Folair feeding seems to be one of the easiest ways of increasing yield, growth speed, and quality in a well vented space, with or without elevated CO2 levels. Just prepare a tea of worm castings, fish emulsion, bat guano, or most any other plant food right for the job and feed in vegetative and early flowering stages. It is not recommended for late flowering, or you will be eating the sprayed-on material later. Stop foliar feeding 2-3 weeks before harvesting. Wash off the leaves with straight water every week to prevent clogging the stomata of the leaves. Feed daily or every other day.
Best times of day to Foliar feed are 7-10Am and after 5 in the evening.
This is because the stomata on the underside of the leaves are open then. Also, the best temperature is about 72 degrees, and over 80, they may not be open at all. So find the cooler part of the day if it’s hot, and the warmer part of the day if it’s cold out. You may need to spray at 2AM if that’s the coolest time available. The sprayer used should atomize the solution to a very fine mist; find your best sprayer and use it for this. Make sure the PH is between 7 and 6.2. Use baking soda to make the solution higher PH, and vinegar to make the solution lower PH. It’s better to spray more often and use less, than to drench the hemp plants infrequently. Use a wetting agent to prevent the water from beading up, and thereby burning the leaves as they act as small prisms.Make sure you don’t spray a hot bulb; better yet, spray only when the bulb has cooled.
Perhaps the best foliar feeding includes using seltzer water and plant food at the same time. This way, CO2 and nutrients are feed directly to the leaves in the same spray.
Foliar feeding is recognized in most of the literature as being a good way to get nutrients to the plant later when nutrient lockup problems could start to reduce intake from the roots.
WARNING!: It is important to wash leaves that are harvested before they are dried, if you intend to eat them, since they may have nitrate salts on them.
NOTE: One grower who reviewed this document comments: “Fish emulsion smells. Bat guano could be highly unsanitary. Stick to the Rapid-Gro, MgSO4 (epsom salts), hydroponic trace element solution. Nitrate salts (The “N” in NPK) are unhealthy to smoke. Personally, I never foliar feed.”
Above is a great comment, and there is great wisdom in an organic, non-toxic garden. Personally, I use only CO2 on my indoor hydroponic hemp plants, and never folar feed. It simply does not seem to be necessary when using hydroponics.