Blueberries, a high-value crop, require precise environmental conditions to thrive and produce a successful harvest. The blueberry bush is highly sensitive to the amount of moisture in the air, known as relative humidity. This moisture balance is a defining factor in growth, fruit quality, and susceptibility to disease pressure throughout the growing season. This environmental factor interacts closely with temperature, shaping the plant’s physiological processes and determining its overall health.
Optimal Humidity and Temperature Range
Blueberries generally prefer a moderate to high level of atmospheric moisture, ideally between 50% and 70% relative humidity during the main growing season. This range mimics the conditions of their native, temperate woodland habitats, supporting healthy leaf function without promoting excessive moisture buildup. Within this moisture range, the bushes also require an optimal temperature band for photosynthesis and development. The ideal temperatures for active growth and fruit maturation for highbush blueberries are typically between 65°F and 80°F (18°C to 27°C). Consistent conditions allow the plant to function efficiently, ensuring a steady rate of water and nutrient uptake from the soil.
Maintaining this optimal range is a constant balancing act, as temperatures and humidity levels are intrinsically linked. Warm air can hold more moisture, meaning a high temperature can quickly lead to high relative humidity if the air is not moving. Deviation outside of this ideal zone, whether too high or too low, introduces stress to the blueberry plant. This stress can manifest either by causing excessive water loss or by creating an environment where pathogens can easily proliferate.
The Consequences of Low Humidity
When the air becomes too dry, the blueberry plant faces a phenomenon called water stress, which forces it to conserve its internal moisture. Low relative humidity increases the vapor pressure deficit (VPD), which drives a rapid increase in the rate of transpiration, or water loss through the leaves. To combat this, the plant’s leaves will close their stomata—tiny pores responsible for gas exchange—to reduce water vapor escaping into the dry air.
While stomatal closure is a survival mechanism, it comes at the cost of production, as it simultaneously limits the intake of carbon dioxide necessary for photosynthesis. Reduced photosynthesis slows the overall growth rate of the bush and negatively impacts the development of fruit. Prolonged exposure to excessively dry air can lead to the desiccation of delicate plant parts, such as developing flowers and young fruit. This desiccation can result in leaf scorch, fruit drop, and a reduction in the final berry size and weight, leading to a lower harvest yield.
The Risks of Excessive Humidity
While a certain level of humidity is necessary, excessive moisture in the air, particularly above 70%, poses a major threat by fostering the growth of fungal and bacterial pathogens. Fungal diseases like Botrytis fruit rot, often called gray mold, thrive in high-humidity environments, especially when there is prolonged free water on the plant surface. This fungus can cause blossoms to turn brown and die off, and later, it produces a fuzzy gray growth on infected fruit, leading to substantial crop loss.
Another concern is Anthracnose fruit rot, which is favored by warm, wet conditions and can cause infections that remain latent until the berry is nearly mature. Mummy Berry disease is also exacerbated by high moisture, leading to the infection of new shoots and the subsequent mummification of the developing fruit. Beyond disease, excessively high humidity can also interfere with the process of pollination; air that is too moist can cause pollen grains to clump, reducing their viability and hindering the activity of pollinating insects.
Managing Humidity in Different Growing Environments
Effective management of air moisture is achieved through cultural practices that either introduce moisture or enhance air movement. In dry climates, the primary goal is to increase the moisture available to the plant and slow water loss from the soil. Heavy mulching with organic materials like pine bark or wood chips retains soil moisture and minimizes evaporation from the surface.
Using drip irrigation systems instead of overhead sprinklers ensures water is delivered directly to the roots without wetting the leaves. This minimizes evaporative stress.
In humid environments or protected structures like greenhouses, the focus shifts to reducing moisture and increasing air circulation to prevent disease outbreaks. Pruning the bushes to create a more open canopy allows for better airflow, helping the foliage dry quickly after rain or dew. Strategic plant spacing promotes ventilation between rows, which helps prevent fungal spread. Watering should occur in the early morning so the plants use the moisture during the day, allowing the leaves and soil surface to dry completely before evening.