Boron is an essential element for plant growth — a small amount is needed — but if it is present in amounts greater than needed, it becomes toxic.
For example, some plants need 0.2 milligrams of boron per liter, but 1 to 2 milligrams per liter may be toxic.
Municipal water seldom contains enough boron to be toxic, but surface water, well water and springs occasionally contain toxic amounts, especially near earthquake faults. Boron problems originating from the water are far more likely than those originating in the soil.
Although boron toxicity can affect nearly all plants, there are some that tolerate much higher levels of the element.
Boron toxicity symptoms usually appear first on older leaves as a yellowing, spotting or drying of the leaf at the tips and edges. Both chlorosis — yellowing between the veins on leaves, which interferes with the plant’s ability to produce carbohydrates — and drying appear as more boron accumulates with time.
On seriously afflicted trees, such as almonds and other tree crops, which do not show typical leaf symptoms, a gum or a sticky fluid is often noticeable on the limbs or trunk.
Here’s an incomplete list of landscape plants that are more or less tolerant of high boron levels:
• Pineapple guava, photinia, rosemary, viburnum
• Japanese boxwood, bottlebrush, wax leaf privet, oleander, podocarpus, Indian hawthorn, brush cherry
In the kitchen garden, among the plants that are most sensitive to boron are lemons, blackberries, avocados, grapefruit, oranges, apricots, peaches, cherries, plums, persimmons, figs, grapes, walnuts, pecans and onions.
A little more tolerant — able to handle 0.75 to 1 milligram per liter — are garlic, sweet potatoes, sunflowers, beans, strawberries, artichokes and beans.
Some moderately sensitive plants are peppers, peas, carrots, radishes, potatoes, and cucumbers. These can cope with boron levels of 1 to 2 milligrams per liter.
Lettuce, cabbage, squash, artichokes, tomatoes, parsley and beets are moderately tolerant, growing well with 2 to 4 milligrams of boron per liter.
Asparagus is very tolerant of boron and thrives even with levels of 4 to 6 milligrams per liter.
• The Green Thumb is a column by Tracy’s master gardeners. University of California-certified master gardeners are available to answer gardening questions from 9 a.m. to noon Tuesdays, Wednesdays and Thursdays at 953-6112 or firstname.lastname@example.org. Questions for Heather Hamilton can be sent to email@example.com.