Evidence remains inconclusive about the efficacy of foliar treatments
By Jennifer Martin
Every serious plant cultivator is familiar with the foliar application directions on liquid nutrient bottles, as if that approach is a simple and viable way of getting dissolved solids into plants.
Cannabis growers are often prone to avoiding sprays when possible, simply because the higher humidity caused by sprays can increase the likelihood of powdery mildew outbreaks, and especially during late flowering, can impact the health and flavor of the flowers, including sometimes being a contributing factor to botrytis (also known as bud rot).
The September 2015 issue of Marijuana Venture featured an article on pest management that emphasized the importance of atomizing the treatment into tiny micron-sized liquid particles, simply because bugs represent an external presence on leaf tissue. If a single leaf is not coated by a foliar treatment, the entire exercise could fail.
However, feeding with nutrients and bio-stimulants does not require atomization because any part of a plant’s surface can absorb moisture, and if the nutrient or growth stimulant is truly dissolved, it will get absorbed along with the water that’s acting as its carrier. So, while any kind of sprayer will work with foliar nutrient applications, the automated atomizers are still very handy because you just turn them on, leave the room, and come back a few hours later for clean-up.
There hasn’t been any scientific research published regarding the efficacy of foliar treatments specific to cannabis. Different plant species are known to react differently to foliar spraying, but aggregate data from research on other crop types can offer some helpful information for the cannabis industry.
One research study found that the spraying of “microbial fertilizer” on soybean crops increased yields by 5.05% (“Effect of foliar nutrition on soybean yield and quality in organic production,” published by Vera Petrovic of Serbia’s Institute of Field and Vegetable Crops).
In contrast, a five-part study on the foliar feeding of cotton crops showed no difference in yields or plant health (“Effects of early-season foliar fertilization on cotton growth, yield, and nutrient concentration,” published in Journal of Plant Nutrition by K.L. Edmisten, C.W. Wood and C.H. Burmester).
The facts around foliar treatment viability become more complex the more closely they are examined, and even world-renowned plant scientists disagree over what works and why.
What is generally agreed upon is that foliar spraying is not a substitute for root-based nutrient applications, and it is considered much more effective in the case of micronutrient feeding, simply due to the fact that only tiny amounts of micronutrients are necessary for plants, making a foliar application, with its limited systemic reach, more plausible. Foliar spraying, in contrast with root-based feeding, does not allow for any storage of nutrients for later use by the plants; it only treats a temporary shortage at the leaf surface level.
In particular, the micronutrient iron (Fe) is the most widely recommended nutrient for foliar use. This is because, among the seven essential micronutrients, iron is the least soluble and easily binds up in soil, so foliar applications of iron act as a complementary approach to the soil solubility problem. There are kelp supplements that are enriched with iron, and other bio-stimulants can also be mixed into foliar recipes in order to gain multiple advantages with one application. The pH of the solution should be adjusted to 5.9-6.1, which is the same pH that cannabis prefers in the root zone.
In sum, the jury is still out when it comes to the efficacy of foliar nutrients applications with cannabis. The fact that it is such a fast-growing crop makes it more heavily dependent on stored nutrients, and less likely to gain lasting improvement from nutrient sprays. In the case of microbial or pesticide treatments, however, foliar spraying is known to be very effective because absorption into leaf tissue is not a requirement of success.
Jennifer Martin is the winner of the 1998 San Francisco Bay Area Cannabis Cup, and is a pioneer in the US marijuana clone industry. She currently speaks at national conferences and consults for the legal cannabis industry. She can be reached through her website MarijuanaPropagation.com.