The document summarizes the concept of companion planting, which involves growing plants near each other to provide benefits. It discusses the limited scientific evidence but notes some potential benefits like nitrogen fixation, pest trapping, and attracting beneficial insects. It recommends experimenting with methods that have evidence like the Three Sisters garden of corn, beans, and squash. The summary cautions that many companion planting claims lack scientific proof but encourages trying some techniques and having fun in gardens.

1. Companion Planting:
The Real Story
Janet Young, PhD

2. College of
Agriculture and
Natural Resources
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3. Please Silence Your Cell Phone
Thank you!

4. Companion Planting
Outline
 Definition
 Disclaimer
 Brief History
 Biological Benefits
 Practical Considerations
 The Last Word
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5. Companion Planting
Definition
Growing plants near or next to each other to
provide a particular benefit to one or both
plants.
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6. Companion Planting - Disclaimer
WHOA!
There is very little scientific evidence for
certain types of companion planting. This
talk does not propose to ensure success with
any of the methods presented!
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7. Brief History – Ancient Rome
 Varro, a Roman agronomist - 37BC
 Roman Farm Management - Cato
 “Full grown walnut trees render sterile
all the surrounding land.”
 Pliny the Elder – 23 to 79AD
 Naturalis Historia
 “The (highly toxic) plant rue is a ‘very
friendly’ companion to figs.”
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8. Brief History – China
Growing Mosquito Fern in Rice Fields >1000 YR
 Hosts a cyanobacterium that fixes nitrogen from air
 Blocks light from getting to competing plants
Mosquito fern Rice
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9. Brief History – North America
Native American “Three Sisters” Garden
 Variant of method used throughout North America
 Mounds 12” x 20”, maize first, then squash, beans
 Maize=pole ; Beans=N2 ; Squash=mulch/protection
Maize Beans Squash
CA 9

10. Biological Benefits
 Nitrogen fixation – cover crops
 Physical assistance – shade, mulch, support
 Pest trapping – attract pest with other plants
 Host finding disruption – using plant decoys
 Attract beneficials - predators, pollinators
 Repel pests – WHOA!
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11. Biological Benefits
Nitrogen Fixation
 Plants e.g., clover, collect nitrogen from the air
 Special bacteria assist the plant in converting
this nitrogen to usable nitrogen in root nodules
 When the plant decomposes, the nitrogen in the
roots becomes available to neighboring plants
 Three Sisters – beans provide this benefit
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12. Biological Benefits
Physical Assistance
 Three Sisters: maize, beans,
squash
 Mosquito fern and rice –
prevents weeds
 Salad greens as a cover crop
– retain moisture, prevent
soil erosion
http://www.birdclan.org/threesisters.htm
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13. Biological Benefits - Pest Trapping
 Pelargonium geraniums grown
with roses draw away Japanese
beetles; geraniums are toxic to
the Japanese beetles.
 Alfalfa grown with cotton draws
away lygus bugs.
Japanese beetle
Lygus bug
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14. Biological Benefits - Pest Trapping
 Rye is used to protect soybeans
from corn seeding maggots.
 Sesbania is used to protect Corn seeding maggot
soybeans from stink bugs.
Stink bug
B 14

15. Biological Benefits
Host Finding Disruption
How Predatory Insects Find Host Plants
 Host Plant required by predatory insect for feeding,
reproduction
 Attracted by plant odor, and color green, avoid brown
 Fly to several leaves of plant to assess suitability if
host plant found
 Chance of success increased if only a host plant is
available
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16. Biological Benefits
Host Finding Disruption
CONCLUSION: adding non-host green plants
among host plants decreases host finding
success, and insect moves to a different area
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17. Biological Benefits
Host Finding Disruption
Cabbage without clover Cabbage with clover
Biologist (2003) 50 (3), p. 132 17

18. Biological Benefits
Host Finding Disruption
How Predatory Insects Find Host Plants
Cabbage root fly
searching for host.
From: Fig. 4,
Biologist (2003)
50 (3), p. 132
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19. Biological Benefits
Attract Beneficial Insects
 Flowers and herbs are known to attract
pollinators and predators of crop-
destroying insects
 Examples are chrysanthemums,
sunflowers, coneflowers, alyssum, and
flowers and herbs of carrot family
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20. Biological Benefits
Repel Pests - WHOA!
 Chrysanthemum – (may) repel aphids, bed
bugs (Cimex lectularius), leafhoppers, spider
mites, harlequin bugs, ticks, pickleworms and
imported cabbage worms
 Nasturtium – (may) help to control squash
bugs, cucumber beetles, and whiteflies.
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21. Biological Benefits
Repel Pests - Scientific Evidence?
 Pests of the Garden and Small Farm, Mary Louise Flint,
PhD, UC Davis Department of Entomology
“While the concept of companion planting is
appealing, research in this area has consistently
shown that companion plants offer no pest
control benefits under controlled conditions.
While many plants have chemical components
themselves that repel pests, this effect is
localized and benefits are not extended to
neighboring plants of different species.”
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22. Biological Benefits
Repel Pests - Scientific Evidence?
 “The Myth of Companion Planting”, Linda Chalker-
Scott, PhD, Extension Horticulturist, Washington State
University
“There is no scientific basis, however, for any
of the several lists that exist describing
‘traditional companion plants’. Like
horoscopes, these lists may be fun to use, but
they should not be perceived or promoted as
scientifically valid any more than astrology.”
I
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23. Biological Benefits
Repel Pests - True?
 Tomatoes love marigolds
 What does this mean?
 Should one plant marigolds in between
(intercrop) tomato plants?
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24. Biological Benefits – Repel Pests – True?
The Problem
 Nematodes are
unsegmented
roundworms, usually
microscopic in size
 Plant-parasitic
nematodes can kill
Galls from root-knot nematodes
crops - University of MD Extension FS 825
 Plant-parasitic nematodes are found in soil and are
very difficult to control
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25. Biological Benefits – Repel Pests – True?
The Experiment
 Cowpeas seeded in a field with nematodes
 Marigolds then planted next to the cowpeas
in one part of the field
 Nematode numbers, cowpea height,
number of leaves, seeds/pod measured
– African Crop Science Conf Proc,.v. 8:1075, 2007
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26. Biological Benefits – Repel Pests- True!
The Results
 Marigolds DID control nematodes in soil
 Cowpea yield was doubled if plants were
intercropped with marigolds in fields with
nematodes
– African Crop Science Conf Proc,.v. 8:1075, 2003
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27. Biological Benefits – Repel Pests- YES
The Conclusions
 Authors stated that a bio-nematicide may be
released into the soil through the roots of the
marigold plants – African Crop Science Conf Proc,.v. 8:1075, 2003
 More recent studies show that marigold roots release
the chemical alpha-terthienyl -- University of Florida Extension
 In the Florida study only a cover crop of marigold
proved protective
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28. Biological Benefits – Repel Pests- True!
The Conclusions (cont’d)
 Therefore, marigolds may be beneficial to
tomatoes (IF they are grown in soil with plant-
parasitic nematodes)
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29. Practical Considerations
Use what we know works!
 Nitrogen fixation – cover crops
 Physical assistance – shade, mulch, support
 Pest trapping plants
 Host finding disruption
 Plants that attract pollinators and beneficial
predatory insects (know thy friends and
avoid killing them!)
 Some plants known to repel pests from
neighboring plants
CO 29

30. Gardening Plans - Three Sisters
From: http://www.reneesgarden.com/articles/3sisters.html 30

31. Gardening Plans - Three Sisters
From: http://www.reneesgarden.com/articles/3sisters.html
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32. Three Sisters Garden
Corn is planted
the last day of
April.
Beans and squash
are planted mid-
May.
From: http://www.imrisk.com/threesisters/threesisters.htm
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33. Three Sisters Garden
Plants are growing well the
first week of June.
From: http://www.imrisk.com/threesisters/threesisters.htm
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34. Three Sisters Garden
Plants are well established
by the end of June.
Zucchini squash
Butternut squash
From: http://www.imrisk.com/threesisters/threesisters.htm 34

35. Three Sisters Garden
A More Mature Three Sisters Garden
From: http://theexchange.thegrowers-exchange.com/herb-gardening-resolutions-
part-2-companion-planting/ 35

36. Companion Planting - Final Words
 Try using what we know works
 Experiment in your own garden
 Be open to trying folk lore remedies
 Don’t believe
everything you
read
 And most
important of
all, have fun!
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37. Companion Planting – Selected Research References
 Companion Planting: Basic Concepts and Resources by ATTRA,
funded by USDA, https://attra.ncat.org/attra-
pub/summaries/summary.php?pub=72
 Companion Planting: Ecogardening Fact Sheet #10, Cornell U., 1994:
http://www.gardening.cornell.edu/factsheets/ecogardening/complant.html
 Companion Planting: http://en.wikipedia.org/wiki/Companion_planting
 Insects Can See Clearly Now the Weeds Have Gone - by Stan Finch and
Rosemary Collier, Horticulture Research International, Wellesbourne, UK
http://www2.warwick.ac.uk/fac/sci/lifesci/wcc/research/pests/companionpl
anting/biologist_jun03.pdf
 Marigold (Targetes erecta L.) as interplant with Cowpea for the
control of nematode pests, by T.I. Olabiyi and E.E.A. Oyedunmade,
African Crop Science Conference Proc. V. 8, p. 1075-78, 2003:
http://www.acss.ws/Upload/XML/Research/389.pdf
 The Myth of Companion Planting
http://www.puyallup.wsu.edu/~linda%20chalker-
scott/horticultural%20myths_files/Myths/Companion%20plants.pdf
 Three Sisters (agriculture):
http://en.wikipedia.org/wiki/Three_Sisters_%28agriculture%29
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38. Companion Planting – Popular Books
 Cunningham, Sally Jean. Great Gardening Companions.
Rodale Press, Inc., 1998.
 Mayer, Dale. The Complete Guide to Companion Planting.
Atlantic Publishing GroupInc., 2011.
 Riotte, Louise. Carrots Love Tomatoes. Storey Publishing,
LLC., 1998.
 Riotte, Louise. Roses Love Garlic. Storey Publishing, LLC.,
1998.
 Roberts, Margaret. Companion Planting. Briza Publications,
2011.
Note: There is very little scientific evidence for certain types of companion planting discussed in
these very popular books. Listing here is for informational purposes, and in no way ensures
success with any of the methods presented!
M
WHOA! 38

39. This program was brought to you by
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Montgomery County
University of Maryland Extension