It’s kind of surprising that an element as rare as boron turns out to be an essential micro-nutrient of Cannabis. The compounds that boron forms with other elements tend to be soluble and transportable by water. So, while borates are uniformly distributed worldwide in infinitesimal soil quantities, a handful of dense concentrations or “mother lodes” have been created by the evaporation of ancient seas over geologic time. In it’s mineralized and crystallized salt forms, mostly borates like the highly soluble Borax; these large accumulated deposits have been discovered and are mined almost exclusively in Turkey and California.
Boron is required by cannabis to regulate carbohydrate production, stimulate flower development, facilitate root elongation for mycorrhizal fungi and build healthy cell walls; yet, as with manganese, too much of a good thing is highly toxic. In fact, there seems to be a rather fine line between deficient levels of boron and potentially toxic excessive levels. In both deficiency and toxicity, the subtle cannabis leaf necrotic spotting and twisted, stunted, even dead meristem (growing tip) symptoms tend to go unnoticed until it is too late for corrective action, so I think water and soil testing is the best way to monitor.
Beware of irrigating with high salt content well water as a potential source of excessive boron, as little as 2 ppm could be harmful. Outdoors, the process of choosing your cannabis grow site should include a complete soil analysis which includes a report on boron (B). Guidelines for tomato field selection suggest avoiding locations with soil boron in excess of 5 ppm. While mobile in the phloem of a very few fruit and vegetable crops, boron is considered immobile in most plants (including cannabis), as well as my favorite research proxy — the tomato. The difference has to do with what sort of complex sugars are produced during photosynthesis. Since boron cannot be scrounged from older growth, a small but constant supply is required for healthy development.
If your testing comes up boron deficient there are several inexpensive products on the market you can apply. One third to one pound of boron per acre should be plenty; Solubor, for instance, is 20.5% boron so a little goes a long way. Boric acid, often sold as an insecticide or fungicide, is a much cheaper home remedy even at only 16.5% boron. Please watch your pH carefully when supplementing with substances containing boron!
Any advice and opinions about the cultivation of Cannabis offered by Bruce N. Goren are his own and do not represent the University of California or the Master Gardener Program.