Feed It Through the Leaves

Photos by Joshua Burman Thayer

By Joshua Burman Thayer

In spite of challenges like poor soil and drought, the Bay Area is a great place to grow your own food. Organic gardening practitioners like me are constantly working to build our soil by amending with organic matter and green mulch from nitrogen-fixing cover crops. These techniques produce marked improvement to soil tilth over seasons of cultivation. But we can also work on this season’s growth. One way is by enhancing plant nutrient uptake, not via the root zone, but rather through the plant’s leaves directly. If boosting your production by applying foliar spray piques your interest, read on.

A Short Lesson in Drought Adaptation

Our California landscape is home to over 6,000 arid-adapted species. These plants have become masters at slowing their metabolism and retaining moisture by regulating the size of the stomata (small pores) on their leaves, which adjust in relation to light, temperature, and presence of water or humidity. Their strategy is to temporarily open the stomata during winter storms to maximize growth, then hunker down and conserve during drought, like we typically have for six or more months each year here in the Bay Area.

To conserve agricultural water, many growers have switched away from overhead sprayers to drip irrigation, which greatly lessens water lost to evaporation by delivering the water directly to the root zone or rhizosphere. It’s a logical strategy, but it bypasses a key function of a plant’s biology: If you also deliver moisture to the leaves, the stomata will open, and any nutrients in the spray will be available immediately to the plant.


Foliar spraying


Compost Tea for Foliar Feeding

Compost tea, which is most often applied as a root drench, works wonders in foliar feeding for growth potential in production crops. When it’s sprayed on the leaves, it also delivers microbes that can neutralize and even consume overabundant bacteria and fungi, thus improving the plant’s immune system. It’s like preventative medicine that in the same action provides nutrients straight to the vasculature. Spraying should be done once per week, and it’s best done before 10am or after 3pm, since midday sun magnified by water droplets on leaves can lead to burning.



Mildew and Aphids Begone

Summer drought stress as well as fog can compromise plant health. Squash, cucumbers, and tomato plants are particularly vulnerable to mold and mildew, but weekly foliar spraying can help the plants produce healthy fruit and growth well into September. Powdery mildew is especially problematic on brassicas, the plant family that includes kale, cabbage, bok choy, etc. When overnight our broccolini or collard greens become caked in the pasty-white coating of powdery mildew, it’s as if mother nature is culling the cool-season greens. If nothing is done, in two to three days the scourge will cover a whole portion of the garden and “freeze up” growth.

For this “red alert” level of powdery mildew, there’s a solution that can be found in your health food store. Look for Pure Lime by Lakewood Organics. Using this undiluted food-grade lime juice as a foliar spray works wonders on powdery mildew. I use half of a 12.5-ounce bottle of the juice diluted into a 750ml spray bottle of water. This will burn off the mildew spores and coat the leaves in a pH that prevents the mildew from regrowing.


Aphids on a kohlrabi leaf.


To discourage both aphids and powdery mildew on my leafy greens year-round, I have had great luck by spraying once per week just before sundown with a solution of 10 to 15 drops of peppermint essential oil added to a 750ml spray bottle of water. The peppermint oil lingers on the leaf, creating an environment that neither mold nor bugs seem to like. Brassicas in particular respond especially well to this regimen. Furthermore, it doesn’t damage the flavor of the soon-to-be harvested produce. (Many horticulturalists recommend spraying neem oil, but in my opinion it gives food crops a strange flavor even after a thorough rinse.)

Right when we are hoping to harvest our fall tomatoes and grapes, there is heightened potential for the plants to become infected with bacteria and mold, along with greater chance of infestation by insects that prey upon weakened plants. One reason for this might be changing pH as sugar content increases in ripening fruit. A comparison might be the candida outbreaks experienced by diabetics: An organism with a pH that is off becomes easier to infect or infest. It also happens because of the shortening days of autumn. Here on the coast, we can feel the dew point increasing each autumn evening as the solar period lessens and the soil’s ambient temperature decreases.

Foliar feeding can be helpful year round, and now is a great time to make it a habit.


Give Your Cuttings an Edge

Foliar feeding works especially well when propagating cuttings. Some years ago I worked with Captain Carl, a master plant propagator who taught me to root cuttings in pure coconut coir fiber. This inert medium keeps the cuttings free from soil-borne pathogens. The plants then receive their nutrients when misted. A dilute solution of kelp meal, for instance, brings those micronutrients directly to the cutting’s leaves, where they are immediately absorbed. In side-by-side trials, I have found a much better rooting success rate when propagating herbaceous perennials with this method rather than relying on the soil mix.

Permaculture designer and educator Joshua Burman Thayer is a regular contributor to Edible East Bay. In his monthly Gardener’s Notebook feature in Edible East Bay’s free e-newsletter, he offers lots more advice on how to implement gardening ideas like this one. Sign up for the newsletter here. Josh has also written for Mother Earth News and Edible Silicon Valley. Find him and his work at nativesungardens.com, and follow him on Twitter at @nativesungarden.