Crispy brown edges curled across my tomato leaves in the raised beds along the south fence. It showed up two days after I mixed the spray too strong. I cut the next batch to half the rate. The new growth came in clean and green. That burn points to the main disadvantages of foliar feeding: low uptake, real leaf burn risk, and amounts far too small to feed a whole plant.
The first of the foliar feeding drawbacks is how little a leaf takes in. Leaves absorb only about 15% to 20% of what you spray on them. The rest just drips off or dries on the surface. The next rain washes most of that away. So even a careful spray wastes the bulk of what you mix up.
I noticed the same gap on my own squash one summer. I sprayed the whole bed twice a week for a month. The leaves looked a touch greener for it. Then I checked the harvest and it came in exactly the same as the year before. Most of my mix had never made it into the plant at all.
The amounts that do get through stay tiny. Your leaf can pull in micronutrients like zinc or manganese in the small doses it needs. It cannot pull in the bulk nitrogen, phosphorus, and potassium your plant burns through. Those big needs still have to come up from your roots and soil. No spray you buy changes that basic limit.
Push your concentration up to deliver more and you scorch the leaf. The salts in your solution pull water back out of the leaf tissue. That is what crisped my tomatoes along the fence. High concentrations cause salt induced leaf burn. And a damaged leaf feeds your plant less, not more.
Stress makes the whole thing worse. A plant short on water keeps its stomata shut. It also moves nutrients slowly through its tissue. So your spray on a wilting plant barely reaches the rest of it. The solution just sits on the leaf and does almost nothing useful for you.
Watch out for a color response that fools you. Leaves can green up within a day of a spray. They can still give you no real gain in fruit or harvest. The plant looks fed, but the yield does not move at all. A greener leaf is not proof the spray actually paid off. My squash bed greened up and still gave the same crop.
Then there is your time. One spray rarely lasts more than a few days. So you head back out again and again with your sprayer. Each round adds labor and cost for the mix and your effort. Your bill climbs fast across a full season of repeat passes.
You can blunt every one of these problems. Confirm a real deficiency first with a tissue or soil test. That way you treat a true shortage and not a guess. Dilute well and start weak, since a thin mix never burns the way a strong one does. You can always step the rate up later if the plant handles it fine.
Timing protects the leaf too. Spray in cool conditions near 72 degrees Fahrenheit (22 degrees Celsius). Aim for the early morning while the air is still calm. The solution stays wet on the leaf longer in that cool window. And the gentle heat keeps your burn risk low. Do those three things and foliar feeding earns its small, real place in your garden.
Read the full article: The Truth About Foliar Feeding
