Boxwood with Boron Toxicity Symptoms

Dendro and Codit the tree detectives looked at a property, in which some boxwoods (Buxus spp.) were "off-coloured". The shrubs in question were located in the backyard, adjacent to the pool. The owner spends a lot of time taking care of the pool. Using a hand lens and a small knife, Codit examined the leaves and scraped bark from a few branches to see if the shrubs was alive (they were alive). The margin of the leaves exhibited tissue necrosis, and appeared to be scorched. But there did not seem to be any pests or diseases on the leaves. Interestingly, in a row of boxwood, only the ones at either end of the row showed the exact same symptoms. That would usually point to a pest or pathogen, however, the scorching on the leaves points to an abiotic factor. It was not drought steers, as there was an irrigation system, not cold desiccation either, as it was not winter at that time. In addition, there was no heat sources like barbecue grills nearby. After a short conversation with the owner, he revealed that the pool was fresh water, there was no leaks and he used a variety of tools and pool products when taking care of his pool. One of the tools and chemicals are a water optimizer to improve water clarity and a pool cleaning robot. The latter was submerged in the pool to clean it, and the owner drained the robot during and after operation. The spots where he drained the robot were at either ends, where the boxwoods showing the scorching symptoms were located. The water optimizer contains sodium tetraborate pentahydrate (or the cleaning compound borax). Boron as a micro-nutrient is needed by plants but it would accumulate at the leaf margins when there are too much. Other toxicity symptoms include necrosis and uniform yellowing, followed by decline and leaf drop. Dendro advised the owner not to drain the pool water at the boxwoods and the soil should be flushed with clean water to leach out the boron. His landscaper could apply sulphur or gypsum to the soil, and they could help in some cases.

Reference: Arborist News, Oct 2023, pages 17, 18, 54, 55