Boron Toxicity: Challenges and Solutions for the Gardener

 Last spring I had the pleasure of creating a permaculture-style keyhole garden design for a client in southeast Sonoma. The vision for this organic-style garden was a place to rest, to wander the winding paths, and to steward veggie, cut flower, and water-wise, ornamental planting beds. Like many places around here we had to consider deer and gophers. But we also had another less common challenge to contend with – high levels of boron. 

Keyhole Garden and High Boron Tolerant Plants

 Boron is essential in small quantities to plant growth. In larger quantities it becomes toxic to plants. Signs of boron toxicity often start with a browning, yellowing and drying of leaf tips.  Eventually these symptoms progress to the entire leaf. Some crop-bearing trees display an oozing of a gummy substance on the trunk or branches rather than leaf damage. Some plants may not bear as much fruit as they should. Overall growth is often stunted.

 There are swaths of land in Sonoma Valley and the rest of the state that are known to have high boron conditions. Although boron is found in our soils high boron that leads to toxicity is more likely to be found in well water or spring water. If you use that water to irrigate your plants you may have a problem.

Your water can be tested for boron. It is typically measured in milligrams(mgs)/liter (roughly equivalent to parts/million). Less than 0.5 mgs/liter is fine, 0.5 – 1.0 mgs/liter is considered mildly to moderately toxic to certain plants and greater than 1.0 mgs/liter is considered severe. My client used Caltest Labs in Napa, CA and got a reading well over 1 mgs/liter.  

 Ornamental and crop plants vary in their ability to tolerate high boron levels. Grape vines and roses seem to stand up to boron with aplomb. But some of our other favorites such as camellias, azaleas, penstemons, peaches, olives, figs and citrus really struggle with high boron irrigation water. This is very disappointing to plant lovers. We don’t like to be limited.

 So how do we know what we can and can’t plant if we have a boron problem? I did some local research and found Janet Rude and her expert staff at Wedekind’s Garden Center here in Sonoma to be very helpful in advising me about boron tolerant plants. Some informal rules of thumb are: a) stay away from acid loving plants; b) choose grey leafed plants; and, 3) choose perennials that die back each winter (and are therefore not as susceptible to boron accumulation in their leaves).

There are plant lists based on scientific studies in books and online. In the book Abiotic Disorders of Landscape Plants by Laurence R. Costello there are tables that list salt and boron tolerance of landscape plants. UC Davis has a publication called Landscape Plant Salt Tolerance Selection Guide for Recycled Water Irrigation which includes a modest list of boron tolerant plants at: The Food and Agriculture Organization (FAO) of the United Nations has a plant list which can be found at:   

 If your boron content is super-high and nothing grows well you might consider switching your irrigation water source to city water. An environmentally friendly option is to collect and store rainwater in a tank or cistern and use it for dry-season irrigation.

The oval shaped keyhole garden design ended up having an outer ring of boron-tolerant, water-wise, fairly deer resistant plants such as pink lemon bottle brush, leucophyllum, lavender, bush germander, “Dark Star” ceanothus, catmint, and dwarf oleander.  In the interior there is a small rose garden.  The cut-flower section includes irises, alstroemeria, yarrow cultivars, daffodils, and red hot poker. The veggie garden is limited in the way of boron tolerant choices. Tomatoes, parsley, beets and asparagus should do well. Lettuces, cabbages, corn, squash and muskmelon and artichokes may be okay.

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11 Responses to Boron Toxicity: Challenges and Solutions for the Gardener

  1. johanna says:

    love your newsletter info-

  2. I see you are measuring Boron in units of Milligrams per liter, is that right? Or is it Micrograms per liter? My well produces 4 Micrograms per liter, so I think that is ok, is it?

    • Karen says:

      Hi Paul, I apologize for this delayed response. For some reason I didn’t get an email notification of your question so this slipped by me. The boron was measured in milligrams/liter. Micrograms are much smaller than milligrams so you are okay with only 4 per liter.

  3. ron eicher says:

    I am finding boron toxicity to be a real issue for Redwood trees. Much of the well water on the Peninsula contains lethal amounts of sodium and boron. Golf courses struggle with this.symptoms in foliage show up as stunted off color foliage and lead to dieback and eventually mortality.

  4. cheryl cozad says:

    I’m on the cusp of redoing landscape due to failures to thrive. Your information plant choices with high Boron in the water is very timely. Thanks. Now on to the info about succulents.

  5. judy marshall says:

    How do peonies do with high boron water?

    • Karen says:

      Hi Judy, I actually don’t know. I looked them up in a boron tolerance chart and they weren’t listed. However, because they are perennials and lose their leaves each winter they many be okay. The boron won’t accumulate in their leaves as with evergreen plants. I’d give one peony a try and see how it performs.

  6. I am really interested in the roles that different plants play in Nature – symbiotic and allelopathic relationships fascinate me. It is my belief that lavender harvests boron from the air and makes it available to neighbouring plants when it is deficient in the soil. Cape weed is widely acknowledged as making calcium available – while it is my belief that caltrop, a weed that farmers in Western Australia hate, makes bismuth available to other plants. Bismuth is never mentioned as being a vital element for plants, however when I looked it up, I read that ‘wheat takes bismuth up from the soil’. A strange comment.
    So where do the elements come from? It is my belief that Nature can do many things that science cannot do as yet, and I suspect that many of the elements are actually transmuted from predominantly nitrogen, the most prolific gas in the atmosphere.
    I am working through a list of weeds to determine their individual roles – and it is hoped that a university will be prepared to undertake some research to see if their science will confirm my pseudoscience.

    • JD Hagger says:

      Elements do not change form except with enormous transfers of energy. Almost all the elements on earth have never changed form since they got here after being forged in the hearts of super-dense stars. Plants are certainly able to break down and rearrange molecules, but stripping nitrogen of enough protons to turn it into boron would release an enormous amount of energy (for reference, atomic bombs are so devastating because they essentially do this). Most boron ends up in water supplies because the oxidized form that sits in the soil, borate, is soluble in water. So as water seeps through the ground it picks up borate, bringing it to whatever water source you use.

      Hope this helps!

  7. Urban Tree Farm says:

    Can rhus lancea (African sumac) trees live in area with boron in the water?

    • Karen says:

      Sorry but I do not see Rhus lancea on the list of boron tolerant plants in My UC Abiotic Disorders of Landscape Plants book. I found a couple of boron tolerant shrubs in the Rhus genus such as Rhus aromatica and Rhus trilobata in a University of Nevada Cooperative Extension publication

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