Plant Stress: Senescence Simplified
Does stressing plants out to make them stronger make sense in the long run? Matt Geschke argues that the proof is in the harvest.
When you discuss gardening, the content of nearly all conversations inevitably boils down to a comparison of overall yield and quality. It seems the gardening community generally accepts these two barometers of botanical success as fairly broad yet valid indicators of a gardener’s overall prowess.
In reality, these two variables are often difficult to quantify and are the result of hundreds if not thousands of discrete events, the products themselves of countless independent yet often cascading chemical reactions.
As entertaining and informative as a brief dissertation on the hormone/enzyme interplay responsible for guiding, inducing and regulating plant senescence might be to some, the content of such a discourse would likely plunge you into a boredom-induced stupor that I can’t be certain you would ever recover from.
Gardening is supposed to be fun, after all, so I will refrain from addressing this topic from a biochemical position. I will attempt something that is quite the opposite—I will attempt to address plant aging in a simple and uncomplicated fashion.
My hope is that if I approach this complex topic casually, many readers who might have felt overwhelmed by a dry article about ethylene, abscisic and jasmonic acid, day length, root zone temperatures and incident angles of solar irradiation will continue to read along and perhaps even come untraumatized to a better understanding of the whole complicated subject.
First off, know your cultivar. I know it sounds a bit remedial, but most people base their selection of plants on what they want to grow—not on what their current garden is capable of growing most efficiently.
If you want to grow great plants you need an environment tailored to where your plants evolved to grow indigenously. If your plant’s ancestors evolved in Hawaii, for example, they will probably find the harsh environment of North Dakota in winter to be uncomfortable and frigid. Now take into account the fact that plants are generally sedentary and unable to flee in the event of radical changes in their environment.
Simply put, since the dawn of cultivation plants have been slaves to our lack of understanding. Most terrestrial plants are hypersensitive to minute alterations in their environment and often display dramatic physiological responses to change—this means small changes often yield big results, for better or worse.
Don’t be lured into radically stressing your plants—chemically, physically or environmentally. Although some of these methods might result in a dramatic and nearly immediate positive response, the underlying stress caused to the plant will likely lead to a delayed harvest or a diminished yield.
Transitions from the vegetative stage to the bloom stage in nature are generally incremental and gradual, not dramatic and shocking. Countless books, articles and even peer-reviewed journals reference 12 hours of daylight as the generally accepted day-length duration associated with the triggering of flowering in most fruit-setting plants.
However, the vast majority of gardeners I know change their lighting schedule from veg to bloom over a maximum of just four days, with the first two being days of complete darkness.
A strong flowering response is displayed by many plants exposed to this method of transitioning and this might be a result of the awakening of an ancient genetic trigger responsible for accelerating senescence when exposed to a ‘nuclear winter’-like event that’s typically associated with the impact of something like a meteor with the Earth.
Hundreds of these impacts have been seen in the fossil record, with the most notable event occurring at the K/T boundary. Those of you who took an Earth history class in college might recall that the K/T boundary reflects the impact event responsible for the great dinosaur extinction.
It should then be reasonable from a position of evolutionary benefit that a plant would attempt to ‘seed out’ as soon as possible when exposed to an impact event like the one described above, as this would provide the best probability for successful post-fallout re-colonization.
Although this scenario is plausible and the effect documented, though, stressing a plant into senescence does not allow for the gradual depletion and utilization of stored resources within the plant. Worse yet, trace amounts of unused nutrients trapped within the plant might lead to deleterious changes to the overall taste, quantity or appearance of the end product at harvest time.
Changes in substrate chemistry in nature take place over months and years, not hours and days as is commonly found in most hydroponic systems. Some experts even claim that by stressing your plants by altering their nutrition you will achieve better results than by pampering your plant with a balanced diet. This statement is true to a certain extent—but somewhat misleading.
Although many growers believe that by decreasing the nitrogen constituent of their nutrient solution while simultaneously increasing the ratios of the P and K components they will encourage a strong flowering response, it is important to realize that plants that are shocked continuously throughout their growth cycle will most certainly be more likely to exhibit stunting, delayed harvests, poor harvest volumes and increased incidences of failed fruit setting.
Nitrogen is a critical component, necessary during flowering and senescence—and its wholesale removal will lead to poor, weak results. I find that gradually changing the chemistry of my feed solution over a two-week period from veg to bloom and maintaining a small but consistent amount of nitrogen throughout the flowering cycle produces the best overall results.
Remember, fruit that is forced to finish through increasing environmental stressors will always be inferior in taste and overall quality when compared to the exact same cultivar left to mature naturally, assuming no other variables differ.
Finally, it is important to understand that the ‘if a little bit is good, a whole lot must be better’ approach is not really applicable to gardening. This approach to indoor cultivation is becoming increasingly prevalent in North America and runs diametrically counter to what nature took nearly four billion years to perfect.
It is reckless and arrogant to believe that simple reductionist science will unlock the nearly limitless potential of plants—just remember when pushing your plants to their limits with ultra-high ppms, excessive wattage and cool rooms with low VPDs you run the risk of catastrophic crop failure.
My suggestion is to run the lowest nutrient ppms possible without sacrificing yield, to utilize the smallest amount of wattage necessary for optimum growth and never run your CO2 ppms above 1,250. By following these fairly simple guidelines, your chances of crop failure or disappointing harvests are slim. Bottom line—sometimes the field just can’t take the stress!
Now I am sure I will catch flack from fellow academic types criticizing my shockingly over-simplified summation of senescence and plant maturation, but in the end I don’t write articles of this nature to further confuse an already enigmatic issue—I write them in the hope of fostering understanding. After all, the more you know the more you can grow…