Inside those green leafy specimens that we call plants lies a vast sea of water. Each cell contains up to 90 percent water, which plants absorb from the soil. Transport of minerals, movement of sugars, plant rigidity (turgor) and other major processes rely on water to function. While some water loss can be tolerated, a consistent and adequate supply is needed for plants to grow, reproduce, and ultimately survive.
Respiration is the process by which energy and water stored during photosynthesis are utilized for plant growth and necessary functions. Respiration occurs continually, night and day, therefore it is important that adequate stores of water are obtained from photosynthesis. If too little water is stored, respiration rates will exceed that of photosynthesis and plant vigor will decline. Temperature is a heavy influence during respiration. The higher the temperature the greater the water and energy loss.
Transpiration is the evaporative loss of water vapor from plant leaves through tiny openings called stomata. Stomata allow for free movement of water, carbon dioxide, and oxygen. The presence of light signals stomata to open. Conversely, stomata close during darkness. When stomata are open, water is able to move along a force gradient from the roots of the plant, which have a higher quantity of water, to the leaves and surrounding air, which have less water. In this way a continuous stream of water moves from the soil and roots and proceeds out of the plant. Transpiration rates are dependent on environmental factors such as temperature, relative humidity and wind. Stomata are sensitive to drought and will close under dry conditions. When stomata close water movement is prevented and collection and storage of water and energy during photosynthesis cannot occur.
Evaporation describes water loss at the soil area. Dependent upon heat, relative humidity, wind and sun, the summer months are the time of year where evaporation is highest.
Soil type is often overlooked when it comes to water loss. Sandy soils are notorious for needing more watering. Sand is made up of large soil particles which allow water to percolate quickly. Watering with large amounts of water at one time simply moves the water past the root zone out of reach of the plant. Clay soil is the opposite, with densely formed particles, less water is needed, but it saturates slowly.
Keeping your plants well-watered is important for their overall health and growth. While the average gardener does not have ways to control temperature, sun exposure, wind and humidity, there are a variety of practices that can help diminish water loss. Mulching inhibits evaporation. Amending poor soil is advised to improve water-holding capacity. Watering in the cool morning hours is always recommended to allow for less evaporation. Site selection is another component. Planting near cement and reflective walls attracts heat and precipitates water loss. Many plants can tolerate some shade, and even layering, to maintain moisture and keep the effects of sun, heat, and wind to a minimum.