list Unseen Warfare in the Landscape

In my last column, I mentioned feeding the birds. I neglected to inform you, my readers, of a single phenomenon of which some of you might not be aware. The hulls (technically "pericarps") of sunflower seeds contain an ingredient that is poisonous to most plant life, especially grasses, but even future sunflowers. Beneath most bird feeders that had some sunflower seeds, you will find a telltale bare spot of dead grass and poorly growing weeds.

A couple of years ago I wrote about how plants are in it for themselves, with all they need to defend against rivals, invaders, predators. I wrote, "Remember, what they contain is for their good. In the vitalist sense, they are as gods with powers to nourish, heal and kill. We are the sentimental ones." I mentioned toxic mushrooms, poison ivy, stinging nettles, etc.

The notion that plants are just passive organisms does not hold. They can and do engage in warfare, and, unnoticed by many, is the plant versus plant "chemical" kind.

This worldwide phenomenon was noticed and written about by Theophrastus (ca. 300 BCE), a student and successor to Aristotle and nicknamed "the father of Botany". He wrote about how chickpea plants both exhausted the soil and destroyed weeds. He was writing about allelopathic reactions.

Allelopathy refers to the chemical inhibition of one species by another. The "inhibitory" chemical is released into the environment where it affects the growth and development of neighboring plants.

In the year 1 CE, Pliny the Elder, a Roman scholar and naturalist wrote about how chickpeas, barley and walnut trees were toxic to other plants. The black walnut tree remains the most famous example of a plant that releases phytotoxic molecules into the soil as a means of limiting the growth of other plants, even altering the soil chemistry and the microorganisms that inhabit the soil.

This kind of reminds me (in a dark mirror reflection) of how legumes can extract unusable nitrogen from the atmosphere and convert it to useable nitrogen in root nodules with the help of special bacteria. An example: Alfalfa decomposes and when ploughed into the soil will contribute considerable Nitrogen over the next several years... hence the term green manure. It all seems kind of magical, but, then again, so does this chemical warfare.

The plant parts responsible for producing the allelotoxin, the chemical growth inhibitor, may be leaves, flowers, fruits, nuts, branches, stems, shoots and roots. The poison is also found in the surrounding soil, probably having migrated from the roots. In black walnut (Juglans nigra) the chemical is called Juglone and it is a respiration inhibitor. Nightshade family plants are especially vulnerable, tomatoes being the most sensitive.

There are many other allelopathic plants. Did you ever notice a conspicuous bare zone in a shrub or grassland community? I was visiting a friend a couple of years ago in the Kerhonkson area. A gentle hike on relatively rocky limestone outcroppings revealed a predominant ecological community of mostly sweetfern, a plant not found in a lot of places. Here there was a field of them. Of course it turns out that sweetfern is another allelopathic plant that can predictably alter a biological population through biochemistry.

This past June, I wrote about a local scourge that's replacing native woodland plants and wildflowers at an alarming pace: garlic mustard. A campaign to eradicate this invader includes pulling before seed set while the ground is moist. It, however, uses underground chemicals to achieve its goal of conquering the land, labor free and insidious.

University of Delaware biologists recently unraveled the secret of a notorious invasive wetland plant, the common reed, Phragmites australis, found in every state but Alaska. Its roots produce an acid so toxic that the structural protein in neighboring plants' roots completely disintegrates in twenty minutes in a lab setting. In Delaware alone, tens of thousands of acres of marshland have been taken over by Phragmites australis, and UD plant biologist Harsh Bais has called it a "horticultural disaster".

Over the last one hundred plus years a non-native plant known as spotted knapweed has been claiming hundreds of millions of acres of land all across North America, land once occupied by native grasses and countless other native plant species.

It was in 2003 that researchers uncovered spotted knapweed's secret to success: a potent, previously unknown poison exuded by its roots and killing off its plant neighbors. The pink and purple flowers of the culprit have replaced a diversity of native blooming wildflowers. What has followed in its plague-like path has impacted mightily on populations of cattle, deer, elk, birds and insects. Migration pathways have changed because of the plants inhospitality as a food source.

If you are a regular reader of my column, you might recall my recommending barley straw to control pond algae. It really works. Growth inhibiting toxins, allelochemicals, are released by the straw as it breaks down in the water, similar to the effect of sunflower seed hulls on grass, or black walnut roots on tomatoes. The algae simply begin to die and disintegrate.

It might surprise you to know that deadly secretions from roots or other decomposing plant tissues are common to almost every known plant species. It's about competition, just as for light, nutrients, and moisture. Space is the reward. For every plant that exudes a phytotoxin, there are vulnerable species. There are close to one hundred grasses alone that will resort to poisoning their competitors in order to take over the landscape.

The Tree-of-Heaven (A Tree That Grows in Brooklyn), Ailanthus altissima, a serious non-native invasive weed in urban and suburban areas flourishes because of a potent allelotoxin produced by its root bark. The cereal grass sorghum is another.

Add to these fragrant sumac, tobacco, pea, buckwheat, purslane, ragweed, crabgrass, pine, wormwood, hemlock, cucumbers, oats, alfalfa, sycamore, horse chestnut, firs, hackberry, New England asters, goldenrod, roses, lilacs, viburnums, mock orange, barberry, peach, apple, citrus, most members of the cabbage family, balsam poplar, sassafras, black cherry, sugar maple, southern red oak, rhododendron, tall fescue, foxtail and smooth brome grass, annual and perennial rye, Colonial bentgrass, red fescue and Kentucky bluegrass. Whew! This list isn't exhaustive, either.

Recall some past garden failures, and consider that something else might have been going on that was beyond your control. When organisms compete with one another, chemicals that interfere play a significant role. Molecules are establishing or defending their home space against a competing neighbor. Knowing the intricate ins and outs makes all the difference. It's extremely subtle.

Here comes the best part. Modern science has tapped into a revolutionary idea: using these powerful natural chemicals as weed suppressants that might replace synthetic chemical herbicides. Researchers are learning how hundreds of these phytotoxic interactions work, and hopefully, as they learn more, parks, school yards, golf courses, public spaces may become safer than ever before by using non-chemical alternatives.

New methods of purging weeds from planting beds may soon include using specific cover crops that produce just the right allelochemicals. When tilled into the soil in the fall, the next year might yield beds that are substantially weed free. This is the advent of bioherbicidals.

Future plant selections of natives and specific decorative ornamentals based on this research might eliminate the tedious and laborious chore of weeding. What a benefit. It's a form of better living through plant chemistry. This should work for everybody. Something to look forward to.

From The Garden of Ed. Submitted for publication in The Towne Crier on November 21, 2007

© 2007 Ed Mues. All Rights Reserved.