Oct 15, 2010 As part of Blog-Action Day on Water
Water Science and Gardening
by Ginny Stibolt
Without water life, as we know it, would not exist. Plants and
animals contain high percentages of water and depend upon its unique
properties to survive. Humans are attracted to bodies of water for
both their beauty and their usefulness for many aspects of our
What makes water special and why do we gardeners
Water is made up of two hydrogen atoms and one
oxygen atom giving it the familiar chemical formula of H20.
The hydrogen atoms attach themselves to one side of the oxygen, covering
about 1/3 of a circle, and so that the molecules look very much like Mickey Mouse ears. The side of the
molecule with the hydrogen atoms has a slight positive charge and
the oxygen side is slightly negative. Water molecules act like
little magnets and are attracted to each other and form weak bonds, called hydrogen
bonds. You notice this self-attraction, called
cohesion, when water beads up into droplets as seen on my
sunflower petals in the morning light in the photo above.
Water's polarity makes it a good solvent that can break
apart, absorb, and carry organic materials such as sugars, other carbohydrates,
Water is highly unusual as well, because it can exist simultaneously as
a gas (water vapor), liquid. and a solid (ice). The solid form is
unique because it is less dense than the liquid due to its crystalline
structure and floats on the liquid. Water readily evaporates into the
atmosphere and gathers as mist or clouds.
Knowing how plants deal with and take advantage of water's unique
chemistry makes us better gardeners. So I thought I'd take
advantage of the Blog-Action Day topic of water this year and cover some
of the most important plant-water interactions.
Osmosis and root hairs
Starting at the bottom, near the tip of the roots, there are
thousands of single-cell extensions on the root's surface called root
hairs that absorb
water from the soil. Root-hair cells have a semi-permeable cell membrane that allows water and
some materials that are dissolved in water, such as nutrients, to
flow into the cell. Everything
else is blocked from entry into the plant. Water will equalize
over an area and when there is less water within the root-hair cells
than in the surrounding soil, then the water flows across the cell
membrane to equalize that pressure. Once water fills the
root hair cells, it flows into neighboring cells and builds up pressure
that pushes water up into the plant; this is called root pressure.
Then the process of transpiration, as discussed below, carries the water
farther up the plant.
Root hairs usually last just a few
weeks before they are reabsorbed into the root tissue--a root must be
growing in order to develop new root hairs. As gardeners we
need to keep in mind when irrigating plants, we need to supply water to
the region where new roots are growing. Roots of
established trees and shrubs
might be growing yards away from the trunks or stems.
When a plant is transplanted, most of its delicate root hairs are rubbed
off as soil falls away from roots, so these plants need a lot of
water in the planting hole and frequent irrigation until new root
growth begins and the plant regains its network of root hairs.
The larger the plant, the longer the time period when additional
irrigation is necessary for the establishment of that plant.
For irrigation details and other transplanting information see Trees
and Shrubs: the "Bones" of Your Landscape. Good gardeners want
their plants to survive, so we need to pay close attention during
their establishment period. This is true for drought-tolerant
plants as well--you can't plant it and forget it. A drought-tolerant
plant needs the same TLC after transplanting as other plants.
It only becomes truly drought tolerant after it's fully established
in its new location.
Before we leave water's interaction with roots and soil, let's kill
an old gardeners' tale. For years we've been advised to lay a
thick layer of gravel or potshards in the bottom of our pots and
containers to aid drainage. It was shown 100 years ago that this
is false, but even today, so-called gardening experts and master
gardeners continue to pass this myth on as if it were fact.
Because of water's tendency to hang together, it will be reluctant to
jump the gaps created a coarse medium such as gravel. It won't
move away from the fine medium of soil until it's completely
saturated. In order to create the best drainage in your container
gardens, use taller pots and use all soil with just a screen or leaves
covering the drainage holes. You can test for the effectiveness of
the taller pots with a simple cellulose kitchen sponge: Completely
saturate the sponge and hold it in the horizontal position over the sink
until it stops dripping. Then turn it so that it is vertical and after a short delay the sponge will give up more of its
water. Isn't science fun when you can apply it in your gardening?
After water enters the root tissues, there is pressure that begins to
push the water higher in the plant. But it would not go very far up the
water-carrying cells in plants, called xylem, without the suction effect
evaporation through the pores in the leaves and stems called stomata.
For many years it was thought that capillary action (movement of a liquid up a narrow tube)
accounted for much of this movement, but more advanced measuring
techniques show that capillary action is not a major factor in water
movement through the xylem.
More than 90% of the water that enters a plant runs straight through
it and evaporates
into the air. A full-grown oak tree could transpire more than 400
gallons of water on a summer day. The larger the plant, the higher
the volume of transpiration. The area near large plants is cooled by
the transpiration process and on a hot day; the temperatures may be up
to 20 degrees cooler than nearby spaces without large plants. As
gardeners, we use this information in
designing our outdoor spaces and placing plants near buildings to reduce
the need for air conditioning.
The transpiration rate is also
important in planting our rain gardens.
gardens are designed to collect rainwater in swales. The water will be
both absorbed by
the plants and also percolate into the soil to refresh our aquifers. A
rain garden should not have standing water for more than 3 days--the
more biomass in the rain garden plants, the higher the transpiration
rate and the faster water is sucked from the soil.
The remaining 10% of the water that is absorbed into the plant's cells serves
several purposes: it carries nutrients, it keeps the cells turgid and it
will also be used by the
plant for photosynthesis during the
daylight. When the soil is dry, this whole process slows
down. The guard cells around each stomate are highly sensitive to
water supply and when they become flaccid, the stomata close up, and
the evaporation of water is slowed to protect against severe
wilting. Gardeners need to pay attention to the wilting of plants--especially seedlings--and irrigate before permanent damage is done to the
When the temperature lowers at night, less water evaporates into the
air. Nighttime temperatures put the brakes on the transpiration
rate, but in many types of plants, the root pressure is not immediately reduced, so there is a flow of water from the leaves at night to relieve
the pressure. This is called guttation.
The water is excreted as liquid through specialized pores called
hydathodes at the ends of veins. (The strawberry leaf in this
photo illustrates this process.) If the plant is a salt tolerant plant,
you can often see a build up of salt crystals near these
This excreted water is often confused with dew, but the source of the
water is not the same. The water from guttation comes through the
plant, while dew is formed when water vapor in the air condenses on
plant surfaces in the cool night air. Dewdrops form randomly on
plants and will not form neat droplets at the ends of veins or at the
tips of narrow blade-like leaves. Some of the
water--both from dew and guttation--on plant surfaces may be absorbed
through stomata on plant surfaces the next morning
when the temperature rises again, but most of it will either drop to the
ground or evaporate into the air.
As gardeners, it's good to be aware of this cycle of water as we
keep a close watch on our plants. Most people recommend that
we irrigate, if needed, in the morning so the plants have all day
to cycle the water, dry plant surfaces at night are less vulnerable
to fungus attacks, and so that less water is lost to evaporation
during irrigation. Also, if you water at night much of the
water may be out of reach of the roots by the time the plant is
ready to restart the transpiration cycle.
Water drops act like little magnifying lenses or prisms and if you
look closely you can
often see rainbows in the drops. They are beautiful, and contrary
to the old gardeners' tale, the water
drops on leaves in the full sun will not burn the leaf tissue. So
you may irrigate without fear in the
middle of the day if your plants--again particularly seedlings--are
wilting in the Florida heat.
This magical process wherein green plants combine carbon dioxide (CO2)
and water (H2O) with energy from sunlight to form sugar (C6H12O6)
and oxygen gas (O2). For
most vascular plants, the water is supplied by transpiration flow and the
carbon dioxide is available from the air through the open stomata. (Respiration is
the equal and opposite chemical reaction, and all organisms respire as
they gain energy for living.)
When it gets too hot and plants close up their stomata to preserve
water, most plants cannot continue photosynthesizing because they don't
have ready access to water and carbon dioxide. Some heat-loving
plants have adapted to make better use of the sunlight in the
heat. Read The Science Behind Southern Grasses, Including Turf
to learn how they do it.
Water: it's a limited resource
Before we knew better we
wasted much of our water and used up or spoiled many of our water resources. Water,
especially usable fresh water, is a limited resource and as gardeners we
can do our part and create water-wise landscapes.
For a good review of Florida's water resources, see theFlorida
Native Plant Society's post We
All Live in A Watershed, which is also part of this
Blog-Action Day on water.
Ginny Stibolt would like to hear from readers who have suggestions
and questions. After all, there are more than a few transplanted
gardeners Florida trying to figure out what works and what doesn’t
in planting zone 8/9. She's wrote, "Sustainable Gardening for
Florida," published by University Press of Florida that was
released in 2009. Now she's written "Organic Methods for Growing
Vegetables in Florida" with Melissa Contreras in Miami. The
new book was released in Feb 2013. You may contact her or read extra
details on her articles and other information posted on her website:
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