over-mulching and planting under trees?

bstewert

bstewert

TreeHouser
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Portland, OR
Does anyone have a good article I can present to homeowners on proper mulching around the root flare on trees? I see it occasionally where the compost is piled up high. Shrubs, too.

Also, something on planting shrubs or trees right under the tree. Seems like not such a good idea to me. Here, our neighbor trenched into the root zone to put sprinklers, then planted three 6' baby firs under this 160' fir. These little grass shrubs are not so bad, I guess, but sometimes people will plant rhodys and such just a few feet from the base. It grows like that in the wild, so maybe it's OK?


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I discourage the understory planting. People are addicted to it. You can't stop it. I try to settle for nothing 2 ft from the trunk and strive for succulents that don't need 4x/week watering.


love
nick
 
ps. keep the irrigation from spraying the trunks.

My guideline for people is the rule of a hand's width. About a hand's with deep in mulch (3-4"), and a hand's width of bare soil around the trunk to discourage growth of potential stem-girdling roots.
 
Definitely keep irrigation away from the root crown to prevent rotting. Be sure to underplant with shade and drought resistant plants.
 
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  • #7
I forgot to mention there are power lines directly above that these new trees will be growing into. The big one has to be pruned back, as it is.
 
I dont have any issue with the understory, except if like other said its causing over watering from the tree's perspective.

We need to just continually teach about proper mulching. Sean's hand rule seems simple enough. I usually explain the concerns of over mulching, I.e. lack of oxygen, and moisture/insect/creature habitat on the trunk. Then I follow up with 4-6" max of chips, and taped to 0" from a 6" radius of the trunk. If your using a finer particulate mulch (reduced pore space) such as compost, 2-3" is max due to reduced oxygen getting down to the roots.

EDIT: trenching sounds like a bad idea, if the trench's are necessary, it sounds like an application for an airspace to ensure minimal impact to the roots.
 
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  • #9
No, they chopped through the roots for the sprinklers. This was a spec house, get in and out as fast and cheap as possible.

Mainly, I was wondering about compost (the super black/rich stuff). We had some guys spread a couple of units around our yard, and in many cases, it was 3-6" up around the trunks of established trees and shrubs. So I am going around, pulling it back away from where the trunk meets the dirt.
 
bstewert said:
.....Mainly, I was wondering about compost (the super black/rich stuff)....
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The example you showed looks like it was poorly thought out and executed but a well done understorey planting of compatible specimens is not only beautiful but beneficial. The activity and bio diversity below ground is a reflection of whats above ground. We always encourage people to plant tree and perennial islands to separate those from the turf grass.

The rich black compost that is currently popular as a top dressing might look beautiful but is totally wrong. You will never see it on the surface in a natural setting, it is the layer that separates the mineral base soil from the upper litter layer and that is were it belongs. Too much oxygen, water and sun will quickly burn up all of its wonderful properties leaving behind a fine particle mass that is of little use.
 
Evan, you touch on a good point about mulches and porosity. Screened mulch seems to compact/ mat too much by comparison to variable sized, larger, coarser chips. My experience is with CnD chips as mulch, largely.

My guess is that it might resist compaction better than screened mulch. Thoughts?
 
Some people think that they need to "freshed up" the mulch year after year. A little raking will fluff, and make more uniform, once again.
 
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DMc said:
The rich black compost that is currently popular as a top dressing might look beautiful but is totally wrong. You will never see it on the surface in a natural setting, it is the layer that separates the mineral base soil from the upper litter layer and that is were it belongs. Too much oxygen, water and sun will quickly burn up all of its wonderful properties leaving behind a fine particle mass that is of little use.
Click to expand...

Dave, that surprises me that you would say it's totally wrong. I agree it's not what a natural setting looks like, but neither is a lawn. We have 39 rhodys, some are 10" at the base and 16' tall, plus azaleas, and other shrubs. All have done better with the compost mulch, and the battle with weeds is considerably reduced. I like the look, especially after the rain. But I would agree that the natural drop stuff is probably best for the plants.
 
I'm envious, Bob. It sounds like you have a lovely garden and I have no doubt that your azaleas and rhododendrons love having that much compost applied. Have you ever tried making compost? If so you will know that it takes careful control of air, water and temperature. Compost is decomposing because of microbial activities, converting organic material into a more elemental form that plants can use. When taken out of its incubation blanket and placed on the surface of the ground, most of the microbes quickly die, some do get leached to a deeper more stable level and continue doing what they do. The rich black compost that you purchased, depending on weather conditions, will within a few days become a shell of what it once was.

Your plants would get even more benefit and for a longer period of time if you used less compost and lightly worked it into the soil and than covered that with a coarser shredded mulch as a top dressing. You are right, it is the contrasting dark color and the rich smell that make compost an easy sell for a top dressing but it is just not the way it was meant to be.
 
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  • #16
It's a trade off, Dave. Personally, I believe the natural collection of debris is best, but it's a hard sell to most people in the city. They think a pile of leaves means you aren't keeping up the yard. So it's a looks thing, for sure, but it seems to help on our plants. We have awful dirt. I'm surprised anything grows. You think the compost top dressing is a waste of time, huh? Maybe I should rethink it.
 
Bob, I think of compost as a soil amendment. As Dave is saying, the soil should be alive, with the living compost.. I see compost used as mulch here, too.

Having some good pictures of what the final chip mulch product is like can help. Portland surely has keeping up with the strait-laced Jones' people, but a lot of the Portland Joneses have city chickens, edible landscapes, eco-this and that and the like, right?
 
I try to think of wood chips and course material are more like a sponge, porous and holding/releasing/helping produce microbes and food in a time release fashion. Slowly turning to dirt. Compost is almost dirt, releases it's benefiting nutrients faster and such. Mixing the two and adding worm castings is prime. Chips to cover.
 
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frothy biochar compost.jpg

This mulch also has compost on top, along with biochar, and looks too deep.

Reminds me of a sandwich with bread on the inside.
 
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