Autotoxicity

Autotoxicity is a form of chemical warfare found in the plant world, and it might be happening in your garden or landscape. Or, it might be an unsubstantiated biological process that looks great on paper, or works out well in a laboratory petri dish, but doesn’t hold as true in the field.

In either case, it is good know what the concept claims, which parts have withstood the tests of time and science, and how it might impact your garden. Let’s start with basic competition for survival.

We know that plants compete with their neighbors for real estate and resources. Those resources include water, sunlight, nutrients, beneficial soil microbes, and access to pollinators. Over time, plants have found different ways to gain an edge over the local competition. Many weeds have evolved to grow faster, in order to find resources first, while other plants grow taller, to commandeer available sunlight. Other forms of competition occur when plants develop deeper or wider spreading roots, which can reach more food and water, or more flamboyant, heavily scented flowers, to increase pollination rates. In some cases, plants have evolved to use chemicals on their neighbors.

Chemical warfare among plants

​Oblivious to the Geneva Convention, some plants release chemicals through their roots, leaves, and stems that impact neighboring plants. When those chemicals affect other species, it is called allelopathy [al-el-ah-path-ee]. Allelopathic plants release chemicals that can either stimulate or inhibit the life processes of other plant species. When those chemicals only affect plants of the same species, it is called autotoxicity. Autotoxicity can only inhibit, and never stimulate, the growth or germination of plants of the same species. But why would a plant want to treat family members that way?

Family feud

The word ‘autotoxicity’ means ‘self-poison’ and it is used to describe the way some chemicals are believed to only negatively impact plants of the same species. From a survival perspective, on the surface, this might make sense. Members of a species all need the same resources to thrive. If there are too many of one species growing in the same place, those resources can be used up before any individual plant has time to reproduce, except that it isn’t that simple. Let’s take a closer look at which plants are said to be autotoxic and how autotoxicity is said to work.

Autotoxic plants

​Alfalfa is acknowledged as an autotoxic plant. How can this be, you might ask. Alfalfa is grown in immense fields, the same way corn, oats, and other similar crops are grown, with other alfalfa plants on every side, isn’t it? It is. But, underground, soon after alfalfa seeds germinate, seedlings start producing toxins that reduce germination and prevent the growth of root hairs in other alfalfa plants. Without root hairs, neighboring plants cannot absorb water or nutrients. So, why aren’t all the nearby alfalfa plants dead?

​The trick lies in the fact that these chemicals are not released until the seedlings have gone through their full lifecycle and died. As the plants were busy maturing, the levels of these toxins continued to increase, but those chemicals only impact the next crop, as the dead plant material decomposes. Because of this, crop rotation is commonly used when growing alfalfa commercially.

According to one Canadian study, several other edible plants are said to be impacted by autotoxicity. Their list includes asparagus, citrus, tomatoes, rice, wheat, corn, cucumber, ginger, soybeans, sugarcane, muskmelon, watermelon, tea, and coffee. The problem with the science behind autotoxicity lies in the fact that some of the chemicals cited as autotoxins, such as p-coumaric acid, are found in many other plants, with no ill effects. Also, in much of the current research, autotoxic chemicals are applied artificially, which may or may not involve all the steps that might occur in nature.

Autotoxic compost?

When we apply compost to an area of the garden or landscape, we are introducing a variety of chemicals. In the case of small home gardens, the diversity of plant materials found in a compost pile generally reduces any potential autotoxic affect to negligible levels. Using no-dig gardening and cutting used up plants off at soil level, rather than pulling them out by the roots, leaves these potentially autotoxic roots in the soil. This may or may not create a problem, but it is something to keep in mind.

Whether autotoxicity is a real problem or not, you now know more about it than most people. And remember, before you accept opinion as gospel, make sure there is enough good science behind it to make it worth your time and effort.