There’s nothing quite as frustrating as having some know-it-all tell
you why your cucumbers aren’t getting bigger than pickles, or why your
tomatoes look like tomatillos. But when that reason seems like some
old wives tale that has no obvious logical basis in science, well,
that’s just annoying.
I’m a scientist at heart, and I spent the better part of three years
sitting in lectures and labs learning all about horticulture and
landscape design. I know which plants attract the same pests and
which will suck the ground dry of water or nutrients. I know how to
identify and treat verticillum wilt, and what to do with a
But now, here I was being told by an older neighbor—someone who I’ll
be the first to admit has more than five times my experience at
growing veggies, even if he is a know-it-all—that I shouldn’t have
planted my tomatoes anywhere near the sunflowers standing sentry
against the back fence.
“Why?” I ask, which is a big mistake because forty-five minutes later
I’m none the wiser and my head is swimming with age-old adages. A
Google search doesn’t help either, on the contrary: I’m left to
believe that sunflowers would make a rather good support for my
tomatoes, but not my beans. Plus, now I have a long list of veggies,
herbs, and flowering plants (and a few trees) that I should or
shouldn’t plant together.
Are these all just old wives' tales, beliefs passed down through the
oral tradition from my gritty, old, farmer neighbor and great Aunt
Mabel to me? Or is there some scientific fact hidden here somewhere?
Many scientists, like me, have questioned these adages over the years
and have uncovered some truths and some pure hokum.
There is no question that plants influence one another. Both Varro (a
Roman agriculturist) and Pliny the Elder (a naturalist, also Roman)
are credited with noting, around two thousand years ago, that nothing
likes to grow around the root zone of a Walnut tree. Walnuts and some
other members of the genus Junglans produce an allelopathic
toxin—a chemical that inhibits the growth of other plants—called
juglone. But through scientific research, we’ve also discovered that
only some plants are susceptible, while others are unaffected by the
Even further back in history, while domesticating corn, beans, and
squash, Native Americans discovered that these three crops grow better
when planted together. According to Iroquois legend, corn, beans, and
squash are three inseparable sisters. The Iroquois believe each of
these three crops are precious gifts from the Great Spirit and are
watched over by one of three sisters' spirits, called the De-o-ha-ko.
Iroquois ceremonies to honor the De-o-ha-ko mark the planting
season and the first harvest.
Even without the scientific awareness to understand why these
companions thrived, the tribes passed on the knowledge—through stories
and annual rituals—that corn, beans, and squash should always be
planted together. And this makes sound environmental sense: The beans
and corn have a symbiotic relationship in which the corn provides a
support for the beans to grow up. In return, the beans provide extra
support for the corn in strong winds. The squash adds to this
partnership by providing ground cover to both conserve water and
repress weeds. In addition, although corn is a hungry feeder, beans
(as all legumes do) take nitrogen from the air rather than the soil
during the growing season, and so don’t compete for nutrients.
Whether the Native Americans tried to grow their beans up sunflowers,
which also were domesticated and grown by tribes more than four
thousand years ago, we’ll never know, but according to my Google list,
that’s not a combination that would achieve great success.
Sunflowers, notably Helianthus annuus species (annual
sunflowers) produce an allelopathic phytotoxin that inhibits seedling
germination and seedling growth in some plants. Scientific studies
have shown that extract from sunflowers are effective in suppressing
the germination and growth of certain weeds, namely littleseed
canarygrass (Phalaris minor), lambsquarters (Chenopodium
albun), lesser swinecress (Coronopis didymus),
toothed dock (Rumex dentatus), and burclover (Medicago
polymorpha). Unfortunately, it doesn’t seem there is anything but
anecdotal evidence (and about 10,000 Google hits) to indicate that
sunflowers stunt the growth of plants like beans and potatoes.