Boron is a micronutrient taken up mainly as uncharged boric acid. It cross-links pectins in cell walls and is needed wherever cells are dividing and expanding: shoot tips, root tips and developing flowers. In cannabis it is largely immobile.
In practice
Deficiency kills or deforms growing points: brittle, thickened or hollow stems, distorted new leaves and dying tips. It can look like calcium deficiency. Toxicity shows as yellowing and burning of older leaf margins. The safe window is narrow, so hydro feeds use small amounts, usually about 0.2-0.5 mg/L (ppm) B, typically from boric acid or a borate. Some tap and well waters carry enough boron to matter; check the water report.
Don't confuse with
- Calcium deficiency: similar new-growth symptoms; check pH, humidity and the feed for both.
- Borax or boric acid as insecticide: a different use at very different concentrations.
- Potassium deficiency: margin burn on older leaves but with a different pattern and history.
Myth check
"A pinch of borax" is a risky home fix. With a narrow margin, measure grams per litre, not pinches. See boron.
A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Boric acid (H3BO3) is the usual boron source in micronutrient mixes, about 17.5% boron; plants absorb boron in this same uncharged form.The zone of dividing cells at the very tip of a shoot (or root) that produces new leaves, nodes and stem; removing it is what topping does.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Potassium (K) is a mobile macronutrient that controls water balance, stomata and enzyme activity; deficiency scorches the edges of older leaves.An immobile nutrient stays where it is first deposited, so its deficiency shows on young leaves and growing tips first: Ca, S, Fe, Mn, B, Cu.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.
