It was the winter of 2019 when I first realised that maybe foliar feeding my orchids wasn’t the best idea under such cold conditions, because a number of them were exhibiting signs of cold stress, and the cold droplets of water on the underside of their leaves certainly didn’t help. I’m not entirely sure whether most indoor gardeners are aware of the pros and cons of foliar feeding, since I’d only delved into the subject when I started growing orchids. That being said, I’m sure most outdoor gardeners are aware of foliar feeding and how it should be used. In this article, I’ll be discussing what foliar feeding is, when foliar feeding should be used, when it shouldn’t be used, and why.
What is foliar feeding?
Foliar feeding is a supplementary method of fertilising plants whereby a grower sprays the leaves of a plant with fertilised water in order to encourage it to absorb nutrients through the leaves. The ability to absorb nutrients through the leaves’ stomata is more limited than the roots’ absorption abilities. However, once nutrients are absorbed, they are immediately available to the plant, which means that it is an extremely efficient method of feeding.
Note that when foliar feeding, the liquid used must be diluted so that it is extremely weak. As I mentioned earlier, the leaves are comparatively less absorbent than the roots, meaning that salt and minerals may end up sitting on leaves once the liquid dries, causing leaf burn. Thus, it should not be attempted on plants with thin, tender leaves. It is also good practice to wipe leaves down more often if one uses foliar feeding as a supplementary feeding method. Foliar feeding can be done with diluted versions of normal commercial houseplant fertilisers, however, kelp solutions for foliar feeding are much more popular, because they contain less N-P-K and have higher concentrations of trace elements, which are more easily absorbed through the leaves’ stomata.
I’ve seen some people do foliar feeding incorrectly, and who can blame them? There’s not as much information on foliar feeding for individual plants out there, because it’s not a very common practice (perhaps with good reason. We’ll get to this later). What’s important is understanding the physiology of your plants so you’re spraying them in the correct places and at the right times. There’s no point spraying the leaves of your monopodial orchids from above: you need to spray the underside of the leaves, because that’s where the stomata are located. There’s no point in spraying CAM plants in the day time, because their stomata only open at night. Know your plant before you foliar feed.
Is foliar feeding for me?
Foliar feeding should be done in hot, dry environments. This means that foliar feeding liquid will dry more quickly, reducing the risk of rotting the leaves of your plant. Foliar fed plants should never be put in direct sunlight, as there is also increased risk of leaf burn. Basically, you have to have the kind of “goldilocks” environment for growing where your environment is hot, more on the dry side, but not too dry (because we don’t want the salts to dry too quickly and thereby burn the leaves, but we also don’t want the liquid to sit for a long time and rot the leaves) if you want to foliar feed. Otherwise, the risk to your plants increases exponentially. Of course, you can still attempt foliar feeding, but you should at least know the risks if you’re going to do so. Cold conditions are not ideal, as the water can lower leaf temperature, especially if there is airflow causing wind chill. This could potentially damage leaves.
If you want to try foliar feeding, test it on one leaf alone to begin with to see how it responds. If it responds poorly, you can always stop, and if it responds well, try foliar feeding a section of the plant. Once again, if it responds well, you can then foliar feed the entire plant.