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.
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: www.slosson.ucdavis.edu/files/66338.pdf. The Food and Agriculture Organization (FAO) of the United Nations has a plant list which can be found at: www.fao.org/DOCREP/003/T0234E/T0234E05.htm#ch4.1.3.
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.