This article was first published in the 2/16/2016 blog
Allelopathy is the name for what happens when one plant
releases chemicals that affects the growth of another plant. In theory this could be either a harmful or
beneficial effect, but in practice, when you hear people talking about
allelopathy, they are generally referring to harmful effects. This article is about plants which can have
negative effects on other plants that we have in the landscape or grow for
food.
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| Sunflowers can inhibit the growth of other plants |
There are many chemicals manufactured by plants for
their defense. They may keep animals
from eating them or the chemicals may be used to prevent competition from other
plants. These competition restraining
chemicals work in many ways, some prevent germination of seeds, some effect
growth of established plants, some change the soil pH, some may inhibit the
Mycorrhizal fungi that grow on plant roots and help the plant obtain water and
nutrients from the soil. Most allopathic
chemicals don’t affect all species of plants, although some plants have very
strong allopathic properties that affect many species. A plant’s chemical secretions can also affect
members of its own species, preventing new seedlings from germinating too close
to the original plant.
Plants release chemicals from their roots and leaf
tissues into the air and soil in gaseous and liquid discharges, and when
microbes decompose their remains. The
release or manufacture of some of those chemicals depends on environmental
conditions, such as heat, exposure to UV rays, availability of soil nutrients,
water, and whether plants are under stress from predators. Some chemicals may only be manufactured and
released at certain growth stages of the plants or in certain seasons. Most allopathic chemicals can linger in the
environment for at least a year after the plant is gone.
The protective chemicals that plants secrete are often
ones that humans use in herbal medications.
What’s bad for other plants is sometimes good for us. But sometimes allopathic chemicals can harm
animals and humans too. Researchers are also studying allopathic chemicals that
plants release to produce organic weed controls and possibly insecticides. But the study of allelopathy and the ability
to isolate and identify specific allopathic chemicals is a fairly recent one
and much more research on the subject needs to be done.
Our ancestors however, had knowledge of what plants
could help or hinder other plants growth and they often utilized them. Native Americans planted squash around their
corn and beans because the squash helped suppress weeds, both by blocking
sunlight and as we now know by producing allopathic chemicals that inhibited
weed germination and growth. Both corn
and beans are not affected by the squash chemicals. Sunflowers were also planted among other
crops and they too, have allopathic properties.
Early farmers soon learned about rotating crops, some crops leave allopathic
chemicals in the soil and the same crop will not grow well there the following
season. (Crops are also rotated because insect and disease can build up in soil
and some crops use more of certain minerals in the soil.) Farmers also planted cover crops like rye
between crops, because rye has chemicals which suppress many weed seeds from
germinating.
Allelopathy is often cited by those who want to remove
non-native plants from the environment.
Some plants new to an environment do inhibit the growth of plants
already present in the environment, sometimes to what seems to be an alarming
rate. But as recent research is finding
out, the allopathic properties of the invasive plants usually fade over
time. The soil microbes and other plants
develop immunity to the chemicals, and the alien plants adjust their chemical
manufacturing from all-out assault status to maintenance as they adjust to the
environment.
You as a gardener should be aware of what plants in the
landscape and in food growing gardens have allopathic properties so you can
give your plants the best environment possible.
Gardeners should also remember that allopathic chemicals can be found in
mulches, compost, and plant residue left from the previous season. These chemicals can affect plants for a year
and sometimes longer. In general not all
plant species will be affected by chemicals from a certain plant. And some plant species do not have known
allopathic properties.
Trees
in the landscape with allopathic properties
The black walnut is the most famous of allopathic
trees. Black walnuts release a chemical
called juglone, which inhibits the growth of many species of plants. This chemical is released from the roots,
from decaying leaves, twigs and nut husks, from wood or bark chipped for mulch,
and even when rain falls on the tree canopy.
Since the root system of trees can extend 3 or more times the width of
the tree canopy, black walnuts can affect plants a good distance away from
them. Plants in the nightshade family,
such as tomatoes, peppers and potatoes are greatly affected by juglone. Their seeds won’t germinate well in soil with
juglone, and plants transplanted into soil contaminated with juglone will start
to grow, then quickly yellow and die as they enter maturity.
Other plants affected by juglone include: apple,
azalea, birch, blackberry, blueberry, chrysanthemum, colchicum, forget-me-not,
grapes, lily-of-the-valley, linden, mountain laurel, peony, pines,
rhododendron, and thyme. Some other
cultivated plants may also be affected.
Horse owners will also want to keep black walnut trees out of pastures
as the leaves and nuts can make horses founder.
Black walnut nut husks are poisonous to dogs if they are consumed.
Other trees that affect the growth of some other plants include; Ailanthus Tree of Heaven (Ailanthus
altissima), balsam popular, black cherry, box elder, hackberry, juniper, oaks, red maple, sassafras, sugar maple, and sycamore (planetree). The effect of these trees is most often seen
on other trees, but they also inhibit some grasses and weeds from growing near
them. Silver maples and honey locust
both inhibit the growth of Kentucky bluegrass.
Shrubs and other woodies that have allopathic chemicals
include bearberry, elderberry, forsythia intermedia, mountain laurel (Kalmia
angustifolia), rhododendron, and sumac.
Pines and spruces and particularly Douglas firs are inhibited by many of
these shrubs. Interestingly since these
are both common in the landscape, Kentucky bluegrass and forsythia inhibit each
other’s growth.
Grasses
and other plants
Grasses that be a problem for some plants include;
Kentucky bluegrass, bentgrass, perennial rye, foxtail, tall and red fescue, and
most of these are common in lawns.
Kentucky bluegrass and red fescue affect azaleas, barberry, forsythia,
flowering dogwood, and yews. Bentgrass
can affect azalea, barberry, yew, and forsythia. Perennial rye affects apple, forsythia, and
flowering dogwood. Tall fescue can
inhibit black walnut growth, which might be a good thing in some places.
Other garden plants that have allopathic chemicals
include most ferns, asters, goldenrod, fennel, lantana, and sunflowers. The bare spot under the bird feeder where
sunflower husks pile up is due to both the mulch effect and allopathic
chemicals. Sunflower roots also inhibit
many plant species and should be used cautiously in mixed borders.
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| Lantana can affect the growth of other plants |
Ferns often inhibit broad leaved plants but monocots,(grasses
and things like lilies) are often not affected. Be cautious using lantana in
containers because many plants do not grow well with it. Goldenrod and perennial asters also suppress
the growth of other plants. Fennels are
being used as ornamental plants now but be aware that many plants do not grow
well near fennel. Geraniums should not
be combined with ornamental peppers or eggplants as they inhibit the growth of
those plants.
In the vegetable garden plants in the cabbage family –
broccoli, Brussels sprouts, cabbage and so on inhibit each other’s growth and
should not be planted following each other and kept separated in the
garden. Cabbage family members also
stunt strawberry plants. Mustard grown
for greens can inhibit the growth of many other plants. Never plant potatoes near sunflowers. In fact sunflowers should probably be grown
in a patch of their own and not mixed with vegetables as they inhibit many
plants.
As mentioned earlier squash- and pumpkins can affect
other plants. They should not be planted
near potatoes, peppers, or tomatoes. Garlic and onions will affect the growth
of beans and peas. Dill will stunt the
growth of tomatoes. Dill should also be
kept away from carrots. Cucumbers may be
affected by sage and other mint family members.
Mint family members and onions should not be grown near asparagus. Fennel should be separated from all other
plants as it is highly allopathic.
When growing food crops you should always rotate crop
families from year to year. Many food
crops produce allopathic chemicals that linger in the soil that affect the same
crop next year even if debris is completely removed and similar plants also use
soil nutrients in the same way, causing deficiencies if the same crop
follows. See the list of
incompatibilities above and avoid planting those crops after an incompatible crop
when rotating crops.
Weeds often have allopathic chemicals and that’s why
they are so successful. It goes without
saying that weeds should be removed from food and flower beds.
As mentioned the science of allelopathy is still new. Some effects we suspect may be due to allelopathy
because of observation may eventually be found to be caused by other
things. If you would like to read more
about allelopathy here are some links.
Some of these references were used to prepare this article.
http://edis.ifas.ufl.edu/hs186 Includes table of allelopathic interactions
http://www.extension.umn.edu/garden/landscaping/implement/trees_turf.html another table of allelopathic interactions
http://www.dmu.dk/1_viden/2_publikationer/3_fagrapporter/rapporter/fr315.pdf Comprehensive review of allelopathy from
Denmark.
http://www.jes2s.com/pdfs/qin_article.pdf Results of an experiment of juglone from
black walnuts on seed germination.
Thank you. That is one of the best articles I have read on the subject. Very informative!
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