The Myth of Allelopathy

It’s not hard to find a Black Walnut tree surrounded by a great variety of native and nonnative plants, which should give an observant person reason to doubt the excessive talk about the negative effects of allelopathy.

ALL TEXT AND PHOTOS © Marlene A. Condon

 

Allelopathy—the biological phenomenon by which a species of plant supposedly inhibits, via biochemicals, the germination, growth, survival, and/or reproduction of a different species of plant—is much touted in invasive-plant circles. The original scientific interest in this subdiscipline of chemical ecology arose from the idea of making use of allelopathy in agriculture. However, the very vocal support of plant nativists (folks who prefer that alien plant species be replaced by native ones) has undoubtedly assisted this field of study to flourish. But is allelopathy just myth?

 

The beauty of the natural world is that it is open to anyone with eyes to see. You can prove for yourself that many of the “facts” repeated ad infinitum about nature don’t bear out in the real world.

 

For example, I live in a forested area where many Black Walnut trees (Juglans nigra) grow. This tree is the most well known example of allelopathy among gardeners because of its reputed toxicity to many species of garden plants.

 

It’s a given that you’ll be exposed to this information if you plan to grow such plants as tomatoes or green peppers. You’ll certainly be told to keep your garden as far away as possible from a Black Walnut because a plant sensitive to the effects of juglone (the chemical contained within the tree) can be poisoned by contact with any part of the tree (including roots, leaves, nuts, and even branches that have fallen to the ground).

 

It may indeed be true that solanaceous (nightshade family) plants are susceptible to the effects of juglone, but why would anyone plant a vegetable requiring full sun under a shade tree? Thus, to me, all the talk about the Black Walnut tree in regards to growing such plants is pointless.

 

Nevertheless, it’s reasonable to ask if the science is right about juglone causing problems for many other kinds of plants. To judge whether conclusions from a scientific paper in the biological sciences is accurate, you really need to read the details of how the study was done. Unfortunately, many scientific papers are most easily available only to other scientists, and the public is essentially locked out from access.

 

Luckily, I did locate some original research online regarding this situation, and found that the effects of Black Walnut on other plants have been contradictory from one study to the next. I also came across a January 2000 paper that listed plants affected and plants not affected by growing near Juglans nigra trees, and the number of plants not affected by walnuts is far longer than the list of plants affected—which didn’t surprise me. I’ve found quite a variety of plants growing under Black Walnut trees in Virginia.

 

http://www.allelopathyjournal.org/Journal_Articles/AJ%207%20(1)%20January,%202000%20(1-55).pdf

 

I’ve documented many native and nonnative herbaceous plants, and woody vines, shrubs, and trees surrounding this tree species. I’ve seen the same for supposedly allelopathic alien herbaceous plants. To me, the more plausible explanation for why some plants perform poorly near “allelopathic” plants are environmental conditions (e.g., soil fertility, soil compactness, etc.).

 

NATURE NOTE:

Garlic Mustard, Ailanthus, Canada Thistle, and Foxtail are nonnative plants deemed—without question—to be allelopathic. I can tell you for a fact it isn’t true. I’ve grown every one of these plants in my yard, and not a one inhibited the growth of other plants around it. If you want to know the truth about alien plants, grow them to learn for yourself if alien-plant lore is true or mythical.

 

Profusive Photinias

From the fall of 2017 to the spring of 2018, a male Red-naped Sapsucker from the western part of the United States made almost daily visits to the author’s Photinias, allowing her to get good looks and photos (which were sent to experts) of this extremely uncommon visitor to the eastern part of the country.

ALL TEXT AND PHOTOS © Marlene A. Condon

I originally wanted to grow the shrub commonly called “Red-tips” or Photinia because the new growth can be quite red—my absolute favorite color. I hadn’t really considered it in terms of wildlife, but not long after planting six bushes along the west side of my deck, I discovered this plant to be superbly suited to many kinds of critters!

 

The plants grew quickly, shooting up so fast that when my stepmother visited my new home, she said I must have “quite a green thumb”. She couldn’t believe that such big plants had been planted such a short time before her visit.

 

The tall, evergreen shrubs were an immediate hit with birds as cold winter weather settled in. I soon discovered that White-throated Sparrows and Dark-eyed Juncos from the northern states and Canada were sleeping among the glossy leaves that helped to shelter them from icy winds and snow.

 

Throughout each day I would see Tufted Titmice, Golden- and Ruby-crowned Kinglets, and Carolina Chickadees searching over the leaves for leftover aphids (a “pest”  according to gardening websites, but in reality, nutritious food for many organisms). Numerous species of songbirds and Gray Squirrels checked out the safety of the deck from those same branches before visiting for seed and water. And much to my delight, Yellow-bellied Sapsuckers have continued to visit every winter to suck sap from my big (about 25 feet tall, now) Photinias.

 

Making neat rows of sap “wells” on the trunks, these northern birds spend hours where I can easily watch them and the variety of other creatures that also desire a sugary treat. Squirrels, insects flying on warmer days, and songbirds (such as cardinals, kinglets, juncos, titmice, and chickadees) all sneak in for a sip when the sapsucker is not looking. These activities continue to this day, 36 years later.

 

The big shrubs also provide protective cover for birds as they flee predators. More than once I have watched with surprise as a Sharp-shinned or Cooper’s Hawk (two kinds of raptors that feed mainly upon songbirds) tried to maneuver through the thickly branched bushes to catch a bird. The hawks have never been successful, but they have provided me with the opportunity to see them well, something that is not often easy to do! I have even seen an Eastern Screech Owl in the Photinia during years when a pair is nesting in one of my wildlife boxes.

 

During the warm months of the year, cardinals have nested in the Photinias, and many hard-to-see avian summer inhabitants have stopped in for a quick visit. Yellow-billed cuckoos, birds that are rather secretive in habit and are more often heard than seen, will sometimes fly into the Photinias where I get to view them for a few moments. I have gotten a great look at a Louisiana Waterthrush and a Wood Thrush when these summer neighbors flew into the bushes and looked around a bit.

 

In late summer and fall as birds are beginning to migrate farther south, I often catch sight of warblers in the Photinias. Many of these birds are wonderfully colored and a beautiful sight to behold, and thus always cause for great excitement.

 

I had read that the flat heads of small white blossoms on these spring-blooming plants developed into bright red berries (yes, yet more red!) that were eaten by wildlife. After several fruitless years, I had given up, thinking that perhaps the male and female flowers were on separate plants and that I must have all males. But lo and behold, eight years after I had planted the Photinias, one of my bushes finally held beautifully red berries in November, and all the plants began to flower and fruit every year. Flocks of American Robins and Cedar Waxwings have landed to eat these little red delights (to their stomachs and to my eyes), and Gray Squirrels enjoy the fruits too.

 

Much to my surprise, I have seen squirrels eating the lustrously waxy Photinia leaves during the winter. And in springtime, the Photinia flowers are a magnet for many kinds of bees and other insects, proving that they are loaded with nectar and qualifying the Photinia as a good wildlife plant in yet another way.

 

The Chinese Photinia (there are said to be 40-60 species) that I bought locally is the most common Photinia grown in the southern part of the United States. It can reach a possible height of 36 feet, but you rarely see them allowed to grow to their full height and form. That’s a shame because these trees, though nonnative, truly bring in a profusion of wildlife for your viewing pleasure.

 

 

NATURE ADVICE: 

Horticulturalists advise pruning Photinias, but my advice is to just leave them to their own devices for maximum wildlife benefit. Whether you plant just one or several, place the plant(s) where they can fulfill their destiny by growing tall and wide in a sunny location.