Plants need large quantities of water for growth. The most important factor driving water movement in plants is a process known as transpiration. Transpiration is the loss of water from plants in the form of vapor (evaporation). Plants utilize most of the water absorbed from the soil for transpiration (95%), but a small portion of the water absorbed is used during photosynthesis for producing the carbohydrates necessary for plant growth (5%). The rate of transpiration is dependent on water availability within the plant (and soil) and on sufficient energy to vaporize water. Most energy supporting transpiration is derived directly from the sun (solar radiation). Sunny, hot weather increases the rate of transpiration and thus the risk for wilting if adequate water is not available.
Water typically makes up 80 – 95% of the mass of growing plant tissues. Mature woody plant tissue water content ranges from 45 – 50% while herbaceous plant water content ranges from 70 – 95%. Plants have cell walls that allow the build up of turgor pressure within each cell. Turgor pressure contributes to rigidity and mechanical stability of non-woody plant tissue and is essential for many physiological processes including cell enlargement (plant growth), gas exchange in the leaves, transport of water and sugars, and many other processes.
Plants have adapted over time to tolerate extremes in water availability. Plant water availability is influenced by soil moisture. The texture and structure of soils and container substrates influence their relative capacities to retain water. Plant water uptake does not always keep up with transpirational water loss rates, even if soil moisture is adequate. Temporary midday wilting is common during hot, sunny afternoons, but plants can rehydrate over night when lower temperatures result in decreased transpirational water losses. If the soil/substrate dries without addition of water from precipitation or irrigation, permanent wilting may occur, resulting in plant death. It is critical to manage the water status of nursery crops and to irrigate based on soil moisture and plant needs.
Growth is dramatically affected by the timing and amount of water applied during production. Certain stages of plant growth are more sensitive to water stress than others. Plant vigor and overall resistance to stress from insects and/or disease are influenced by water status. Water management is the most important cultural practice of nursery growers whether growing field or container crops. Therefore, optimum growth and quality of nursery plants can only be achieved if water is properly managed. In the nursery industry the goal is not simply plant survival but ultimately the production of quality plants in the shortest amount of time, using minimal production space, with least impact on the environment, and with the most efficient and effective use of other resources so that bottom line costs are reduced.