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Re: [SHADEGARDENS]

Sheila,

IMHO the texture of the soil you place around and over a tree's root system
makes a difference.  If you plop sodden heavy clay subsoil over a tree's
roots and stomp it into a nice compacted mess, it will inhibit air from
percolating through the soil and injure the root system no matter how
shallow the layer is.

If you add a foot or more of soil of any kind you run the same risk.  We
don't tend to think of it, but soil has a lot of air in it and tree roots
need this and have placed themselves where they need to be to take
advantage of it.  Optimally one would not disturb soil levels around
existing mature trees, but this is seldom actually practical when you are
trying to make a garden.  However, making a flower bed around part of a
tree is different from substantially raising or lowering the grade level
under the entire perimeter of a tree's canopy.

If you add 6" or 8" of  *really* loose organic mix over the existing grade
you're not going to create a problem, especially if this is only on one
side of the tree or for a small percentage of the total tree root area.

What I'm talking about is a mix of things like compost (which will have
some soil in it), rotted wood chips, leafmould, pinebark fines, coarse sand
or grit.  This gives you something to plant perennials or annuals
into....more like a potting mix than "soil".  Their roots will work down
into the native soil under this mix if they want to.  I also take a garden
fork and disturb the native soil to encourage this.  Generally, you can't
actually "dig" in close proximity to a large tree, but you can work your
fork into the soil and jiggle it around as much as you can without tearing
out the tree roots.  If you tear a few feeder roots, it will not kill the
tree; the tree will make more.  You want to roughen  the existing soil up
as much as you can so that your "mix" will slip into the crevices and your
small plant's roots will follow.

I've done this for over 20 years under various maples, oaks, black locusts,
dogwoods, poplars, and other indigenous trees in my garden with no apparent
harm to the trees.  I have always considered that they were quite
enthusiastic about it by the way they sent their roots up to colonize the
mix.  But, the perennials that I plant in the mix have a chance to get
established before the tree roots take over.

Beds like these require watering in summer in my climate because the tree
roots will take the lion's share of all water and nutrients provided.

You have to take into consideration what kind of tree you're dealing with
and how its roots are structured and you have to avoid adding anything with
moisture holding capacity up around the bark of the tree to avoid rotting
it.

What you are describing does not sound to me like it should injure your
tree at all if you use good common sense about what you make your bed of.
If you have qualms, put down a layer of gravel an inch or so thick under
your infill, making the total no more than 6" or 8" thick.  This will aid
air flow in the soil and your small plants won't mind a bit...most plants
love gravel.

Marge Talt, zone 7 Maryland
mtalt@clark.net
Editor:  Gardening in Shade
http://www.suite101.com/frontpage/frontpage.cfm?topicID=222
Gardening Topic Index for Suite101:
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----------
> From: Michael D. Cook <mikecook@PIPELINE.COM>
> Date: Friday, February 20, 1998 1:24 PM
>
> Hi; I am not trying to beat a dead horse, but about tree root zones:
>
> OK, I am supposed to increase the soil level, should I desire to do so,
in
> a tree's root zone by no more than 1 or 2 inches per year.  Picture this:
> the tree has substantial knees, from which the soil level drops away,
maybe
> not dramatically, but at least significantly.  If I only increased the
soil
> level on about 1/3d the perimeter of the tree, in such a way as to not
> affect or cover the knees or trunk, and only in the first few feet
outward
> from the trunk, could I increase the soil level by 4-6" instead?  Then it
> has almost it's entire root zone left undisturbed, with just a yard-wide
> swath over one section.  I hope you all are not tired of talking about
> this; I really need to know.  Thanks.
>
> Sheila Smith
> mikecook@pipeline.com
> Z 5/6
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