Heat and Moisture StressAlthough not technically a deficiency, heat stress symptoms are similar to nutrient deficiencies and will be covered in this section. Visual symptoms will vary depending on the severity and longevity of the stress. Soil salts can become a problem in dry soil and cause root injury, which in turn causes wilting of shoots and leaf scorch. Sometimes scorch of the leaf margins may develop on immature or nearly mature leaves; especially those exposed to direct sunlight.
Heat stress is often noticed after a period of cloudy, rainy weather promoting succulent top growth, followed by hot, dry, sunny days. The lack of oxygen in the soil caused by excessive rainfall (and possible poor drainage) puts the plant under stress to withstand the quick respiration of hot weather. Wilting of young growth may occur at this time.
Bull nosed bloom formation of roses is essentially a temperature stress problem. Red flowers are more likely to develop bull noses than light colors. The darker pigments apparently absorb more heat during the sunlight hours and re-radiate more heat at night establishing a greater temperature fluctuation within the bud.
Petal edge burn is more noticeable in red flowers and is associated with the buildup of heat in the petals. This is more evident after extended periods of cloudy weather followed by hot, sunny days.
When air temperature is high, relative humidity low and air is moving across the leaf surface, the demand for water is greatest. Under these conditions, it is essential to provide water to the soil to reduce the total stress on the plant. During the hottest part of the day, it may be beneficial to water overhead. This will reduce the air temperature while providing moisture for the plant.