in exchange for making nutrients available to the plants. Organic material is required to keep that cycle going and mulch is a cost-effective way to begin.
- Weed suppression. Landscape fabric is initially porous but dust eventually clogs it and weeds are quite happy to grow on top of them. Mulch helps to prevent light from getting to the seeds that are in most soils so they are not able to germinate.
TYPE OF MULCH:
The type of mulch matters; bad mulch accomplishes few to none of the goals for using it. Bad mulch doesn’t improve the soil structure, in fact it can make it worse. But the biggest reason is that is doesn’t add nutrition.
- Most mulch that is colored is a waste product. It is usually made of re-cycled wood such as pallets or old decking and most of these have been treated with insecticides or pressure treated in order to prolong its usefulness for it’s first job and that makes it bad at being beneficial to the soil. It is dyed for 2 reasons: first, it makes it more attractive to buyers because it’s uniformity makes it look like outdoor carpet and that tidiness is hard to resist. The other reason it’s dyed is to make it harder to see that it is actually lumber not tree trimmings and so you are less likely to suspect how dry and lightweight it actually will be on the ground. Think of it as the equivalent of processed food…. The less you are able to visually identify its origin, the more highly processed it is and the less nutritional content for the soil organisms and plants.
- What type is good? Arborist trimmings. These are just like the pieces that would normally fall to the forest floor—twigs, leaves, branches. They will still have much of the nutrition that the tree created through it’s photosynthesis and much of the moisture. Some irregularity in size of pieces is important because it allows that sifting through of rainwater; if the pieces are too small (humus mulches) is knits together so tightly that the water doesn’t easily penetrate. You can get this directly from arborists if they happen to work in your neighborhood or buy it from Rocky Top Resources—ask for Natural Fine Tree.
Which Types to Avoid?
- The stranger the color, the worse the mulch. Shredded tire black mulch? Red colored pallets from international shipping? No thanks, keep their pollution off your property.
- Gorilla hair and other exterior bark mulches. Bark is a barrier… it works to repel water, fire, insects, and disease. That’s great for the “skin” of a living tree but you don’t want to keep these things out of the soil; we need them to break down the wood into organic soil particles. Gorilla hair (aka Cascade cedar) is shipped in from the west coast and is the most expensive, maybe that’s why it’s often recommended by the people who sell mulch? Skip it.
- Very fine humus mulches are tempting; they look dark and rich but they are almost like putting soil on top of soil and I find that once they get dry they almost repel water.
You don’t have to take my word for it: Resources
Some of this is my experience and observations on many local properties, but this is not just my opinion. Below are some of the soil experts who I have read, taken classes from, listened to, and learned from. Check out their work if you wish to learn more:
Soil Experts:
- Jean Reeder, Ph.D. soil scientist for USDA and CSU
- Jeff Lowenfels, “Teaming with Microbes: The Organic Gardener’s Guide to the Soil Food Web” 2010
- David Montgomery, Ph.D., “Growing a Revolution: Bringing Our Soil Back to Life” 2018
Podcasts:
- Joe Gardener, by Joe Lampel
- A Way to Garden by Margaret Roach
- You Bet your Garden by Mike McGrath
OK, ok I’ll get some mulch. When should I put it down?
I like to do it between late spring and early fall. Put down a thick layer 4″-5” so it doesn’t blow away, but do it while you can place it around all your plants. Doing it in winter can risk smothering small, new plants when they are totally dormant and you can’t see where they are. If you do it while you can see them, you typically leave a little space by the stems. Leave a good margin (6” plus) around tree trunks, you don’t want too much moisture or insects right against the trunk.