The amount of water that can be held by the substrate in a given container with a specific irrigation method is the effective water-holding capacity (EWHC). It may be desirable to maintain the moisture content of the medium below EWHC in order to regulate plant growth or reduce the risk of soil-borne diseases.
Overall, it is important to irrigate thoroughly. The amount of water applied must be sufficient to re-wet the entire volume of growing medium. If the medium is dry and not enough water is applied, only part of the mix will be re-wetted.
In irrigating thoroughly, the method of delivery plays an important role. With top-down irrigation, water will be pulled downward by the force of gravity. As it moves downward, some will be held by the growing medium.
Usually, some water will escape from the bottom of the pot before the medium is thoroughly wet. At the same time, water collects at the bottom of the pot and then moves back up. This can easily be seen if a saucer is placed under a pot with a very dry growing medium. When water is applied to the surface, it will quickly flow through to the saucer, but it then moves back up into the medium.
The relationship between the amount and rate of application of water and the amount retained can be rather complex, but generally, the slower the application, the higher the proportion that is retained. Applying small amounts of water repeatedly with pauses between applications (pulsed irrigation) allows for greater water retention and reduced leaching.
Irrigation System Breakdown
In the most basic terms, an irrigation system consists of three components: a source of water; a delivery system to get it to the growing medium; and valves to control water flow. The goal for any irrigation system is to deliver water to the growing medium as uniformly and efficiently as possible.
Irrigation systems can be divided into two basic categories depending on how water is delivered to the growing medium. Top-down systems deliver water to the surface of the growing mediu
Top-down systems can be divided into three sub-categories: hand watering, which may target individual containers or larger areas; overhead systems such as sprinklers or booms, which deliver water as a spray covering an area; and drip or trickle systems, which deliver water to individual pots.
Hand watering is most efficient for plug trays and flats, small containers that are tightly spaced. Hand watering is least efficient for potted plants, when they are at final spacing and the canopy is closed, because it is harder to get the water to the growing medium, and it takes longer to deliver water to each container.
Even with careful application by experienced workers, as much as 50 percent of the water that exits a hose ends up on the floor. Efficiency and uniformity can be greatly improved by using water-saver saucers or trays that catch much of the water that would otherwise miss the pot. These trays retain water so it can be absorbed from the bottom up.
Boom systems are useful for plug trays and flats. Mist nozzles can deliver water consistently at a low rate, resulting in uniform wetting and good water retention. The spray is under low pressure so compaction and washout is minimized. However, drips and uneven spray can take a toll on small containers such as plug trays.
Booms can also be used for larger containers, especially when pots are still tightly spaced. Booms typically have selectable nozzles for different spray patterns, allowing them to be used for misting during propagation and for higher-volume delivery for irrigation. They can be programmed to make repeated passes over the same area or to shut on and off while passing over different areas in their path. Shutting off some nozzles on a boom allows for selective irrigation within a block, like when spot-watering pots on the perimeter of a block that dry out more rapidly than the interior.
Sprinkler systems are often used for outdoor production areas. Uniformity is a problem outdoors, as sprinklers are designed for turf or field irrigation and don’t distribute water evenly, especially in windy conditions. Furthermore, considerable amounts of water can be wasted as runoff, depending on pot spacing.
Sprinkler systems can be used for irrigation inside the greenhouse, as well. As with booms, different nozzles can be used for propagation mist or potted plants. Like all overhead systems, they are most efficient with tight spacing and an open canopy.
Drip systems deliver water from individual emitters. In drip tape, the emitters are incorporated at regular intervals in the tape. Drip tape is widely used for irrigation of cut flower crops in soil beds but can be used in a simple setup for containers. The tape is stretched in long runs at the height of the pots, which are placed under the emitters. Drip tape is also pressure compensated, which means water does not come out of emitters until the tube is uniformly pressurized. This ensures a uniform flow rate from all emitters. A pressure regulator is used to avoid excess pressure that could burst the tape.
The most familiar form of a drip system uses flexible “spaghetti tubing” to deliver water to individual containers. The delivery end of the tubing has a weight or stake to hold the tubing in the pot. Pot weights may be designed to shut off individual tubes when not in use, thus conserving water. In large containers, it can be difficult to spread water across the entire surface. Here, individual “spray stakes” are used for better water distribution.
A pressure-compensating outlet ensures uniform flow rates from each emitter. Multiple outlets can be connected to the same pressure compensator. If the length and diameter of the tubing from each outlet is the same, then the flow rates will be uniform. If a multiple outlet system has tubes of varying lengths, then a flow restriction device at the end of each tube is needed to obtain uniform flow rates.
Pressure-compensated drip systems can be used for pulsed irrigation. For example, if the normal irrigation run time would be 30 minutes, the system can be run for six cycles of 5 minutes on, 10 minutes off. Irrigation systems controlled by soil moisture sensors can be programmed to pause long enough for soil moisture to equilibrate after each pulse.
Hanging baskets present a special problem for greenhouse irrigation. When baskets are hung on a transport system, drippers won’t work. A sophisticated system equips each basket with a funnel to ensure that the water ends up where it’s intended.
There are many overall advantages of top-down watering. For example, hand watering requires no installation of special equipment, so it is often used by seasonal operations. Hand watering is also the most flexible system. Very large potted plants, hanging baskets and plugs and flats may not be suitable for flood systems, so top-down watering must be used.
There are, however, limitations to top-down watering. For example, overhead systems result in wet foliage, which is conducive to foliar diseases. The water itself may be damaging to sensitive flowers and bracts. Plus, water and fertilizer residues may be left behind on leaves and flowers, which can be unattractive to consumers. Hand-watering requires labor each time plants need water. Drip systems require less labor on a day-to-day basis but require recurring labor to set up and maintain. Drip emitters can also clog, leading to non-uniform irrigation.