IT’S GOOD TO LEARN HOW TO GARDEN THE OLD-FASHIONED WAY, WITH
knowledge passed down through generations—mostly. But much has changed in the
gardening world in the past decades. Horticulture science, the landscaping profession,
and the entire “green industry”—from hybridizers to growers to marketers—have evolved
and that evolution should influence what gardeners do on their properties, regardless of
Science now says: no more
Many people today remember learning to
cut off a broken tree limb flush with the trunk,
in a straight line. Others just cut tree limbs off
wherever the branches stick out too far. Or they
leave a broken branch stub where it broke, thinking
that trees can fix themselves. These are all wrong
In the 1970s, scientist Alex Shigo discovered
and explained that trees handle wounds using a
process called compartmentalization. It changed
forever how arborists and homeowners should
prune tree limbs. The process, known as CODIT
(Compartmentalization of Disease in Trees) works
like this: when a tree is wounded, certain cells are
spurred into action to form a series of walls around
the wound. Different kinds of cells plug tissues,
produce anti-fungal substances, and form barriers
so that fungal or bacterial infections can’t penetrate the tree beyond that wound site.
Simply put, the process seals off the wound.
Wrong: Cutting the limb flush with the trunk, or cutting a limb in the middle (leaving
a stub) prevents an effective sealing off process. Decay will proceed from the wound
through the whole plant.
Right: If a tree is pruned just outside the branch bark collar or branch bark ridge (the
thick part where a branch meets the trunk), the right cells are in the right place to begin
the compartmentalization process.
Science now says: soil is alive!
Former soil textbooks focused on the mineral components of soil and the differences
between sandy, loamy, clay, and various soil combinations. Farmers and gardeners knew
about adding organic matter—called humus in old textbooks—to improve soil texture,
and appreciated the aeration and decomposition that earthworms achieved. But soil
teachers weren’t talking about microorganisms, or mycorrhizae very much until (maybe)
the 1980s, depending upon where the learning was taking place.
Today, most gardeners have probably heard something about life in the soil—both
macroorganisms (visible creatures such as worms, beetles, and centipedes) and
microorganisms (invisible living things including bacteria, actinomycetes, fungi, algae,
and others). In gardening classes, students now learn to add compost and other forms of
organic matter to improve soil texture and to feed and stimulate soil organisms so that
they can provide nutrition for plants. This concept of soil life has changed how we treat
Wrong: Compacting soil by parking on it or walking on it when it’s wet is destructive
of texture and soil life; the organisms need air and moisture as well as organic matter.
Tilling soil until it is powdery is equally destructive. Many chemical fertilizers disrupt the
community of organisms in soil, as well.
Right: Since the soil is alive, stimulate
and increase that life by “feeding” it with
compost and other organic matter such
as manures and chopped leaves. Make
or purchase compost with an awareness
that all composts aren’t equal; some have
different communities of microorganisms.
( This is an intense learning area for
everyone, including compost experts and
Science now says: the roots are
not where you think they are
If you remember seeing Presidents
Kennedy and Nixon on a TV with rabbit
ears, you probably also remember your
schoolteacher showing an illustration of
a tree with a round top and an equally
round root structure below the soil line.
That was the belief, including the idea
As the garden
WHAT GARDENERS USED TO DO AND
THINK—AND WHAT WE KNOW NOW
BY SALLY CUNNINGHAM
If a tree is pruned
just outside the
branch bark collar
or branch bark ridge
(the thick part where
a branch meets the
trunk), the right
cells are in the right
place to begin the