Sick leaves – All of the leaves in this photo came from the same plant. All leaves show signs of overfertilization.

When it comes to fertilization, experience with specific varieties and growing systems will tell growers more than anything else. There are hundreds of N-P-K mixes, and they all work! When choosing a fertilizer, make sure to read the entire label, and know what the fertilizer claims it can do. Do not be afraid to ask the retail clerk questions or to contact the manufacturer with questions.

Once you have an idea of how often to fertilize, put the garden on a regular feeding schedule. A schedule usually works very well, but it must be combined with a vigilant, caring eye that looks for over-fertilization and signs of nutrient deficiency.

Leach soil with 1–2 gallons (4–8 liters) of mild nutrient solution per gallon of soil every month. This is the best form of preventive maintenance against toxic salt buildup in the soil.

Foliar Feeding

Spray foliage from underneath so that the spray is able to penetrate the stomata located on the leaf’s underside.

Foliar feeding means to spray the nutrients or bio-stimulants onto the foliage to augment available nutrients, vitamins, hormones, etc. Cannabis foliage absorption via the stomata is limited, and the timing is crucial to achieving the best coverage and absorption. Foliar sprays enter a plant when the stomata are open. The best times to spray are when the stomata are open as widely as possible. However, stomata are constantly balancing humidity, CO2 levels, temperature, and light levels, all of which signal stomata to open or close.

Cystolith hairs – Leaves and stalks have waxy, cystolith hairs – that act like feathers on a duck – and shed water.

The waxy (cuticle) surface coating (cystolith hairs and resin) on cannabis foliage makes them very poor water absorbers. This barrier wards off pest and disease attacks, but it also slows the penetration of sprays.

Young leaves are more permeable than older leaves. Nutrients and additives penetrate immature leaves faster than tougher, older leaves, and they are easier to damage with strong sprays.

Stomata openings on the cannabis plant are on the bottom of the leaf’s surface, so sprays must be applied upwards.

Foliar feed the cannabis plants only when specific deficiency symptoms manifest. Foliar feeding is a quick fix only and is easy to overdo. High levels of nutrients in the foliage stop the roots from taking in more; this is confusing for the plant. Foliar sprays are cumulative and build up in the foliage. Never spray more than once every 10 days, and keep the spray concentration to less than 500 ppm or with an EC of less than 1.0.

Stomata close when there is:

• Too much CO2
• Low humidity
• Roots are dry

Stomata open when there is:

• High light
• Low CO2
• High humidity

Spreader-Stickers

Smart growers use a surfactant, surface-active substance, or (adjuvant) helping by supplementing to increase effectiveness of foliar fertilization.

Spreaders (wetting agents) reduce the surface tension of sprays and keep them from beading up and rolling off the foliage. Big, bulbous drops on the leaves mean you need to use a spreader. Flat drops that slide off the foliage mean there is too much spreader. There are nonionic, antionic, and cationic spreaders. The nonionic spreaders that do not ionize in water are the most common, and they do not react with most pesticides. Antionic and cationic spreaders are not used often.

Stickers keep the spray adhered to the leaf after spraying so that it does not wash off when it rains or when dew forms. Stickers not only increase adhesion, they slow evaporation, and impart a waterproof coating. Some stickers are spreaders, too. Spreader-stickers allow the stomata on the leaves to be penetrated.

Extender (stabilizing agents) protects applied sprays against UV radiation and heat that degrade the sprays.

Liquid and powder soaps and detergents act as surfactants, too. But, they are not nearly as effective as horticultural surficants. Biodegradable surfactants disappear the fastest. Silicone surficants are also mild insecticides that work to impair pest functions.

Foliar spray concentration is cumulative. Nutrients delivered via the foliage can cause a buildup of salts in and around the leaves. This is similar to the way the salts accumulate in the soil.

Chart: How to get the most out of your spray.

1. Spray the bottom of the leaves. Spray with a fine mist, and do not create droplets on the leaves. Fine mist is electrically attracted by the foliage. Even young marijuana plants have waxy hairs that impair liquid penetration.

2. Do not spray plants that are hot or when the atmosphere is too dry. Spray in low light, either before the lights go off or just as they are coming on. If spraying in hot conditions, first spray everything with plain water until the temperatures of the room and foliage drop, before applying the real spray. Spraying when the plant foliage is hot causes the spray to crystallize on the surface, and it stops the penetration. Spraying with water 10 minutes afterward often increases the penetration. Mobile nutrients move freely within a plant. Immobile nutrients move slowly, but once deposited, they stay.

3. Apply mobile nutrients sparingly. Immobile nutrients—sulfur, boron, copper, iron, manganese, molybdenum, sulfur, and zinc—often require two or three applications. Calcium and boron are poor candidates for foliar feeding because they translocate poorly. But urea nitrogen applied as a spray in high humidity, penetrates almost instantly into leaves. Be careful when spraying urea-based fertilizers, and keep them diluted. Urea also carries other nutrients into the plant and works well for a base to the mix. Foliar feeding should turn the plant around in less than a week. A second spray could be necessary at the week’s end to ensure the cure sticks.

4. Boron, calcium, and iron move slowly during flowering. A supplemental foliar dose often speeds the growth when it slows. A foliar spray of potassium can also help flowering, especially if the temperatures dip below 50° F (10° or above 80° F (25° C).

5. Always spray new growth. The thin, waxy layer and a few trichomes allow for good penetration.

6. Measure the pH of the spray, and keep it between 7 and 8.5. Potassium phosphate (K2HPO4) becomes phytotoxic below pH 4 and above 8.5. Stomata are signaled to close within these pH ranges.

7. Use a surficant with all sprays, and apply these as per the instructions on the label.

8. Add the proper amount of surficant so droplets do not form on the leaves. Once formed, the droplets roll off the foliage, rendering it ineffective.

9. Stop the application before the droplets form on the leaves. Make a test spray on a mirror to ensure the spray is even and does not form droplets that roll off the mirror.

10. Spray with as fine of a mist as possible to minimize the size of the drop.

DropFalling – Without a spreader-sticker sprays often bead up and roll off the foliage which makes them ineffective.

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