The stomata will open up at various times to allow the plant to transpire water and oxygen and intake carbon dioxide. After a plant is sprayed with water or a fertilizer mixture the stomata open and actually absorb some of that water or fertilizer mixture from the leaf’s surface. Foliar feeding may be a valuable tool for the indoor horticulturist to quickly correct a nutrient deficiency or further customize plant nutrition.

Growers will only gain the benefits of foliar feeding if they understand the parameters of foliar feeding.

Open Stomata

In order for the plant to absorb the minerals from foliar feeding the stomata on the plant must be open. Stomata are triggered by the first intense light. This means right when horticultural lighting comes on or as the sun rises.

To increase absorption, a grower can foliar feed right before his or her lights are set to come on. This way the plant is saturated with the desired fertilizer as the stomata initially open for the day.

Stomata are also triggered open by intense light. This is to maximize the amount of carbon dioxide the plant can take in so photosynthesis happens at the fastest rate possible. Although the stomata would probably be open, it is not always a great idea to foliar spray after the plants have absorbed the radiant energy from the horticultural lighting.

The temperature differential between the leaf surface and the liquid sprayed on the leaves may cause severe burning. If growers want to take advantage of the open stomata during the lights-on cycle they should lift the light fixture a few feet off the plant canopy for an hour or so before spraying to allow the plant’s leaves to cool.

Stomata are also affected by temperature and humidity. At temperatures above 82 degrees F many varieties of high-value plants will close their stomata to retain water. The same thing happens during very low humidity conditions (which usually correspond to high temperatures).

pH and PPM for Foliar Feeding

One of the biggest mistakes growers of high-value plants make when trying to foliar feed is that they treat the solution the same as their root feeding solution. The ideal pH for foliar feeding is 7.0; this differs greatly from the ideal pH in hydroponics (around 5.8) or soil (around 6.5). The PPM or nutrient concentration differs greatly in foliar feeding as well. Growers should expect to use 1/4-1/2 the amount of fertilizer for foliar treatments that they would use in normal root applications.

Make it Wet

One of the best ways an indoor horticulturist can increase the effectiveness of foliar feeding is to use an emulsifier. Better known to the hydroponic community as wetting agents, emulsifiers help to increase the amount of time a compound stays active on the leaf’s surface. Organic Wet Betty by Advanced Nutrients has always been my emulsifier of choice. A little goes a long way with this product and I have had good success using 1/2 of the manufacturer’s recommended dose.

Whether it is to quickly correct a deficiency or to maximize total nutrient absorption foliar feeding can be an invaluable tool for growers of high-value plants. By paying attention to the routine of the plant’s stomata, adjusting pH and PPM to the correct levels, and adding a wetting agent, any indoor horticulturist can maximize the efficiency of foliar feeding.

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