High Humidity & Hidden Calcium/Boron Problems

This week, Dr. Josh Henry leans into his expertise in nutrition to tackle some fall crop issues that the Technical Services team dealt with in late-August and September.

PROBLEM: Over the past several weeks, our tech team has received numerous questions involving distorted growth in common fall crops such as pansies, kale and mums. While the symptoms have varied by crop, many of the affected plants share a common set of characteristics including stunted growth, distorted or “clubby” new leaves, marginal necrosis on young foliage, leaf curling and tip atrophy.

JOSH’S TIP: These symptoms are consistent with both calcium (Ca) and boron (B) deficiencies but can also be mistaken for broad mite feeding or herbicide injury. Under the microscope, none of the samples have shown presence of broad mites, and while herbicide is hard to rule out, there have not been any obvious sources of contamination or drift in any of these cases. However, a common thread has emerged this season: the growers reporting these issues have also experienced prolonged periods of rainfall and unusually high humidity throughout August and September.

The explanation lies in plant physiology. Both Ca and B move through the plant primarily via transpiration. When humidity remains high, especially overnight, transpiration slows dramatically. Even when adequate levels of Ca and B are present in the substrate, plants struggle to move sufficient quantities into the youngest developing tissues where demand is highest.

As a result, plants can develop symptoms that closely resemble calcium or boron deficiencies despite having acceptable substrate and tissue test results. In fact, by the time symptoms are observed and samples are collected, tissue analyses often show Ca and B levels that are low but technically adequate. This occurs because the environmental conditions that limited nutrient movement may have happened days or even weeks earlier during a critical stage of leaf or flower development.

This type of issue is most likely to occur during extended periods of cloudy, rainy weather when relative humidity remains elevated for much of the day and night. Crops with rapidly developing growing points, flowers and young foliage tend to be the most susceptible since those tissues rely heavily on a continuous supply of Ca and B. Supplying adequate B (0.5 to 1 ppm total at every irrigation) in the plug stage and fertilizing with a high rate (e.g. 200 ppm) of a soft feed like 20-10-20 immediately prior to transplant can help prevent this issue from occurring in finished production.

While it can be difficult to grow out of once symptoms develop, growers can reduce future risk by promoting transpiration whenever possible. Good air movement, horizontal airflow fans, strategic venting and moisture management practices that prevent crops from remaining excessively wet overnight all help maintain nutrient movement during prolonged periods of humid weather.