Containerized plants are by definition constrained with regard to root growth. As a result, plant growth in containers is impacted by a variety of factors including container style, container substrate, weather, and irrigation uniformity. Water is a critical component of plant growth, and all of the other factors mentioned above influence the amount of water available to container nursery plants. Various container styles and substrate types have different capacities for holding water. On cloudy or overcast days, transpirational water losses decline. The amount and frequency of water applied by irrigation is adjusted to offset plant water losses in the context of all of the other factors that might increase or decrease plant water use.
Irrigation systems most commonly used in container nurseries fall into two main categories:
Each irrigation system has strengths and weaknesses. System selection depends upon: intended use, topography of the site, water supply (i.e., water quality and quantity), availability of funding, and desired profitability.
Overhead sprinkler systems are based on two types of spray heads - rotary sprinkler heads or stationary spray heads. Rotary sprinkler heads utilize a rotating nozel that projects a stream of water over the production area while stationary heads direct fast moving water against a deflection plate. This results in a specific constant pattern of spray.
Overhead sprinkler systems are traditionally the most popular systems
for nursery irrigation. Unfortunately, they are also the most wasteful
systems. Primary disadvantages of overhead systems are the need for high operating pump pressures and poor application efficiency. Higher operating pressures require more energy and fuel. Water application is inefficient because the containers occupy a small percentage of the bed area leading to water losses that approach 80 percent. Water efficiency is also reduced by plant foliage, which can direct water away from, rather than into the container substrate.
Large amounts of water can be lost via evaporation as water travels through the air to the plants. Wind may also direct water away from plants.
Water use efficiency can be improved by channeling runoff from production areas into water collection ponds. Channeling can be accomplished through construction of sloped beds that have been covered with plastic or some other impermeable material. Some nurseries place containers on gravel that has plastic underneath. Collected water can be re-circulated into the irrigation system.
Water reuse can lead to problems such as accidental spread of pathogens, chemicals, salinity and/or fertilizers. If a nursery is totally dependent on ponds as the primary source of water, salinity could become a serious problem during a prolonged drought when plants have a high water demand. Wetting of foliage can also wash off applied chemicals (e.g., fungicides). Overhead systems, even when efficiently managed, still waste water and energy.
Microirrigation systems may consist of capillary mat irrigation, micro-sprayers, micro-sprinklers, and spaghetti tubes. Microirrigation systems are increasing in popularity because they are more efficient than other systems, they are effective at low operating pressures, and they utilize low rates of water application. When managed correctly, these can be the most efficient systems.
There are some disadvantages to microirrigation systems including emitter clogging by substrate particles, algae, and chemicals, emitter displacement from the container, and lack of frost protection.
Various methods of capillary watering are now used in both the greenhouse and nursery industry. One method involves the use of line-source tubes that carry water to a capillary mat. Water moves through the mat by capillary action somewhat like a sponge. The containers are placed atop the saturated mat. This allows the medium in the container to soak up water. Containers must be saturated before being placed on capillary mat systems to ensure effective water uptake. Capillary mat irrigation systems can use 60% less water than overhead irrigation systems. Capillary mats have the limitation as other sub-irrigation types in that salts can build up in substrate over time without periodic leaching events.
Micro-sprayers or spray stakes are very efficient. Water is delivered directly to the soil in the pot through drip tubes attached to spray stakes. This avoids deflection from plant leaves. Ideally, water is applied to the root zone in quantities that are approximately equal to the amount of water required to meet the needs of the plant. Production in smaller containers using microirrigation is cost and labor prohibitive because of higher cost of installation and maintenance when compared to overhead irrigation systems. As a result, this system has been used mainly to produce larger plants. Directed microirrigation is the most efficient irrigation method for large containers because they intercept the lowest percentage of applied overhead irrigation (wide spacing and large plant canopy).
Spaghetti tube systems transport water directly into the container using small tubes. Tube size controls the rate at which water is emitted. Tubes are cut to the proper length for water distribution to each container, and a small weight at the end of the tube insures that the tube stays in place. Because the water is applied in a drip-like pattern, the movement of water through the entire medium mass depends on capillary action (i.e., movement from one tiny pore to another). Therefore, a uniformly mixed, well-graded medium must be used. This is a very efficient system, but water distribution will be poor if the substrate dries between waterings.