Foliar Spraying for Improved Plant Health

Reader Contribution by Joshua Burman Thayer and Native Sun Gardens
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Many of us deal with less than ideal soil. Whether too much clay, too much sand or a history of contamination, dealing with poor soil can hinder potential harvests. As organic gardening practitioners, we are in the business of ongoing soil building. By amending the soil with organic matter and by green mulching on site, we can see marked improvement to soil tilth over seasons of cultivation.

Please see my past article on soil building here. While building soil is a constant practice for long-lasting organic farming success, this article focuses on this season’s growth by enhancing plant nutrient uptake, not by the rhizosphere, but rather by the plant’s leaf directly. If getting an added boost in your production by applying weekly foliar spray piques your interest, read on.

Plant Stomata Regulate Growth Rates

In his book Plant Life in the World’s Mediterranean Climates, author Peter Dallman describes the way that plants have adapted to surviving drought. “Leaves lose water mainly through small surface pores that are lined by specialized guard cells, regulating the size of the opening to a chamber underneath.”[1] These stomata “function as hydraulic valves, adjusting pore size according to; light intensity, temperature and humidity.”[2]

It seems these seldom discussed valves on the undersides of leaves are key ways that plants adjust to the ever-changing weather. Much like our pupils dilating with changing light conditions, so too do plants adjust in how they exchange gasses.

In the case of arid-adapted species, such as the 6,000+ plants native to California, these plants have become masters of closing these stomata to slow metabolism and to retain as much moisture as possible. This allows them to temporarily open the stomata in winter storms to maximize growth, then to hunker down and conserve for the long drought.

In other wetter areas of the globe, such as the tropics, a plant can grow much faster and exchange much more carbon dioxide/oxygen by keeping these stomata wide open to maximize gas exchange. How does this apply to the crops we grow? Keep reading.

As a measure to conserve agricultural water, some growers have switched to drip irrigation for many production crops. While this greatly lessens water wasted by overhead sprayers, this keeps the water delivered to the root zone or rhizosphere of the plants in question, while leaving the leaves dry above. For dry-adapted crops such as pomegranate and olive, this may be preferred.

However, for many of our water loving coveted fruits and vegetables, they now find themselves in drier than desired air *(here in California and the arid West). Since stomata are responsible for regulating metabolism, those of us saving water on drip irrigation can boost plant health by spraying foliar nutrients. For those of you unfamiliar with this term, foliar feeding means to apply nutrients in a liquid spray that is applied directly to the leaves of plants. In this way, nutrients are absorbed immediately through the vasculature of the plant. With restricted water and less than ideal soil tilth, we can still see increased growth capacity with weekly foliar feeding.

Foliar Feeding for Propagating Cuttings       

A master plant propagator I learned from, Captain Carl taught me on the North Coast of California to root my cuttings in pure coconut coir fiber when propagating plant cuttings. This inert medium keeps the cuttings free from soil borne pathogens. The plants then receive their nutrients when misted with a dilute solution of kelp meal. This solution brings micronutrients directly to the baby cutting’s leaves, to be immediately absorbed.

In doing side-by-side trials, I have found a much better rooting success rate when propagating herbaceous perennials with this method than any other soil mix. The fact that cuttings in inert coco coir outperform those in a nutrient soil shows that plants can assimilate nutrients through their stomata much better than we may have assumed.

Foliar Feeding for Compost Tea Application

Compost tea works wonders for plant growth potential whether in a root drench or in foliar feeding production crops. The tea sprayed once per week just before sunset allows for the microbes to neutralize the leaves and consume any bacterial or fungal imbalance while increasing it’s immune system.

In this way, the plant is given preventative medicine while in the same action providing nutrients straight into the vasculature. I am quite the believer that compost tea offers plants an accelerated advancement of their genetics to unlock latent potential.

Peppermint Essential Oil Foliar Feeding for Reducing Pestilence

With the shortening of days in autumn comes a heightened potential for infection from bacteria and mold, as well as infestation of insects who pray on the weakened plants. Especially with fruiting bodies, from tomatoes to peaches to grapes, when the sugar content in a fruit increases so too does the risk of infection in that high sugar environment. Not unlike candida outbreaks in diabetics, once the pH of an organism is off, it becomes easier to infest.

On the coast of California, we can feel the dew point increase each evening as the solar period lessens and the soil’s ambient temperature diminishes. At this point each fall the goal is to get the fruit to mature without damage from bacteria, fungus or insect. I have had great luck with a few simple products.

One of my favorites is Peppermint essential oil. Ten to 15 drops diluted into a 750ml spray bottle of water once a week and applied just before sundown allows me to avoid aphids and powdery mildew year-round on my leafy greens. The brassicas in particular respond especially well to this weekly spray regimen. Many horticulturalists recommend spraying neem oil, but in my opinion, it gives food crops a strange flavor even after thoroughly rinsing. The peppermint oil, however, lingers on the leaf creating an environment that mold nor bugs seem to like.


Lime Juice for Powdery Mildew Outbreaks

Like clockwork, when our weather becomes over 80 degrees Fahrenheit our brassicas begin to come down with powdery mildew. It is as if mother nature is culling the week cool season greens that are not equip to deal with hotter temperatures. Overnight, a crop of broccolini or collard greens can come down caked in the pasty, powdery mildew’s white heavy coating. If nothing is done to remedy this, in 2-3 days, it will cover a hole portion of the garden and “freeze up” the growth of said brassicas. For this “Red Alert” level of powdery mildew, I have a solution and it can be found in your health food store.

Lakewood Organics produces a lime juice that works wonders on powdery mildew. I use half of a 12.5 small-sized bottle diluted into a 750ml spray bottle of water, so that one juice *(~$4.00) fills 2 spray bottles. This lime juice burns off the three-dimensional mildew and coats the leaves in a pH that the mildew will not regrow. Further, it doesn’t damage the flavor of the soon-to-be harvested produce.

I value this readership here at MOTHER EARTH NEWS. So as to further this dialogue on foliar spraying for plant health, please share this article and feel free to leave comments below on what has worked well for you.



[1] Dallman, Peter R. Plant Life in the World’s Mediterranean Climates.  University of California Press.  1998. (page 30).

[2] Dallman, Peter R. Plant Life in the World’s Mediterranean Climates.  University of California Press.  1998. (page 30).

Joshua Burman Thayer is a landscape designer and permaculture consultant with Native Sun Gardens. He is the Urban Agriculture Supervisor for Tenderloin Neighborhood Development Corporation in San Francisco, Calif. Find him at Native Sun Gardens and read his other MOTHER EARTH NEWS posts here.

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