Boron in Well Water: Is It Killing Your Plants?

Brown tips on your citrus leaves. Grapevines that aren’t producing the way they used to. Avocado trees that look stressed despite regular watering and feeding. If you’ve been troubleshooting these problems without a clear answer, your well water may be worth a closer look, and specifically for boron.

Boron is one of the most misunderstood irrigation water problems. It shows up on water test reports, but many homeowners aren’t sure what the number means or whether it’s a problem. In this video, Gerry from Clean Water Store walks through exactly what boron does to plants, how to interpret your water test results, and what treatment options are available.

Article last updated July 6th, 2026.

What Is Boron and Why Do Plants Need It?

Boron is an essential micronutrient and plants require it in very small amounts for cell wall formation, pollen germination, fruit set, sugar transport, and protein synthesis. Without any boron at all, plants show deficiency symptoms including hollow stems, poor fruit set, and distorted new growth.

The problem is that the range between too little and too much is extremely narrow. Boron is one of the few plant nutrients where the difference between deficiency and toxicity is measured in fractions of a part per million. What helps at trace levels becomes toxic at slightly elevated levels and that elevated level can come directly from your irrigation water.

When Does Boron Become Toxic to Plants?

According to the University of California Integrated Pest Management Program, boron toxicity in most landscape plants occurs when irrigation water exceeds approximately 0.5 ppm boron. The Pennsylvania State University Extension similarly notes that boron toxicity may occur when irrigation water exceeds 0.5 to 1.0 mg/L, particularly with long-term slow-growing crops.

These are low thresholds. Well water in arid and semi-arid regions, particularly in California, Nevada, Arizona, and the southwestern United States, frequently contains boron levels that exceed these limits. University of Nevada Reno Extension notes that well water near geothermal areas or earthquake faults may contain particularly high boron concentrations.

Boron also accumulates in soil over time. Each irrigation cycle deposits more boron into the root zone, and as water evaporates the boron concentrates further. A well water source that tests borderline may still cause cumulative damage over a full growing season.

How Boron Toxicity Is Different From SAR Problems

Homeowners dealing with irrigation water problems often see both boron and sodium on their water test report and aren’t sure which one is causing the damage. They are different problems with different mechanisms.

SAR (Sodium Adsorption Ratio) measures sodium relative to calcium and magnesium in your water. High SAR causes sodium to displace beneficial minerals in soil, degrading soil structure over time. The damage is indirect and  compacted, poorly draining soil suffocates roots. Plants can look wilted or drought-stressed even with regular irrigation.

Boron toxicity is a direct ion toxicity. Boron is absorbed through plant roots and accumulates in leaf tissue. Once it reaches a toxic concentration in the leaf, it causes direct cellular damage. The visual symptoms are different and a water test is the only reliable way to know which problem you are dealing with, or whether both are present simultaneously.

What Boron Toxicity Looks Like on Plants

Boron accumulates in the oldest leaf tissue first, which means symptoms appear on older leaves before newer growth. Classic boron toxicity symptoms include:

• • Brown or scorched leaf tips and margins — starting at the edges and progressing inward as damage increases
  • Blackened or irregularly shaped spots between leaf veins on more severely affected leaves
  • Yellowing of older leaves before browning and die-back
  • Poor fruit set and reduced yield on fruit trees and vines
  • Lower fruit quality — smaller size, off flavors, premature drop
  • Needle die-back from the tips downward on conifers, with most damage on older needles

One important diagnostic note: these symptoms look similar to salt burn, chloride toxicity, and certain nutrient deficiencies. Visual symptoms alone cannot definitively identify boron toxicity. A water test and in some cases a plant tissue test are needed for a definitive diagnosis.

Which Plants Are Most Sensitive to Boron?

Plant sensitivity to boron varies enormously by species. This is why you may see damage on some plants in your garden while others right next to them on the same irrigation zone appear perfectly healthy.

Most sensitive — show damage at low boron levels:

• • Citrus (lemon, orange, grapefruit)
  • Avocado
  • Grapevines
  • Stone fruits (peach, cherry, plum, apricot)
  • Strawberries
  • Beans and peas
  • Many ornamental shrubs and trees

Moderately sensitive:

• • Tomatoes and peppers
  • Corn
  • Lettuce and leafy greens
  • Walnut — note that walnut trees irrigated with moderate boron levels may produce nuts without obvious fruit damage but show severe leaf toxicity symptoms by end of season, reducing long-term tree vigor

More tolerant:

• • Asparagus
  • Beets and sugar beets
  • Bermuda grass and some turf varieties
  • Certain native and drought-tolerant shrubs

How to Read a Boron Result on Your Water Test

Boron on a water test report is expressed in milligrams per liter (mg/L) or parts per million (ppm) — these are equivalent units.

A simple interpretation framework:

• • Below 0.5 ppm — generally safe for most plants including sensitive species
  • 0.5 to 1.0 ppm — marginal range; sensitive plants may show symptoms with long-term irrigation, tolerant plants usually unaffected
  • 1.0 to 2.0 ppm — problematic for most landscape plants and fruit trees; treatment should be considered
  • Above 2.0 ppm — high risk for most crops and ornamentals; treatment required for sensitive species

Important: a general salinity test that measures only TDS or electrical conductivity does not tell you your boron concentration. Boron requires a specific test. Request a water test that explicitly includes boron alongside TDS, sodium, SAR, chloride, pH, and alkalinity for a complete irrigation water evaluation.

Treatment Options for High Boron Irrigation Water

Reverse Osmosis

Reverse osmosis is one of the most effective technologies for boron removal. RO membranes physically reject boron molecules, reducing levels significantly. One important factor: boron removal by RO is pH-dependent. At lower pH levels, boron exists primarily as boric acid which passes through membranes more readily. At higher pH levels (above 8.5 to 9), boron converts to borate ion which RO rejects more effectively. pH adjustment upstream of an RO system can significantly improve boron removal efficiency.

Anion Exchange

Boron-selective anion exchange resins can effectively reduce boron levels in irrigation water. Standard water softeners do not remove boron, a specialized resin designed for boron removal is required. This approach can be effective for irrigation applications where treating larger water volumes is necessary.

Blending Water Sources

For some properties, blending high-boron well water with a lower-boron source like municipal water, rainwater capture, or a secondary well, is a practical and cost-effective solution. Diluting the boron concentration below the toxicity threshold for your specific plants can solve the problem without a full treatment system. The feasibility depends on your property, water availability, and the boron level in your well.

Plant Selection

Where treatment is not practical, selecting boron-tolerant plant species for areas irrigated with high-boron water is the most straightforward management approach. This is not a fix for existing sensitive plants but is worth considering for new landscaping installations.

The Right Starting Point: A Water Test

If your well water is used for irrigation and you’re experiencing unexplained leaf damage, declining fruit production, or landscaping problems that don’t respond to fertilizer or watering adjustments, boron is worth testing for specifically.

Ask about a water test that includes boron along with the full range of parameters relevant to irrigation water quality. Once you know what you’re working with, the right solution becomes clear.

Whether you buy from us or not, we hope understanding your water better helps you give your garden, orchard, and landscaping the best possible chance.