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University
of Rhode Island GreenShare Factsheets
Companion
Planting
Most
people think of plants as very passive organisms. They grow almost
unperceptively, and only once a year do they flower or produce edible
products. However, plants are very active in ways that are not so
obvious to the casual observer. Plants both change the chemistry
of the soil and influence the types of microorganisms that grow
around them. Plants actively compete with other plants for space.
Some will poison their neighbor's offspring to maintain a competitive
advantage, while others change the environment in ways that benefit
other species. Plants wage a constant battle with insects, relying
heavily on chemical warfare. Naturalists
have known about these properties of plants for thousands of years.
About 2,000 years ago the Roman agriculturalist Varro, declared "Large walnut trees close by, make the border of the farm sterile."
Chemicals in oak leaves retard the development of insects that feed
on them. Some insecticides are derived from plants; examples include
rotenone, sabadilla and ryania. Not all effects of plants are deleterious
on other organisms. Alfalfa and clover enrich the soil with nitrogen
that they capture from the air. Certain trees move groundwater to
the soil surface where shallow-rooted plants can grow even under
drought conditions. Groups of plants which grow well together are
called "companions." Perhaps
the best historical example of companion planting is the "Three
Sisters" in which corn, beans, and squash are planted together
in a hill. Native Americans developed this system to provide food
for
a balanced diet from a single plot of land. Each of the crops is
compatible with the others in some way. The tall corn stalks provide
a support structure for the climbing beans. The beans do not compete
strongly with the corn for nutrients since as legumes, they can
supply their own nitrogen. Squash provides a dense ground cover
that shades out many weeds which otherwise would compete with the
corn and beans. Modern
agriculture tends to rely heavily upon specialized machinery and
synthetic inputs, and have rendered companion systems such
as the "Three Sisters" obsolete. Obviously, it would be difficult
to harvest corn, beans and squash simultaneously with a machine,
especially
when they are not planted separately in rows. However, interest
is growing in using these special properties of plants to our
advantage
when growing food. Home gardeners, unencumbered by the need for
specialized equipment or row crop production, have rediscovered
some of the beneficial interrelationships among plants. This
knowledge,
coupled with a long tradition of folklore, is being utilized to
improve home garden production.
Certain cover crops concentrate specific nutrients in their tissues.
Deeply rooted plants move nutrients from the subsoil to the aboveground
parts, and when the plants decompose, the nutrients become available
for subsequent crops. Potassium levels can be increased significantly
by selecting a good preplant cover crop. Buckwheat, grain rye, and
sudangrass are good preplant covers. Plants in the legume
family are capable of gathering unusable nitrogen from the
air and converting it into usable nitrogen in root nodules,
with the help of special bacteria. Legumes increase soil fertility
as they decompose, thus releasing the stored nitrogen. An alfalfa
sod that is plowed under will provide 150 to 200 lbs of nitrogen
per acre the following year, 60 to 80 lbs the year after, and
30
to 40 lbs the year after that. In fact, any cover crop that is
plowed under will release nitrogen as the crop decomposes.
This is the
origin of the term "green manure." Many
plants produce substances that are toxic to other plants. The study
of this phenomenon is called "allelopathy." Juglone, for example,
is a natural herbicide produced by the roots of walnut trees.
Many
plants have allelopathic effects, including sunflowers, cucumbers,
oats, alfalfa, rye and tobacco. When these crops are planted
prior
to other crops, weed pressure is reduced. Maple
trees can move groundwater from their lower roots to the upper roots,
where it is exuded into the soil. Herbaceous plants can use this
groundwater when conditions are dry. Shade tolerant plants often
grow better under the trees than away from them. Certain
garden plants grow better if provided with some shade, while others
need to be elevated above the ground to capture sunlight. Leaf lettuce
grows well in the shade provided by taller crops. Rhododendrons
and azaleas thrive under pine trees. Corn growers will often seed
clover between rows so it will germinate after the corn is established.
The clover grows throughout the fall and winter after the corn is
harvested, increasing soil nitrogen when it decomposes the following
spring. Grasses
often are planted between rows of perennial crops such as fruit
trees. The grass alleys cool the soil, prevent erosion, improve
water penetration, exclude weeds and harbor beneficial insects.
Most
plants produce defensive chemicals that help fend off insects and
diseases. These chemicals may be insect poisons, feeding deterrents
or have fungicidal properties. The roots of some French and African
marigolds contain a substance which is toxic to certain types of
nematodes. Nematodes are soil inhabiting microscopic roundworms
that damage many species of plants. Certain nematodes can be eliminated
from a site by growing a thick crop of marigolds for one season
prior to planting the vegetable or fruit crop, or by interplanting
marigolds between crop rows. Destructive
insects often locate their food by smell. Many plants, especially
culinary herbs, produce strong scents which may confuse insect pests
looking for a host to feed on. Garden vegetable plants such as garlic,
onions, chives, and herbs such as catnip, horehound, wormwood, basil,
tansy, and mints all produce scents which seem to repel insects
or mask the scents which attract insects. A certain level of insect
protection can be achieved by carefully interplanting some of these
as companions to vegetables. Many
insect pests have specific food preferences while others feed on
a wide assortment of hosts. Even those species which feed on
a wide
variety of hosts, such as Japanese beetles, have preferences for
certain plants. It is possible to plant a preferred host as
a trap
crop near the plant that is being protected. Once the insects have
settled on the "trap" crop, they can be killed periodically
by spraying, without having to treat the protected plants.
Many insects are helpful because they eat or parasitize harmful
insects. Most species of wasps and spiders are beneficial as are
ground beetles, praying mantids, lady bugs, pirate bugs, and several
species of flies. It is possible to attract beneficial insects by
planting flowers near the garden. Dill, parsley, carrot, coriander,
angelica, and parsnip feature flat topped clusters of small flowers
that have strong fragrances. They also seem to attract large numbers
of beneficial insects, particularly predatory wasps and flies. This
characteristic makes them good candidates for companion planting. Avoid
monoculture in terms of space and time. A one-hundred foot long
row of broccoli presents a large target for a cabbage moth that
is flying by, but the same number of cabbage plants scattered over
several thousand square feet, and interplanted with other crops,
is less obvious and attractive to the insect. Pests which routinely
plague large, commercial plantings of crops may never be a problem
in the diversified home garden. Learn
to recognize beneficial insects as well as pests, and note which
plants are attractive to beneficial insects. Less than 1% of insects
are garden pests. Plant dill, marigolds, chives, onions, parsley,
basil and other flowers throughout the garden. Allow parsley, carrot
and celery to remain in the ground over the winter. They will produce
flowers the second season and attract beneficial insects. Plant
strong smelling herbs among vegetable crops. Some examples are:
• Chives planted at the base of roses may repel aphids • Garlic could
be planted at the base of peach trees to repel borers • Basil planted
among tomatoes may repel tomato hornworms • Nasturtiums
grown near squash may repel squash bugs • Tomatoes planted
among asparagus may repel asparagus beetles • Marigolds,
mint, thyme, or chamomile may repel cabbage moths. • Radishes make
excellent trap crops for cucumber beetles among squash and
cucumbers. Radishes also attract flea beetles when planted
near
cole crops. • Garden borders
planted with low growing thyme or lavender may deter slugs. • Tansy and pennyroyal
repel ants. Observe
your plantings carefully, and write down combinations that seem
to work for pest control and growth enhancement. Communicate your
observations with others. Try to replicate your observations or
have others try the same combinations. Testimonials that are shared
by many observers often turn out to be valid. Scientists have not
spent much time looking at these relationships among plants and
their community; furthermore, the number of possible combinations
is enormous. You can be the first one to discover a new set of compatible
plants! Adapted
from Robert Beyfuss and Marvin Pritts, Cornell University, 2001
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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