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Small
planter, downtown Miami: Four foot wide planters surrounded by structural
fill. These trees will definitely blow over.
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It looks
like we may get through another hurricane season without a bad storm.
So far we have not had to deal with any blown over trees or what is known
in the arboricultural industry as windthrow. Well, this is certainly not
the time to be complacent. Now is the time to review and perhaps change
our bad planting and horticultural habits so we have less windthrow and
the resultant damage in the next storm.
It is always
interesting to hear about the reasons why trees fall down in a hurricane.
I hear quite often, “It wasn’t a native tree” or perhaps,
“that tree wasn’t in the right place”. Ever see a picture
of someplace in the Pacific that just had a typhoon? Did you see any trees
left standing? I’ll bet you did, and I would also wager that the
trees left standing were not native to South Florida. In Guam I have seen
some of their native plants like the palm, Heterospathe elata and the
Cycad, Cycas micronesica withstand a super typhoon (Category 5 in our
side of the planet). So what should you plant and how should you plant
it?
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The
arrows show the root flare and where the circling roots should be
cut off from this Pachira aquatica.
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First, what
anchors the tree to the ground? The roots of course; the root system is
what keeps the tree from blowing over. What do roots need the most? They
need oxygen and water which they seek out by growing through the tiny
pore spaces within and around the soil particles. When there is no more
oxygen or water, the roots stop growing and often start to die.
Ever see trees planted
on a newly constructed building site? The “soil” that they
most likely are planted into is the structural fill that was placed onto
the site and compacted before the construction began. The original fill
was taken out because there was too much organic material in it and/or
the rest of the soil was not homogenous in nature, meaning it will not
compact well. Structural fill has consistent particle size so that it
can be compacted to the point of having almost no voids between the soil
particles. This compacted structural fill is critical because it will
have to support the weight of the building, cars in the parking lot etc,
so that there will be minimal settlement of the structures. No one wants
cracked walls, saggy roof or a new parking lot full of potholes.
Well, what about the tree roots? If there is no oxygen or water because
the roots can not push through the compacted fill they will stay close
to the tree and hope that there is a great irrigation system and every
once in a while someone will throw some fertilizer on them. Often on landscaping
projects, “landscape soil” is thrown into the freshly dug
hole (in the structural fill) along with the tree. That’s great!
Now the roots are being encouraged to stay there.
In an act of desperation the roots usually work their way elsewhere like
underneath driveways, roads, curbs and sidewalks. Why is this? These aforementioned
structures are just laid upon the structural fill (for buildings and other
structures a foundation is dug into the structural fill). This leaves
an interface between the hard stuff (concrete and asphalt) and the structural
fill below. The interface is a space that provides oxygen and moisture
so that roots can grow underneath and beyond; hence bumpy roads and sidewalks
next to large trees. Up north with the changes in season the expansion
and contraction of concrete and asphalt is very noticeable (I recently
learned of frost heaves) and favorable to root growth. In south Florida,
not as dramatic but happens just as much. Roots also will follow pipes.
The soil isn’t compacted around pipes or conduits and if the pipes
leak water, the roots will be fat and happy and can then end up underneath
your house or building. They didn’t push through that structural
fill if it was properly compacted, so don’t blame the tree.
In the last few hurricanes
I am sure you saw trees blown over with a flat root plate still attached
and very few roots still in the ground. Parking lots were a great place
to see this. The roots were never able to penetrate the surrounding soil
because it was so compacted. The trees grew and maybe even had a large
canopy because of irrigation systems. When the winds got strong the tiny
root system could not support the movement of the canopy and the tree
succumbed to windthrow. Start thinking about planting spaces a minimum
of 10 feet wide with better soil.
Other horticultural
practices that often are the cause of windthrow, or just tree decline
and gradual death, are planting too deep and planting root bound trees.
When a tree or shrub is planted either in the ground or into a container,
the upper most roots that come directly out of the trunk need to be exposed,
just like in nature. This is called the root flare or root collar. It
will likely necessitate the removal of several inches to a foot of extra
soil from the top of the root ball before planting. Why do you not want
to plant too deep? It will be more difficult for oxygen to diffuse into
the ground that deep and less water will percolate to that area, which
will mean fewer roots to feed and hold up the tree. Also, the bark on
many species of trees and shrubs will not tolerate the excessive moisture
held against it by soil, or mulch, and rot will enter the tree.
Plants from nurseries
seem to do well planted deeply in a container but remember they used a
special soil mix with great oxygen and water holding capacity. This mix
is only for the short term and will eventually decompose. Ever take a
plant out of a pot only to find surface roots and roots circling around
the inside of the container and none in the middle. The roots had to go
where the oxygen was; on top and to the interface of the pot and soil.
The rest died most likely from the lack of oxygen.
Lastly I would like
to point out what happens to these circling roots that you noticed when
you pulled the plant out of the pot before planting it into another pot
or in the ground. Check out the photos with this article. It shows a root
that does not radiate out but crosses and may even circle around the trunk.
This root will never do anything to support the tree. It will eventually
damage the tree and definitely make it more vulnerable to windthrow. As
this root grows into the trunk, it will actually crush the xylem tissue.
The xylem tissue is the vascular system that brings water and nutrients
from the root system to the foliage. On larger trees with these circling
roots you will often see a bulge on the trunk just above the root. This
is where the sugars and food from the foliage that is passed back to the
root system via the phloem system has been clogged up. My recommendation
is to cut these circling roots before planting. The next time you see
one side of a tree in decline, start digging. And remember, all of these
issues have very little to do with the country of origin for a particular
tree species.
Jeff Shimonski
November 2007
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