Are You ready for Spring? Companion Planting

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Garden Season 2019 is underway! Like many, I perused the catalogues over the winter to try
and decide what plants to hunt for, what seeds to start, and what shrubs or trees to plant. In
one of the catalogues, I stumbled across something I had really never given much
consideration to, though have practiced somewhat through the years: companion planting.
Initially when I thought of ‘companion planting’, I thought of plants that have similar needs and
look good together, like coleus and sweet potato vine. And that is companion planting, but this
article talked about growing plants together not because of how they look, but how they
benefit each other.
The idea of companion planting isn’t a new thing. Early European settlers learned about Three
Sisters Gardening from Native Americans. Threes Sisters gardening is the practice of planting
corn, pole beans, and squash together. The corn supports the pole beans, the pole beans fix
nitrogen into the ground, and the squash vines spread out keeping the ground cool and moist
along with providing weed control. A dynamic trio! Three plants benefiting each other, and
providing highly nourishing food for Native Americans and early settlers to boot.
For a long time, and I can’t really say why, I have planted marigolds with my tomato plants. My
grandmother did it, my parents did it, and now I do it. I think it was because someone
somewhere along the way said marigolds keep pests away from tomato plants. And now that
I’m thinking about companion planting, I decided to do a little research. What I found was very
In an article called Companion Planting by Dorrie Minigar, a Penn State Master Gardener, she
wrote that marigolds produce a chemical that repels nematodes in the soil. Eureka! Granny
was on to something.
Then I stumbled upon another article (Marigold and Root-Knot Nematode, by Charles
Overstreet, LSU Ag Center) that says ‘Although planting marigolds as companion plants for
susceptible crops sounds like a good idea, it just doesn't work.’ The article goes on to say that
nematodes entering a marigold will die, but it won’t prevent nematodes from entering a
susceptible companion plant.
So confusing!
There are many, many charts and articles out there about companion planting; what to plant
together, what not to plant together, what will attract beneficial insects, what will repel pesky
insects, etc. etc. While very interesting and fun to read, I wasn’t finding any scientific claims in
support of companion planting.
Then I stumbled upon ‘Companion Planting: Anecdotal or Tried and Tested?’ by Grant McCarty
from University of Illinois Extension. The article starts by addressing the goals of companion
planting: managing insect pests, attracting beneficial insects, adding nitrogen to the soil, weed
control, and more. It then leads to how most support of companion planting is anecdotal. How
one person can plant two things together, have great results, and decide the companion
planting worked. When instead it could have nothing to do with the ‘companion planting’ but
other factors causing the desired results.
And then the author shared that there is scientific evidence supporting companion planting in
an Iowa State University research article called ‘Companion Planting’ (Read about it here).
The ISU article goes into great detail about growing five vegetable plants with five companion
plants. I could go into the details, but instead will just jump to what I had long been searching
for: “Conclusively, all the control plots showed the most insect and pest damage, which
indicates any intercropping with a companion plant is more advantageous than none.” There it
is, scientific evidence supporting companion planting.
But what I really learned from all my research is this: companion planting does no harm, and
could cause great good. That good could be in using non-chemical methods in controlling
insects, or using living mulch for weed control, or using natural methods to increase nitrogen in
my garden soil. And even if companion planting has no benefit at all, marigolds still look good
with tomato plants.
Submitted by Julie Miller, Warren County Master Gardener
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