Foliar Plant Feeding

Foliar Plant Feeding Overview
Foliar nutrient sprays are used to bypass the root system and directly introduce liquid nutrients into plants via pores in plant leaves called stomata.  When nutrients are sprayed on the leaf canopy, this permits the liquid to enter the open leaf stomata and effectively nourish the plant.  This highly advantageous approach to plant feeding is a simple yet effective method of ensuring that gardens receive adequate nourishment and reach their full growth potential.
Many gardeners, however, overlook this highly beneficial yield increasing practice.  This is unfortunate because foliar feeding allows for the rapid entry of energizing nutrients into the upper regions of plants, which in turn leads to improved overall plant health and larger, more abundant fruits and flowers. Additionally, the nutrient residue left behind on leaves from a consistent foliar spray regimen reduces pathogens and parasites like mites, leaf miners, and powder mold.
Effective though it is, foliar spraying should be viewed as a supplemental form of plant feeding to compliment a well-balanced nutrient mix being administered to the grow medium.  When practicing this supplemental feeding technique, the best results are usually achieved by applying foliar nutrients after sundown as photosynthesis is ending and plants are entering the dark cycle when their stomata are fully open.

Why Foliar Feed?
Hydroponic gardening allows the modern gardener to control the growing environment, which in turn increases the potential for improved yields.  But with greater control also comes a larger number of influencing factors which decrease the margin for error.
To start with, a nutrient growth solution must contain a very delicate balance of nutrients, correct reservoir temperature, and correct pH levels in order for the plants to be sufficiently supplied with nutrients. Sometimes, even when it appears that all of the conditions are optimized, the plants may not receive all of the nutrients they need.  Therefore, it’s wise to incorporate a foliar feeding regimen along with conventional feeding programs to ensure that plants get all of the nutrients they need to grow to full potential.
According to conventional wisdom, plants receive the vast majority of nutrients via root zone uptake.  But in the 1950s, scientists working at Michigan State University amassed conclusive data demonstrating that plants could effectively absorb substantial quantities of nutrients through their leaves and foliage as well.  Since then, expert horticulturalists have taken advantage of foliar feeding as an effective means of providing nutrients that their plants might not otherwise receive.
So the question arises, when should hydroponic growers use this supplemental feeding technique?  Many experts employ foliar feeding as simply an effective method to boost plants with the extra nutrients they need to grow to their maximum potential.  This is particularly sensible for smaller growers; because of all the methods used to increase crop yields, this is the most efficient and affordable.  With foliar feeding, all you need to give your plants an extra surge of nutrients is a simple spray bottle and well-balanced foliar mix.
A second reason for foliar spraying is that the balance of factors in a hydroponic garden is rather delicate, so if your garden experiences even a slight imbalance your plants may suffer from a degree of nutrient deficiency.  Perhaps the nutrient balance isn’t consistently optimized, or the pH level occasionally fluctuates outside the optimum range.  Then again, at times improper growth medium temperatures impede nutrient uptake from the root zone, or the medium retains insufficient moisture. But in all conditions, foliar feeding is assurance against nutrient deficiency.
And a third reason is to overcome the nutrient uptake restrictions brought on by an impaired root zone.  Root damage can be devastating to your garden.  Most commonly, it is caused by root rot from overwatering, but it can also be caused by disease or the presence of pests.  Once roots are significantly damaged, plants become incapable of soaking up the nutrients they need to stay healthy, and the crop yield with be disappointing at best.
Expert growers who identify root damage early will immediately begin foliar feeding on a regular basis.  Doing so ensures that the plants continue to receive the nutrients they need to grow to their full potential as the root zone struggles to heal and recover.

Proper Foliar Feeding
How quickly and effectively your plants are able to absorb nutrients through their leaves will depend upon a number of factors.  Importantly, just as the the nutrient solution being used in a hydroponic reservoir should fall within a pH range of 5.6 – 6.2, so also should the foliar solution being applied to your leaves.  And the temperature of the foliar mix should be in the moderate range of 65 – 70 degrees F.
It’s also important to maintain a temperate environment in the grow room.  If the air temperature exceeds 80 degrees Fahrenheit, the foliar spray may quickly evaporate and therefore be less effective.
The nutrient solution should be applied using a finely atomized mist.  The fine mist should be thoroughly applied until it begins to run off of the leaves.  And make certain that both sides of each leaf receive ample coverage.
Plants are able to effectively absorb the nutrient spray through surface because of pores in the leaves known as stomata.  And although the primary function of stomata is to facilitate gas exchange, stomata will also readily take in liquid nutrients once they become fully open during hours of darkness.  This is because plants normally respire and grow in darkness, and perform photosynthesis when there’s daylight.  Noteworthy too is that foliar feeding should only be done in slightly cooler conditions; because if the atmosphere becomes hot the stomata will close to protect the plant from dehydration and your plants will receive very little of the solution; or possibly none at all.
And to ensure that the solution adheres to the leaves, and thus maximize nutrient intake, the solution should include a non-ionic wetting or sticking agent to prevent it from beading up and rolling of the leaf’s surface.

Foliar Feeding Guidelines 
In order to increase the effectiveness of foliar feeding programs, there are a number of environmental conditions which should be present.  By following the below guidelines, your garden will experience the maximum benefit of a foliar feeding program.
Increase Humidity –While using a foliar fertilizer, your plants will benefit from an environment that is slightly more humid and cooler than your grow room may normally be.  The purpose for these atmospheric adjustments is to bring about the most conducive conditions for the foliar fertilizer to be absorbed into the plant as possible, while allowing the least amount to be evaporated or respired. With an increased humidity level and a temperature level kept below 80 degrees, the odds increase of the nutrients being utilized by your plant, instead of simply dissipating into the air.
Spray Leaves with Water Afterwards – After the initial application of foliar fertilizer, the spray will slowly dry up and leave a small nutrient residue on the leaves.  To make certain that your leaves take up as many of the nutrients from the spray as possible, spray the leaves with temperate RO water over the next couple of days.  The water will re-liquefy the dried residue allowing the leaves to absorb the residual nutrients.  As always, make sure the water that is used is properly pH balanced.
Emphasize the Undersides of Leaves –The reason foliar fertilization works so well is because plant leaves are porous, a feature which allows them to efficiently respire, or breathe.  Much of the respiring occurs during hours of darkness when the lack of light energy is insufficient for plants to conduct photosynthesis.  So when sprayed with a nutrient solution, these open pores can readily absorb nutrients. And since the majority of these pores, known as stomata, are on the underside of the leaves, extra attention should be given when spraying that section.  And because the underside is also where the majority of the runoff occurs, the nutrient mix should contain a surfactant which functions as a liquid sticking agent.
Use a Fine Mist – Utilize a spray bottle that allows the foliar fertilizer to be applied in the form of a very fine, drifting mist.  This will help ensure more even, thorough, gentler applications, and also helps avoid wasteful over-spraying.
It is a scientific fact that gardens thrive when being foliar fed.  But as always, experiment with one plant before trying it on your entire crop.  Because it is also true, and not at all uncommon to feed plants too many nutrients, possibly causing them to suffer some of the ailments that under fertilized plants can experience.