From BASF: The Chemical Company Website
By Paul Pilon
During the winter months and into the early spring, it is not uncommon to have extended periods of dark, cloudy and often rainy conditions. Although, most growers are producing plants inside various types of growing facilities which provide some protection from the outside elements, these conditions frequently have adverse affects on crop production. When there are extended periods of consistently cloudy weather, the naturally low light levels which occur during this time of the year are reduced even further.
Low light levels adversely affect plant growth and development as well as several quality attributes of greenhouse crops. During propagation, growers can expect delayed rooting and slowed growth and development during dark, cloudy weather. When the light levels are very low, growers finishing crops can expect decreased branching, more stem elongation (taller crops), and increased finishing times. To avoid quality issues and increased production times, many growers supply supplemental lighting. Supplemental lighting is commonly provided in stock plant and propagation houses.
There are several plant pathogens including Botrytis, numerous leaf spot diseases, powdery mildew and various root rots that frequently arise under cool and dark growing conditions. Of these diseases, Botrytis is the most commonly observed foliar pathogen observed under these growing conditions.
Under dark and cloudy growing conditions, the relative humidity is typically high and greenhouse ventilation is often reduced causing moist air to become trapped inside. These conditions result in long drying times. For example, following an overhead irrigation, the foliage stays wet considerably longer than an irrigation applied on a sunny day. Several plant pathogens can attack plants after only a few hours of free moisture on plant surfaces.
Although most foliar diseases don’t cause plant mortality, their presence greatly reduces the appearance and marketability of the plants they infect. The occurrence of these pathogens can be reduced by avoiding periods of high humidity (>80%), providing adequate air movement and keeping the foliage as dry as possible. Pageant is good fungicide for controlling Botrytis and other foliar diseases that may arise under adverse growing conditions.
Root rots are another common occurrence during extended dark periods. They typically arise as the result of stressful growing conditions such as being kept consistently wet, high soluble salts, or due to injury following extreme wet-dry cycles. During these periods, it is helpful to reduce the amount or fertilizers being applied as well as maintaining the plants on the dry side (these factors also improve some of the negative effects of low light levels discussed above).
Problems associated with dark growing conditions can be reduced through environmental management. Providing adequate crop spacing, good air movement, controlling the humidity, and maintain good fertility and irrigation practices will go a long way to alleviate these production issues.