Sources and Symptoms of Boron Toxicity in Container Grown Woody Ornamentals

Symptoms

The early symptoms of B toxicity on both Rhododendron and Taxus were characterized by leaf-tip yellowing (Fig. 1). This concurs with reported data on other crops (1,2,3). As the toxicity progressed, marginal chlorosis was observed with Rhododendron (Fig. 2), followed by tip burning and premature leaf drop. With Taxus, no marginal chlorosis developed. Leaf tips turned yellow, followed by tip burn. Often it appeared that the leaf tips or margins had been scorched or burned (Fig. 3). In some cases of severe toxicity, the tips turned brown almost immediately. These injury symptoms occurred because B is transported in the transpiration flow and accumulates in the leaf tips and margins as water is lost to the atmosphere (3,4).

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Fig. 1.

Early symptoms of B toxicity on Taxus media.

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Fig. 2.

Marginal chlorosis caused by B toxicity on Rhododendron × Catawbiense.

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Fig. 3.

Scorched or burned appearance of leaf-tip with the later stages of B toxicity on Rhododendron × Cataw-biense.

If the B toxicity is not corrected, new growth the following season may be affected. Terminal growth is generally rosetted or twig dieback occurs (Fig. 4). Leaves are dwarfed, curled and arise from shortened internodes. Flower buds, if present, generally die with acute B toxicity.

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Fig. 4.

Dwarfed leaves arising from shortened internodes on Rhododendron as a result of B toxicity on Rhododendron × Catawbiense.

Early stage symptoms of B toxicity are similar to those produced by numerous other conditions, overwatering, soluble salts, etc. As a result, the first step to correcting a potential B problem is to analyze foliar tissue to positively identify if B toxicity exists. Currently research is underway to establish B levels in the tissue which cause B toxicity, however, based on available data from samples taken from growers and some research data, approximately 80-100 ppm B seems to be a critical level. Boron levels above this range in the foliage result in toxicity symptoms while B levels below this range seem to result in normal growth. Further research is underway to determine more precisely critical B levels and the growth reductions with excess B.

Sources of boron

Most B toxicity problems in the nursery industry are caused by over fertilization with B fertilizers. The major source of B in a container medium is the micronutrient fertilizer. Seven micronutrient sources are presented in Table 1, and four of these have B levels of 1,000 ppm or higher. These sources alone will not generally produce B toxicity because the B becomes available over an extended period of time. Many of the other pre-plant mix ingredients contain B. For example, in Table 2 a number of common pre-plant ingredients are listed with the rate per cubic yard of medium, % B, and the grams of B per container. Clearly, some of the fertilizers are quite high in B. For example, one pine bark medium studied contained .25 ppm B, and Canadian peat contained approximately .06 ppm B. In many cases, total B may exceed 2 grams per container when all additives are considered.

Avoiding boron toxicity

One way to avoid B toxicity is to select a micronutrient fertilizer without B or one with low B levels. Our data show that adequate B is available as impurities in preplant medium ingredients (Table 2), and is not needed in the micronutrient fertilizer.

Boron toxicity may also be minimized by proper pH levels. Availability of B decreases as the pH increases. If B toxicity has been a problem, a pH range of 6.0-6.5 may adequately correct the problem. Most B toxicity problems occur when the pH is in the 4.5-5.5 range. As a result, we often see B toxicity on plants grown in this pH range (rhododendrons, azaleas, etc.).

To further complicate the problem of B toxicity, post plant fertilizers may contain B. For example, Peters standard fertilizer (20-20-20) has .0068% B, which means that when 200 ppm N is applied, approximately .68 ppm B is also applied. Some slow release fertilizers also contain B. Growers with B problems should request from the technical representative of the fertilizer company the data on B levels. We have found that many of the technical representatives were not aware of any B in their product. Government regulation for fertilizers only covers minimum levels in the fertilizer bag. Furthermore, the state agency which monitors fertilizers may not test for B, as is the situation in Ohio.

Previously it was mentioned that tissue analysis is necessary to confirm B toxicity. Even then, tissue analysis may not provide positive identification of B toxicity. Boron is very active in the soil and complexes with many other elements within the medium. Copper (Cu) is one element with which B complexes readily. As a result of the complex, Cu may often be low or deficient in the tissue sample where B toxicity exists. In fact, B toxicity may induce Cu deficiency. Also, zinc (Zn) may appear to be low in the tissue sample.