Boron Toxicity

We've got an interesting water situation in certain areas of the Lowcountry- excess boron. It was a surprising discovery for me and it took me a long time to figure out. When a landscape is being irrigated from a municipal source, you don't expect a toxicity problem.....so I never tested.
Well, finally I did. And as it turned out, the aquifer that a certain municipality pulls water from has high boron levels (it is safe for human consumption) . I have found excessive boron in the well and municipal water in Mount Pleasant, Meggett, Edisto and John's Island.
Boron toxicity is fairly easy to diagnose, but it requires a water or foliar test for confirmation.
WHAT YOU SHOULD KNOW:
1. It occurs on the older growth. It takes a while for the element to accumulate in the leaves, so only the older leaves will show injury.
See how the new growth on these Sago Palms is unaffected? This is a typical injury pattern:

2. A marginal "burn" shows up on the leaves of dicots, while a "tip dieback" happens on monocots. The excessive boron accumulates at the very ends of the leaf veins. In a monocot, the veins run parallel and straight to the tips of the leaves. A good example is shown on 'Evergreen Giant' Liriope:It can also be seen at the tips of palms, like this Windmill Palm (Trachycarpus fortunei):
When a dicot like a Hydrangea is suffering from excessive boron in the water, the edges become burned. The necrosis will sometimes look wavy:
Magnolia is extremely sensitive to high boron levels and the injury is very distinctive. In fact, I commonly use this species as my "indicator plant" in a landscape if I suspect high boron.3. It generally doesn't persist in the soil. If the pH is high or there is a lot of calcium in the soil, boron can be bound to the soil. But generally, boron easily leaches from the soil profile. Once the contaminated water is removed as the irrigation source, the plants will recover.
4. Contaminated soil and certain fertilizers can be responsible for excessive boron levels. In my experience, this has never been the source, but it should not be overlooked. If you have fertilized with potassium chloride or amended the soil with marine sediment or animal manure, this may be the cause of the toxicity.
5. Some plants are more sensitive than others. Magnolia, Hydrangea, Palm, Liriope, Cycads and Viburnum show substantial leaf burn. Other plants, like Fatsia and some Camellia cultivars become slightly "off color", but don't have distinctive symptoms. And then there are a few species like Ligustrum and Cleyera (Ternstroemia gymnanthera) that don't seem to be affected at all.
WHAT SHOULD YOU DO?
The take-home message is that soil and water tests are a good idea- they eliminate a lot of guess work. As you can imagine, boron toxicity is often mistaken for drought or fertilizer burn.
If the tests confirm that boron is to blame, the best option you have is to limit irrigation. Turn off the irrigation system in the dormant season and run it no more than once per week during the summer.
Use a rain barrel to collect rain water for supplemental irrigation (this is very important for house plants).