Applying Foliar Spray on Cannabis Plants and Which Sprays to Use
Using a foliar spray is a great way to give your plants a quick nutrient boost or a dose of insect protection, but careful with that sprayer as too much of good thing can do more harm than good to your prized plants.
One of the easiest ways to give your cannabis plants a quick dose of needed nutrients (or to handle a small-scale pest outbreak) is to give them a foliar spray. A foliar spray is simply any application of nutrients, pesticides, growth regulators, or any other amendment that is delivered through the leaves (more specifically through the stomata) of the plant instead of placed in or on the soil for absorption by the plant’s roots.
There are several benefits to this kind of application and it is fairly easy. There are some key points in mind to effectively apply foliar spray to your cannabis plants. So long as you are mindful about when, where, and how you spray, there is no reason a foliar spray won’t give your plants just the boost they need to thrive.
Benefits of Foliar Spraying
Foliar spraying confers many benefits to cannabis plants other methods can’t boast. The primary benefit to foliar spraying is the plant receives the product much quicker than with a soil drench. When used for fertilizing, they receive the nutrients faster and more directly; when dealing with a pesticide, it makes direct contact with the pest and offers quicker protection to the plant. Results can be observed much faster as well than with soil applications.
Applying a foliar spray helps reach every cell of your plant. Because it is possible to obtain total spray coverage of the plant, it is possible to reach every cell versus the likelihood that some are missed when applications are absorbed through the roots. This can help cannabis crops boost their yields. Some nutrients, like iron, are better absorbed through the leaves than through the roots to begin with, and plants are in a better position to efficiently use the nutrients they are given. In the case of new clones (which do not have a root system at all), foliar applications are the only way to deliver the product in a way is needed and can be absorbed.
Plant stress can be reduced by applying foliar sprays versus through the soil or drip line. In cannabis, stress can turn your girls into hermaphrodites. When applying with a foliar spray, this chance is greatly reduced. It takes some of the pressure off the root system as well. When the plant does not have to totally rely on the root system’s ability to pull and transport all the needed nutrients from the soil, the plant is better poised to succeed. It also reduces the possibility that your plants’ roots buildup too much salt or excess nutrients. It avoids nutrient buildup in the soil or reservoir too.
None of this is to say foliar applications can or should take the place of applications at the soil level. It should be seen as a supplement to a regularly scheduled feeding regiment. In some instances, it is also used for surgical or strategic applications of materials in places or at times when soil application is not possible or practical.
When to Foliar Spray
To take full advantage of the benefits of foliar sprays on your cannabis crops, it is important they are done at opportune times and applied properly. The stage at which the plants are in, as well as the time of day for applications, is important to know. Cannabis plants will make the most of foliar sprays during the vegetative stage when they are actively growing. Wait until they have left the seedling stage before making the first foliar application. From this point all the way through the onset of the flowering stage, foliar applications can be made as often as once per week. Spraying during the flowering phase will contaminate the flower and should be avoided.
You can also spray your plants as a remedial measure upon noticing nutrient deficiencies. These are typically nitrogen, calcium, or magnesium. These sprays should be targeted to just the affected areas. Having a source of these on hand that can be quickly made up and applied is “cheap insurance” so problems can be addressed immediately.
Though the actual time of day to apply foliar sprays for outdoor plants and indoor plants may be slightly different, the principles are the same. Spraying of field-grown foliage should be done in the morning, ideally just before or at sunrise. It needs to occur well before the hottest part of the day occurs, so there is time for the spray to be absorbed and any excess to be evaporated or dried. It is not ideal, and can backfire, but if a spray application absolutely has to go on later in the day, it should be done once the sun starts to set, and only if the night is warm and dry enough to do so. Sprays should be avoided when rain is forecast and when temperatures are extreme.
Indoor applications of foliar sprays should be done when the lighting is first turned on for the day or just before. Fans should be turned off for up to two hours so the spray can be fully absorbed by the plants. The temperature in the growroom should not exceed 79°F (26°C).
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How to Apply Foliar Spray
Applying foliar applications is a relatively easy process. The key to successful applications lies in the formulation of the product and calibration of the sprayer. The droplet size is an important consideration. Medium-sized droplets, between 100 and 175 microns, are ideal for foliar applications. The volume of spray is important too.
Unlike when making “traditional” pesticide applications, which usually involve spraying to the point of run-off, foliar applications should be more reserved. Once you are satisfied the entire canopy and undersides of the leaves have been sprayed, go no further. It is important to include the undersides as this is where more of the leaves’ stomata are located. If trying to address pest issues, many insects hide on the undersides of leaves as well.
Make sure the dosing of your spray is appropriate for foliar feeding. This should be a far weaker concentration of nutrients or pesticides than would normally be applied. In the absence of specific directions for foliar applications, use no more than half of the recommended rate for traditional applications.
Types of Foliar Sprays
As with so much about cultivating cannabis, there is no single “best” foliar spray to use. There are so many different types and formulations that it would be impossible to devote space to all the available concoctions. Instead, let’s consider some of the more practical and popular foliar sprays used to address specific issues or problems.
First, make sure that the water you are using to mix your spray with is clean. Use only pH-neutral water, distilled water, deionized water, or water that has gone through a reverse osmosis (RO) system. Tap water contains various minerals (often unwanted) that can cause undue harm to your plants. Make sure the water is always at least 73°F (23°C). Water that is too cold or too hot can shock or burn your foliage. Adding a surfactant will help to make whatever your application is stick to the foliage better.
Look for fertilizers and pesticides that are crafted to dissolve in water or are already in a liquid state. These are easier to mix up and use in a sprayer and are in a form your cannabis foliage can more easily absorb. Make sure to follow the label’s instructions for mixing to the letter so you don’t risk making a solution that will harm or even kill your plants. Adhere to the recommendations for personal protective equipment so you don’t risk harming yourself as well.
Many growers avoid using chemical applications because of the taste that can be imparted onto the cannabis when it is consumed, and to reduce the chance they will harm their plants inadvertently. For those looking to use natural or organic formulations for foliar applications to avoid this, there are many to choose from.
You can even mix up your own. Some common ingredients for foliar sprays include potassium soap, neem oil, or aloe vera juice to repel insects. A once-weekly foliar spray of these, especially neem oil, will help to keep insects and diseases at bay. Other good, natural ingredients are derived from plant oils. Cayenne pepper, cinnamon, and garlic oils are other natural insect and pathogen repellants. Baking soda, and even plain milk can be used to treat the onset of diseases like powdery mildew.
Types of Foliar Sprayers
There are hundreds of available sprayer models on the market that can be used for foliar applications. Most of these, however, fall into one of two main types of sprayers: spray bottles and pump sprayers (tank and backpack models). The pump sprayers typically have a plunger type of pump and a wand that the spray comes out of. Usually these are made of some type of plastic, but occasionally for smaller models, glass is available.
If in general, you only have a few plants to spray, the spray bottle sprayer is just fine. If you intend to do a lot of foliar applications or have many plants, then some type of a pump sprayer is in order. If your application needs to be more “surgical,” such as to avoid buds, a spray bottle may be the better option. If you need to saturate your foliage to the point where the spray is running off the leaves, then a pump sprayer is the better option. This type of sprayer is usually better suited to make applications on the undersides of leaves as well.
There is no one “best” type of sprayer for all growers for all applications. Many growers find it easier to have different types of sprayers on hand for different applications depending on what needs to be sprayed and how much. Each type of sprayer has its pros and cons. Regardless of the type of sprayer selected, it is critical to rinse it out properly and thoroughly each time between uses and to make sure a sprayer that has been used for herbicides is never used when foliar applications need to be made.
Foliar sprays are a great way to quickly get your cannabis plants what they need, when they need it. This does not mean, though, that there aren’t potential drawbacks to consider or times when a foliar spray may not be the best solution. When making foliar applications, there is no buffer between excess nutrients, or concentrated ingredients to protect the plant, like there is with soil applications. Just as the benefits of foliar applications will be realized far sooner than by soil applications, damage caused by improper foliar applications will be seen and occur much sooner as well. Various precautions must be taken. Applications should not be made under direct light. Liquids can magnify the intensity of grow lights and the sun, and burn leaves just like a magnifying glass.
Foliar applications should be made on cloudy days or under dimmed lighting in the growroom. Foliar applications in high-humidity environments should also be avoided. If the excess moisture from the application cannot dry, then it can contribute to the high moisture environment needed to create conditions for mold to develop. Conversely, if it is too dry and the moisture evaporates too quickly before the leaves can properly absorb the nutrients, they can remain on the surface of the leaf and scorch them.
Ultimately, foliar applications are not magic. However, when properly applied in the right ratios and at the right times, they can do your plants a world of good and especially help get them out of temporary stressful situations. They are also very useful to give your plant a boost of nutrients at critical times. Make sure to cease all foliar sprays once your plants start to flower. Remnants of any foliar application can remain on the flower and ultimately contaminate it or otherwise negatively impact the final product.
Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional
Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.