Plants have evolved their own natural defense systems. They kind of had to; after all, they don’t have the option of fight or flight.
Still, though your plants have the built-in capability to protect themselves, there are some things you can do to enhance those natural defenses and help your plants fight off the bad guys. No, this is not an endorsement to teach them “tree kwan do” or “corn fu.” Again, that would be hard due to the lack-of-fight-or-flight thing. Instead, here are several much more useful steps that you, as the plants’ primary caregiver, can do to make sure they are as equipped and prepared as possible to handle any challenge.
The natural ability of plants to fight off pests is far more complex than most people would think. Plants can generate a wide array of hormones and antimicrobial enzymes, chemicals, and proteins in response to the unwanted presence of various pathogens. For example, some plants have shown the ability to manufacture chemicals that either inhibit insects from feeding or at least slow down their appetite. Some plants may also close their stomata to prevent the additional entrance of unwanted pests or diseases. In other instances, some plants can also send nutrients to where they are under stress from insect or disease attack and simply outgrow the nemesis. Some can even kill off and drop the part of their anatomy under attack, kind of like how certain lizards can shed their tails.
Another tool in the plant defense arsenal is the ability to communicate with beneficial insects and even other plants when they sense they are about to be attacked or succumb to a wide range of stressors. To do this, plants emit volatile, organic chemicals to beckon specific beneficial insects based on which pests are attacking. This signaling power is not limited to the leaves or shoots either.
Studies by the US Department of Agriculture show the roots can also emit signals to summon beneficial nematodes and bacteria to attack the pest.
Pleading for help is not the sole use of this communication. The hormones and chemicals released by distressed plants also act as a warning to other plants of what is happening to it. Studies confirm neighboring plants pick up these signals and, in turn, start to ramp up their own defenses in anticipation of the oncoming onslaught.
Responses triggered by the appearance of a pest, pathogen, or other agent are known as induced resistances. There are two types of induced resistance in plants: systemic acquired resistance (SAR) and induced systemic resistance. When a plant is expressing SAR, it will manufacture and produce a hormonal response and generate defensive proteins to protect itself as soon as it is exposed to any pathogen or pest. Much like the human body will release antibodies when it detects an invader, plants release salicylic acid in response to these stressful events in its life. Induced systemic resistance is mostly associated with plants grown in soil as it is dependent on root contact with specific plant growth promoting rhizobacteria (PGPR) in the soil. Once this connection between the roots and PGPR is made, plants produce jasmonate and ethylene. They then release these hormones to both internally protect themselves from pests and externally signal there is a problem.
Once initiated, different plant defense responses occur at different rates. Many of these are instantaneous or take only a few minutes to fully initiate. Others, such as some of the external signaling defenses, can take many hours.
Just as you are not likely to fight off an onslaught of sickness-inducing germs effectively while sleep deprived, hungry, and thirsty, your plants will be better poised to defend themselves from a variety of pests and pathogens if their basic needs are met. So, first off, make sure that the plants are where they want to be. Many growers fight a losing battle by trying to get a plant to grow in an environment that it is not suited for. If your plant needs full sun, make sure it gets full sun. Without it, the plant won’t be able to make enough food for itself. If your plant is meant to live in the shade, it will likely dry out or its foliage may burn if it is planted in full sun.
Also, make sure to locate your plants in a soil or soilless medium that is compatible with their types of root system, eventual root size, and water needs. Some plants can live in water or constantly moist soils; many cannot. If they cannot tolerate wet feet, your plants will not be able to fight off root rot.
Consider the plants’ nutritional needs, too, as your plants have different nutritional needs at different stages of development. Just as a human infant is not ready for steak, your young seedlings cannot tolerate grown-up doses of fertilizer. When your plants are older, they will likely need different levels of certain nutrients as they go through their various vegetative and flowering or fruiting stages.
Once these basic needs are met, your plants have a fighting chance to make use of their own naturally occurring defenses. There is nothing wrong, however, with wanting to even the odds—or even stacking the deck—a bit more in their favor. After all, it is your plants we’re talking about.
Enhancing Your Plant’s Defenses
The market abounds with amendments designed to fix whatever ails your plant. Products meant to specifically bolster natural plant defenses and immunities, however, are relatively hard to come by. “The main scientific challenge is the complexity of the physiological effects of biostimulants,” explains an article in the November 2015 issue of Scientia Horticulturae. “In general terms, the primary effects of biostimulants are to induce physiological responses in the plant. Many of these responses bear on primary metabolism, growth, and development. These processes are subject to tight homeostatic regulations which originate from millions of years of biological evolution and explain why plants occupy specific ecological niches and display characteristic phenotypic responses to fluctuating environments. Acting on such biological processes is challenging and attention should be paid to the many cross talks between processes and pathways in plant organisms in their response to their environment.”
So, what can you use specifically to improve your plants natural defenses? The Rodale Institute, a non-profit dedicated to supporting organic farming research, states the addition of organic matter, specifically properly crafted compost, will boost your plants’ pathogen-fighting microbial populations by up to 1,000-fold. You can also use certain commercially produced plant hormones on your plants. Salicylic acid is available as an amendment to help your plants fight off pathogens and methyl jasmonate is available to help plants cope with stressors. There are also aloe vera-derived solutions sold as plant immune system boosters.
More to Learn
A plant's natural defenses are extremely complex, and we only fully understand a sliver of what there is to know about these responses and processes. However, we do know enough to understand how to enhance your plants’ natural defenses. The best thing you can do for your plants to help them realize optimal health is make sure they have all the nutrition and moisture they need and an ideal environment in which to grow. When they are healthy, your plants can develop and maintain their own defenses and be fully equipped to handle whatever nature throws at them.