Boron (B) in Cannabis – Deficiency and Requirements

In a boron deficiency, the shoot tip becomes stunted and misshapen, and the young leaves turn thick and brittle. Boron is a micronutrient, a mineral the plant needs only in tiny amounts, but too much of it also does damage. That is why a leaf tissue analysis comes before any boron application.

What does boron do in the plant?

Boron holds together the cell wall of young cells, the first and still stretchable wall known as the primary wall. Part of this wall consists of pectin, a gel-like substance made of sugar chains. A particular pectin called rhamnogalacturonan II occurs there in pairs. Each pair of molecules is joined by a firm bridge of borate, which is the form boron takes in the plant. These bridges build the pectin network and carry the strength of the wall. Without boron the bridges are missing, and the wall loses strength before any sign becomes visible.

In most plants, boron hardly moves from old leaves into young ones, and the article on nutrient function and mobility explains what this depends on. In cannabis, the damage starts at the shoot tip in any case.

Supply boron evenly over the whole crop, because the plant hardly relocates boron from old leaves.
If you suspect a deficiency, look at the shoot tips and youngest leaves first.
In high humidity and still air, provide air movement so the plant transpires.

How do I recognise boron deficiency?

Boron deficiency shows first at the shoot tip and on the youngest leaves, while everything below stays green. Calcium deficiency looks similar, because calcium works in the same young tissues and is also hardly relocated. In the intermediate stage, only the leaf margin bends down with calcium, whereas with boron the whole leaf does. In ornamental cuttings, boron deficiency can leave many side shoots below the dead tip and calcium deficiency none. In cannabis this feature does not separate the two, because many side shoots also grew there after calcium deficiency. Only a leaf tissue analysis in the lab tells them apart reliably.

Stage	Appearance
First sign	The shoot tip deforms and growth stalls. New leaves yellow from the tip, much like margin burn.
First sign, leaflets	The individual fingers of the leaf are narrower at the base than at the tip.
Advanced	New growth becomes distorted with kinks and twists, and the leaf margins die.
Advanced, hallmark	The whole leaf rolls downward, and the leaves stick out at odd angles.
Late	The upper leaves feel thick and brittle. In the hemp cultivar Finola they grew thicker with age.
Late, shoot tip	The tip dies. Many side shoots grow below it, and they also die if the deficiency persists.
Very late	The root tips die as well, and the plant wilts.
Flowering	The buds stay clearly smaller, even without signs on the leaves. In severe cases the top cola is lost.

Check whether the whole leaf rolls downward or only the margin.
Feel whether the upper leaves are thick and brittle.
Compare the calcium pattern with the article on calcium.
Send a leaf sample to the lab before feeding more boron.
Judge the effect of an application on new growth. Deformed leaves stay deformed.

Does a hollow stem indicate boron deficiency?

In cannabis, a hollow stem on its own is not a usable sign of boron deficiency. The rule linking a hollow stem to boron deficiency comes from vegetable growing, and it is precisely there that it has become disputed. In three field trials on broccoli, cultivar, plant density and nitrogen supply clearly determined the cavities, while a foliar boron application had no effect. In another trial, cavities formed where the stem grew especially fast in width, and the researchers attribute them to mechanical tension. Older trials, by contrast, had measured an effect of boron applied to the soil.

In cannabis the pith, the soft tissue in the centre of the stem, breaks down anyway as the plant develops normally. A hollow cross-section therefore depends first on the point where you cut, and only after that is it a question of nutrition.

Do not take a hollow stem as a reason to apply boron.
When comparing two plants, cut at the same point on the shoot.
Check a suspicion at the shoot tip and young leaves, not at the stem.

What does too much boron look like?

Too much boron shows on the lower, older leaves, because it accumulates there with the water flow. At ten times the boron rate, the leaf contained 671.75 mg boron per kilogram of dry matter, which is the weight of the dried leaf. The control plants had 64.60 mg. In field and fruit crops, damage usually starts above 250 to 300 mg/kg in the leaf, and the tissue then dries out at the tips and margins. In tobacco the lower leaves wrinkle first; the researchers interpret this as the cells ceasing to expand. Potassium deficiency also starts low on the plant, at the serrations of the leaf margin, whereas salt burn runs more evenly around the whole leaf. Too much boron can come from a mixing error when making up the stock solution, the fertiliser concentrate, or from a misset injector. For garden and park plants, irrigation water with more than about 0.5 ppm boron is considered harmful, with ppm meaning milligrams per litre.

Stage	Appearance with too much boron
First sign	The margins of the lower, older leaves turn yellow.
Advanced	The yellow moves from the margin inward to the midrib, the main vein of each leaflet, and the margin turns brown.
Late	The whole margin of the leaflet is dead. Roots can also die.

Flush with clear water.
Then water with calcium nitrate, a fertiliser that supplies calcium and nitrogen.
Recalculate the stock solution recipe and check the injector setting.
Have the irrigation water tested for boron.
Rule out potassium and salt as the cause, most reliably with a leaf tissue analysis.
Check new growth to see whether the new leaves stay free of yellow margins.

How much boron does cannabis need?

No tested requirement value exists for cannabis, because trials with graded boron rates that measure yield and cannabinoids are still missing. The complete nutrient solutions of two deficiency trials contained 0.42 mg/L boron and, by calculation, 0.49 mg/L boron. The NC State group gives 0.875 to 1.0 ppm and advises taking a lab sample first. Boron can also run short when the fertiliser contains enough of it. One cause is a pH that is too high, the pH value being the measure of how acidic soil or solution is. A lot of calcium and low transpiration in high humidity also inhibit uptake.

You compare the lab value with leaf values from practice, and in hemp samples the middle half of the leaf values fell between 30 and 90 mg/kg boron. In the vegetative trial without boron, the first sign appeared after six weeks. At that point the leaf contained 2.46 mg/kg instead of 58.58 mg/kg, and the plants weighed 28 % less. With boron, the span between deficiency and excess is one of the smallest of all plant nutrients, so applying it by guesswork is riskier than with other micronutrients.

First get a leaf tissue analysis, and take the sample as in the article on nutrient diagnosis.
Check the pH and look up the threshold in the article on nutrient lockout.
Check the calcium share in the fertiliser.
Lower the humidity if it stays high for long periods.
Give boron only once a deficiency is confirmed, in small steps.
Watch new growth, and stop increasing once the signs have gone.
Check other causes of deformed shoot tips against the article on nutrient deficiency and overdose.