Date: Wed, 13 Jun 2001 09:34:57
-0700
From: ron <ron@NVWETLANDS.COM>
Reply-To: BB for Hummingbirds and Gardening for them in the Southeast
<HUMNET-L@listserv.lsu.edu>
To: HUMNET-L@listserv.lsu.edu
Subject: Re: [HUMNET-L] root rot
Van,
My guess is that what you are seeing is your yard developing anoxic
soil
conditions which are killing (suffocating) the roots of your plants.
I don't
think there is necessarily any fungus or bacterium involved in
the problem,
although they may attack the dead roots after they are already
dead. I don't
think there's anything you can do - at least I have never found
anything to
be done The reason you are seeing the wilt first in the sunniest
areas is
that they are wilting there first due to sun, not because those
plants have
lost more roots than others. I would guess the lowest lying parts
of your
yard and the soils with most clay will be hardest hit.
Here's how it works. Soil is
made up of pieces of solid material (sand, silt,
organic matter, etc), and lots of empty spaces (voids). The voids
are
partially filled with water and partially filled with air in a
typical upland
setting like your garden before a tropical storm. The water mostly
clings to
the sides of the void spaces, while air fills the centers of the
voids. The
roots of all plants need oxygen to function, just like animals
do. Most
plants get this oxygen directly from the air spaces in the soil.
In a
typical, dry to moist soil there are enough air spaces that they
are
interconnected, and the oxygen used up by roots is quickly replaced
by oxygen
diffusing through the interconnect air-filled void spaces from
the open air
above the soil. Oxygen diffuses very rapidly through other gases,
like air,
so there is no shortage of oxygen even fairly deep in the soil.
If a soil
gets too much water in it, the water begins to fill up the void
spaces rather
than simply clinging to the sides of the void spaces as it does
under typical
soil moisture levels. Once water fills enough of the void spaces,
oxygen can
no longer diffuse rapidly through void spaces filled with air,
it has todiffuse
through water, which occurs at a rate something like 1 / 10,000
of
the rate of diffusion through air. So in saturated soils, oxygen
supply is
essentially cut off. The plants continue to use up oxygen, but
the oxygen is
not replaced (or replaced very very slowly) so soon the soil becomes
anoxic,
or lacking in oxygen. This means that roots suffocate and die.
This seems to
take a day or two or three, based on my experience with overly
wet gardens
(I once lived in louisiana, land of the overly wet garden)
Soil particles are named by size:
sand is larger than silt is larger than
clay. Smaller particles have smaller sized voids, although the
total volume
of void space in a clay soil is about the same as in a sandy soil.
The
difference is that the smaller clay voids are not all connected
to each
other. The small voids and lack of interconnection means that
clay soils hold
water better than sands or silts, and they are more prone to developing
conditions that limit oxygen diffusion, since the poresare smaller
and aren't
well conected to each other in the first place. This is why I
said clays are
more prone to anoxic conditions than other soils, but even pure
sand can tanoxic
if it is underwater too long. If it is raining for a week, there
are
probably short intervals of drier weather, and during some of
these, some of
the soils in your garden might briefly drain enough to allow oxygen
to
diffuse into the soil. Diffusion through air could quickly return
a soil to
an oxygenated condition, and reset the 1 to 3 day oxygen depletion
clock.
This might mean that certain portions of your yard that are slightly
better
drained due to topographic setting or oarser soils might never
go anoxic,
while nearby spots might be anoxic. There is apparently some variation
between plants, and some tolerate anoxia better than others, even
though most
of our garden plants can't really tolerate anoxic conditions.
Some plants can live in permanently
anoxic soils. Anoxic soils are one of the
defining characteristics of wetlands. Wetland plants get oxygen
to their
roots through their stems. If you cut the stem/leaf of a cattail
or reed (or
iris or canna) you can see a series of tubular structures running
vertically
in the stem. These tubes (calles arenchyma, I think) allow oxygen
to diffuse
through the leaf and down into the roots, allowing the roots to
survive and
function without receiving oxygen from the soil. A few hummingbird
plants are
adapted for wetlands, including iris fulva, canna indica (I'm
not sure about
hybrid canna x generalis), cardinal flower - lobelia cardinalis
-, mimulus
species, maybe bog sage - salvia uliginosa. I'm sure there are
others, too.
Once a plant is impacted by root
rot, there's nothing I know of to do but
hope. It may be that only the deeper roots were killed, and so
the plant is
suffering from a reduction in roots, not a total loss. Such plants
should
recover and might benefit from a light pruning so that they don't
wilt too
badly. Other plants seem to suffer a long death, but inevitably
do die. My
guess is that any microbes (fungi, bacteria) that attack the roots
do so
after the problem arises, and are more of a symptom than a disease.
Van Remsen wrote:
> HUMNET: is there anything
one can do about root rot? Even after a storm
> total of 23.4 inches, garden drainage seemed fine. As usual
after> extended cloudy, rainy weather, lots of plants wilted
slightly with first
> sun. However, a bunch of my best stuff has never recovered,
mainly Costa
> Rica Blue salvia, Salvia mexicana, Abutilon pictum, and Cestrums.
The
> worst cases seem to be in the sunniest areas. I assume that
this is some
> sort of fungal root rot.
>
> Any Real Gardeners out there have solutions? Cut'em back
ASAP? Use
> fungicide (ugh)?
>
> #################################
>
> Van Remsen,
> LSU Museum of Natural Science,
> najames@unix1.sncc.lsu.edu
===========================================================
Date: Wed, 13 Jun 2001 11:03:12
-0700
From: ron <ron@NVWETLANDS.COM>
Reply-To: BB for Hummingbirds and Gardening for them in the Southeast
<HUMNET-L@listserv.lsu.edu>
To: HUMNET-L@listserv.lsu.edu
Subject: Re: [HUMNET-L] root rot
Van, I think that loosening /
poking holes in the soil could help, especially
if done BEFORE damage occurs. If the rain had let up enough that
you could poke holes that would be filled with air, not water,
then poking holes would have been a good idea. Instead of expecting
oxygen to siffuse all the way from the surface, you could allow
it to diffuse only a few inches laterally from the hole to the
roots. For those plants that have lots of smaller roots, I think
any damage done by poking holes all around them would be offset
by benefits. Plants with fewer, largerroots might suffer more
if you cut a root. I don't really know which plants have lots
of small roots, vs fewer large roots, or even if there is such
a difference
between plants. At any rate, I think that once the damage is done,
recovery is slow.
Ron Rovansek
Reno, NV - land of well aerated soils.
=======================================================
Date: Thu, 14 Jun 2001 07:04:30
-0500
From: Tracey Banowetz <banowetz@bellsouth.net>
To: HUMNET-L@listserv.lsu.edu
Subject: [HUMNET-L] [Fwd: Helpful Information for Clients]
Re. the "root rot"
thread of yesterday, I am forwarding the following
message that Baton Rouge Master Gardeners received from County
Agent Bob
Souvestre. The original post is from Dr. Tom Koske. While he refers
mainly to tomatoes and peppers, I imagine the same scenarior is
possible
for perennials as well.
Regards,
Tracey Banowetz
Weyanoke, LA
Bob Souvestre wrote:
> This is from Tom Koske:
>
> after this saturating rain, we should expect
> many plants to wilt and then scald out. especially
> prone to this is peppers and tomatoes. the
> root system is sitting in a low oxygen mud
> because water has displaced the soil atmosphere.
> in this condition, the roots do not work well-
> neither would you. as such they do not take
> up water as well and when a evaporation
> demand is placed on the plants, they show
> drought stress even though their 'feet' are in
> watery mud. as long as it is cloudy and humid,
> the plants hold up but as soon as the sun shines
> strongly, it's over. this physiological wilt then
> slows transpiration. without this evaporation
> from leaf surface, leave burn-up/scald and
> die. once dead: always dead. then to add insult,
> that lack of foliage cover sunburns the fruit
> present. thus pick what u can because 'game over'.
===========================================
Below is Tom's reply to my inquiry about aerating the soil
around wilted plants. Brief and to the point....
Tracey Banowetz
Tom Koske wrote: yes
my salvias are mostly lost-same
reason.
those w some live foliage may slowly recover
but will look bad for a long time- w perennials
that's ok but annuals? trim the dead-what's dead
is dead. arearate/fork up the soil is not a bad
idea to spur recovery if not too far gone. tomk