Growers typically develop methods for determining how much and how often to irrigate. Proper water management enables the production of high quality plants while limiting waste and reducing costs. When managing water and scheduling irrigation, it is important to consider the factors that influence irrigation frequency, the methods for determining when plants need water, irrigation timing, irrigation uniformity, and the negative aspects of overwatering.
There are many factors that influence the frequency and amount of water that need be applied to production areas. These factors not only dictate the frequency of watering but should also be considered when grouping plants in different zones within the production areas. If plants are not grouped, watering frequency will be dictated by the fastest growing crop, and excess water will be applied to the remaining, slower-growing crops. This excess irrigation will result in nutrient leaching and the need for additional fertilizer applications.
The main factors influencing irrigation frequency are discussed below.
Plant species, plant size, and growth rate all have significant impacts on the frequency of irrigation. Some plant species prefer dry conditions while others must be consistently moist. Small young plants with little top growth require less water because they lose less water via transpiration/evaporation than larger plants in similar sized containers. Plants growing rapidly with extensive succulent growth usually wilt sooner because more of the tissue is composed of water. Plants with large, thin leaves absorb and transpire water more rapidly than plants with small, thick leaves (e.g., broad-leaved evergreens, conifers, and succulents). Plants should be grouped according to daily water requirements.
Container size, substrate type, and nutrients influence irrigation needs as well. Pots have fixed volumes that can hold a limited amount of water. Larger containers provide a larger area for water storage. Plants in large pots tend to need water less frequently than plants of the same size in smaller containers. Substrate weight, color, and feel can also be used to determine irrigation scheduling. Some substrates can hold more water for longer periods than others. Many amendments to substrates are being explored to help increase water holding capacity of container substrates. Nutrients influence irrigation because excess irrigation may wash beneficial fertilizer from the soil. Repeated excess irrigation can be a costly mistake. A balance must be reached between water status and nutrient leaching. Typically the leaching fraction or the percentage of the total amount of water applied to a pot that runs out of the bottom of the pot should be maintained at approximately 20%.
transpiration rate is affected by several environmental factors
including intensity of sunlight, wind speed, air
temperature, and relative humidity. Water loss is greatest in full sun
where the heat energy can have the greatest evaporative impact. Wind
causes increased water loss. The moving air helps pull water out of the plant. Water is lost more rapidly when
are high because the water is changed more rapidly into vapor.
Additionally, warm air can hold more moisture than cold air. High
humidity, such as on damp or foggy days, will cause less water loss
Irrigation can be used for frost protection. Frost protection is best achieved by an overhead irrigation system. Water may be applied directly to leaves or it can be applied to a tarp that has been placed over the plants.
The most common method of determining a watering regime is by simply weighing the containers with a scale or by hand on a daily basis. An emerging technology that is showing great potential with regard to ease of application, maintenance, and efficiency is the load cell with a computer interface. The load cell converts force (weight) into an electric output that can be monitored continuously, providing continuous feedback on crop water status. This and other on-demand systems successfully use weight to maintain adequate moisture throughout the day.
Another method for scheduling/determining irrigation frequency is commonly practiced in larger scale nursery operations and is called the water budget procedure. This involves measuring the amount of water lost from a crop by both direct evaporation from the soil surface and transpiration from the plants. This combination of evaporation and transpiration is called evapotranspiration or ET. This value is known as the "crop water requirement." Complete determination of the quantity of water needed must incorporate other losses such as run-off of applied water or percolation of applied water below the root zone. Percolation is of great importance for field-grown nursery crops.
Soil-moisture probes can also be used for irrigation scheduling. While most suitable for field-grown crops, some of this moisture sensing technology is also being adapted to container crop production. In field soils, devices for measuring soil moisture levels have been available for over 20 years. Tensiometers are the most widely used for this purpose. Tensiometers measure the soil moisture status at the soil depth where they are buried. These readings are often best used in conjunction with ET data. The ET data indicates how much to water is lost to the air, and the tensiometer signals when available water is low.
Application of water during the hottest part of the day may indeed cool plants but also result in a large waste of water due to evaporation. Watering at night is more conservative; however, plants with wet foliage (as results with overhead watering) during the coolest part of the day may be predisposed to infection by fungal pathogens. The presence of a wet film on leaves during the cooler night provides ideal conditions for plant penetration and growth of some pathogenic fungi. Early morning watering, in contrast, has the benefit of cool temperatures and reduced evaporation losses during watering. Also, the subsequent temperature rise will dry the foliage, minimizing the chances for fungal infection.
Frequency and timing should be considered when scheduling irrigation. Many producers irrigate with a single pulse of water. However, recent research has shown that this practice can lead to considerable water loss and that cyclic irrigation may be more efficient. A grower that normally applies 0.5 inches of water over a 60 minute irrigation cycle can reduce irrigation volumes and increase irrigation application efficiency by 25% by simply changing to a cyclic irrigation pattern. Irrigation application efficiency is a ratio of the volume of irrigation water stored in the substrate (water available for plant use) compared with the volume of water applied. If the grower used three cycles of 15 minutes each or five cycles of 9 minutes each, significant water savings will accumulate over time. Savings are realized because less water is applied and leached, and a greater percentage of the water applied is retained by the container substrate. This higher retention rate not only saves water but will also reduce nutrient leaching from containers without sacrificing plant health to high substrate salt levels (EC levels).
Irrigation systems should be designed with uniform application patterns to ensure that all plants in each production area are adequately watered. Non-uniform systems may contribute to nutrient and chemical leaching and runoff; this could potentially contaminate both surface and ground water. Additionally, systems that lack uniformity will produce plants that suffer from overwatering, drought stressed plants, and quality plants all in the same production area. This results in significant production losses. Distribution uniformity should be checked at least once a year.
Excess water in the growing substrate fills air spaces, reduces the substrate’s space for oxygen, and limits the oxygen available to plant roots. Excess water in the field may result from flooding and/or poor drainage. In container production, excess water may result from excessive irrigation and/or the use of a medium with low porosity and a high water-holding capacity. The primary effect of excess water on plant roots is oxygen deprivation. Impaired root respiration prevents roots from taking up adequate water. Shoots subsequently will wilt, turn yellow, and drop off because of the drought induced by excessive water. If the plant is not killed by excessive water, it becomes predisposed to infection by disease organisms considered to be weakly parasitic (i.e., the organism would not likely infect a normal, vigorous plant). Excessive water is conducive to Pythium and Phytophthora infection of nursery crops.