Granite mulch on top of wood mulch - rot hazard?

[Dbroek]

Location: Phoenix Arizona.

I have water retention basins surrounding my low-water use trees, such as chinese elm, chinese pistache, mesquite, acacia, and texas mtn. laurel. These trees were planted from 15-gallon pots a few months ago. I have placed 4 to 6 inches of wood chips in the basins. The basins extend 10 to 20 feet from the base of the trees and receive periodic rainwater from gutters off the house rooftop.

I want to cover the chips in the basins with a 3 to 4 inch layer of colored granite, 3/4" or 1" in size. Why? Because it will have better aesthetic appeal and the wood chips won't blow onto the surrounding granite nearby.

Question: Will I be creating an aeration problem for the tree roots?

I realize that the granite will heat things up a bit on the surface, but I expect that the wood chips will insulate the heat coming from the granite above it and will allow the ground underneath to stay cooler in the summer and warmer in the winter. The wood mulch is mostly shredded wood and it is comprised mostly of large wood chunks with twigs. Not very fine at all and lots of air spaces in the mix.

BTW, the trees are planted at the original grade level and on islands within the basins. These islands are at least five feet in diameter, some up to eight feet wide. I expect that the wood mulch will decompose considerably within five years or so and that there will be enough earthworm and microorganism activity in the mulch underneath to make the soil more porous and less hazardous for root rot by the time the tree roots penetrate the area. That's my theory anyway, and it is not placed on any training whatsoever in agriculture or plant science. I'm just hoping I will not end up eventually killing my beautiful trees!


DBroek

 

[JesseHuffman]

Welcome to the buzz. I have been trying to push mulch application for trees here in the Valley for years.
I dont think you should have any problems with root rot or aeration so long as you allow soil to sufficiently dry between waterings. Are you depending solely on rain runnoff or do you have an irrigation system as well? From my experience, the Elm will reqire a bit more water than the others if you want it to thrive, One of my favorites by the way. They can get pretty big here.
Im sure you already know to be carefull about mulch at the root collar. Make sure your tree has a good flare to it at the base. Mulch migrating into the surrounding yards is probaly why most people won't apply it here. A good Monsoon storm can really make a mess of things. The rocks seem like a good idea but will you be reapplying mulch in the future? I geuss it would be easy enough to rake the rock back and reapply. Good luck with the trees and batton down the hatches. Storms any day now.

 

[Sylvia]

Db, I will state that I am a huge fan of mulch. The benefits it gives back to the soil and plants is tremendous.

Here comes the caveat. As with all things, there is a time and place.

Your Chinese elm, Ulmus parvifolia, will love the mulch and needs more moisture, liking moist, well-drained, fertile soil with an adaptability to extremes in pH.

Your mesquite is drought tolerant in the extreme, and giving it too much moisture may limit its rooting ability and reports are that windthrow is common in overwatered plants. As the native habitat of this plant is desert and harsh, I would be hesitant to mulch. Many desert plants are not adapted to handle mulch and, in fact, prefer crushed mineral. But the species and cultivars may vary, which do you have?

Texas Mtn Laurel...likes alkaline soil, well drained and on the dry side. In fact, one source states: "Soak the base of the plant deeply if the soil becomes dry just below the surface during the first year after planting. After the first year, little additional water is needed, as the Texas mountain laurel is very drought-tolerant." Again, too much of a good thing will go bad.

Granite weighs about 166 lbs per cubic foot. For a 5 ft diameter circle, this translates into about 1/2 ton of rock. I'm thinking you won't be shoveling this off very often.

I encourage you to look at each plant and its needs individually. One size does not fit all.

Sylvia

 

[Dbroek]

The four chilean mesquite trees I have all have mulch in the basins. I probably should hold off on laying granite on top of the wood chipped mulch and let it decompose naturally. The trees are now in the ground for the past nine months and thriving well with heavy growth. Maybe I should pull back on the watering now especially when I receive monsoon rain runoff into the basins anytime soon now.

Thw chinese elm and chinese pistache will be happy with the mulch in their basins so I won't worry about them. But the texas mtn laurel and one of the chilean mesquite trees have quite a deep layer of mulch surrounding them. This now worries me. I will probably need to be more mindful to water less often and depend on natural rainfall more than usual for the two mtn laurel and mesquite trees. Hate to remove all the mulch I put in there!

 

[Sylvia]

Db, the Chilean mesquite is quite adaptable in its environment, thriving from the extreme arid deserts to highly maintained turf landscapes. So I wouldn't be overly concerned about what you have done to date.

The biggest concern on the mesquite, in my readings, is the possibility of windthrow due to a heavily developed canopy with the lack of a sufficiently developed root system to anchor it. The first 2 to 3 years are the critical times for root development, which is true for most trees. Let their root system develop. Deep irrigation to encourage depth of rooting is good. The mulch you have already will help keep the moisture from evaporating too quickly.

If you are coming into your monsoon season, it will be getting sufficient moisture. You can then augment the water as necessary during future drought periods.

Sylvia

 

[JesseHuffman]

I have removed many mesquites that have the original drip system grown right into the rootball.
Its amazing that people refuse to move the water away from the base after its planted. The windthrow is also amplified once all the bottom branches are cut, not to mention what that does to lateral root growth.