All commercial and home growers should be feeding their marijuana plants. Under normal circumstances, that means mixing a liquid plant food or fertilizer with water and introducing it to the soil, where the plants will uptake the nutrients they need to grow and thrive through their root systems. Foliar feeds work a little differently.
With foliar feeds, growers apply a less concentrated fertilizer or plant food directly to their plants’ leaves. It’s important to get it right, though, because applying any kind of moisture at the wrong time of day or the wrong point in the season can actually harm the marijuana plants. This article will offer a comprehensive introduction to everything growers need to know to foliar feed correctly, including when to feed, how much to dilute the fertilizer, and how to avoid issues like nutrient burns.
How it Works
During foliar feeding, cannabis plants absorb nutrients through microscopic openings found on the undersides of leaves called stomata. In nature, the stomata’s primary purpose is to absorb CO2 and release the byproducts of respiration: water and oxygen. In a controlled grow, these openings can also provide a more direct path to the leaves for beneficial nutrients.
Stomata don’t just regulate gaseous exchange and, in this case, uptake nutrients. They also open and close depending on the temperature, light levels, and humidity level to regulate the plant’s water levels.
When plants dry out, their stomata close to prevent them from losing more moisture. That can be a problem since plants can’t absorb CO2 through closed stomata. They will be unable to process the nutrients they need to grow until the plant’s internal water levels balance out, but misting them with a heavily diluted plant food can encourage the leaves’ microscopic pores to open back up.
A cannabis plant’s stomata also close at night, typically around 15 minutes after the sun goes down or the lights go off. These factors influence when growers should foliar feed their cannabis plants and what other steps they should take to ensure their plants’ ongoing health.
Benefits of Foliar Feeding
Foliar feeds can’t replace standard plant foods and fertilizers. That doesn’t mean they don’t substantially benefit marijuana plants, though. There are several ways that foliar feeds can encourage healthy growth and address potential problems.
Quickly Address Nutrient Deficiencies
Plants absorb more nutrients through their foliage than they do through their roots. This makes foliar feeding an effective way for growers to address nutrient deficiencies in their plants. Depending on which nutrients the plant is craving, deficiencies may manifest as yellowing of part or all of the leaves, changes in coloring in the leaves and stems, slowed or stunted growth, black spots on the lower or upper leaves, or growing shoots that turn gray and die.
Avoid Plant Loss from Nutrient Lockout
Nutrient lockout occurs when the electrical conductivity (EC) in the soil gets too high or the pH gets off balance. This causes salt and other minerals to build up around the plants’ roots, making it harder for them to uptake nutrients.
Most growers know that the best way to deal with nutrient lockouts is to flush the soil. This gives them a clean slate so they can balance the pH and EC, but it doesn’t address the plants’ immediate needs. Namely, it continues to deprive them of nutrients until growers begin a new feeding schedule.
Foliar feeds provide essential nutrients in the intervening time. Since nutrient absorption through the leaves is three to five times as efficient as root-based uptake, foliar feeding macro- or micro-nutrient deficient plants can get them back on track almost immediately. Growers will then have the time they need to restore healthy soil without worrying about losing their plants.
Perfect for Rooting Clones
Growers who take cuttings from their plants to root as clones can use foliar feeding to give their young plants the water and nutrients they need to root. Since young clones don’t have roots, they can’t uptake nutrients or water through the soil. While clones, which are typically rooted in high-humidity environments, can draw in some water on their own, foliar feeding is the only way to introduce nutrients to these plants.
How to Foliar Feed
The key to effective foliar feeding is full coverage. Since cannabis plants’ stomata are found on the undersides of leaves, it is important to cover the entire surface of the leaves.
Full coverage does not mean saturating the leaves. Instead, use a spray bottle to produce a fine mist of diluted fertilizer or plant food. The smaller droplets will be less likely to burn the leaves and allow growers to achieve optimal results with less fertilizer.
When foliar feeding seedlings, root fertilizers should be diluted to 20% of their suggested strength. For clones or older plants in the vegetative stage, dilute the solution to around 50% of the suggested strength. There’s no need to buy specialized plant foods.
When to Foliar Feed
Some growers only foliar feed plants that are exhibiting signs of nutrient deficiencies. That’s a mistake. It’s best to use this technique as a preventative instead of waiting until there’s already a problem.
Start foliar feeds as soon as the plants have begun to produce fan leaves. With clones, begin foliar feeding immediately after transplanting them into rooting cubes.
Spray the plants once a day, either in the early morning or right before it gets dark. Make sure the temperature isn’t too high, though, as cannabis plants close their stomata when the external temperature reaches around 80 degrees Fahrenheit. Indoor growers can schedule their feeds according to their lighting and temperature schedules.
It’s usually best to stop foliar feeding plants around two weeks after they enter the flowering stage. Soon after buds form, the plants begin to secrete resin to insulate themselves against the heat. Spraying the resin can negatively impact the plant’s ability to keep itself cool.
If plants are exhibiting symptoms of a serious nutrient deficiency during the early flowering stage, indoor growers can still use foliar feeds to reintroduce essential macro- and micro-nutrients. They should be extra careful about avoiding heat stress, though. Only use this technique after decreasing the temperature to avoid mixing plant food into the natural resin, and stop feeds well before harvest.
Foliar feeding flowering plants in the weeks preceding harvest will prevent them from using up the nutrients already stored in their leaves, stems, and roots. This leads to excessive nutrient concentration in the sugar leaves and buds, which negatively impacts the taste and aroma of the finished product.
Common Mistakes
Foliar feeding doesn’t require any specialized skill, but it can take some time to get it right. The worst-case scenario is usually just that the plant can’t absorb all the nutrients due to high temperatures, inadequate internal moisture levels, or other problems, but some mistakes can completely ruin a crop. Here are a few common mistakes to avoid:
Overly Concentrated Foliar Sprays
Don’t just mix traditional fertilizers or plant foods according to the label’s instructions and apply them as foliar sprays. It will be too strong and could lead to nutrient burns.
Excess Humidity
Foliar feeding increases humidity. This can be problematic in some indoor grows, as high humidity can cause problems with mold, pests, and other serious problems, even in the vegetative stage. The need to maintain a low relative humidity during the flowering stage is just one of the reasons growers should avoid foliar feeding flowering plants, as it makes them more susceptible to bud rot.
Heat Stress
Foliar feeding in the middle of the day is never a good idea. The water acts as a lens, amplifying heat and increasing the chances that the plant’s leaves will burn. Plus, the plant needs to use its stomata to regulate its internal temperatures to avoid heat stress and applying a foliar spray will make it more difficult for the stomata to release heat, gas, and water.
Outdoor growers can avoid these problems by feeding in the early morning or near the end of the day. Indoor growers can solve them by reducing the temperature in their grow rooms before and immediately following foliar feeds.
Nutrient Concentration at Harvest
The easiest way to avoid excess nutrient concentration at harvest is to stop spraying plants soon after they start flowering. If the plants start exhibiting symptoms of nutrient deficiencies later in the flowering stage, growers can alter their soil fertilizer mixes or feeding schedules. Novice growers should also note that some yellowing of the leaves is normal in the one to two weeks preceding harvest and is nothing to worry about.
The Bottom Line
Foliar feeds are a great way not just to address nutrient deficiencies as quickly as possible, but also to help clones root and to support overall growth in healthy plants in their seedling and vegetative stages. It’s also a low-risk technique since there’s really no way to damage healthy plants, whether they are from regular or autoflowering seeds, when following the advice above. As long as growers pay attention to environmental factors like light schedules, temperature, and humidity, they shouldn’t encounter any problems. Click here to get in touch with one of our helpful representatives today!
