By Mark J. Fitzpatrick and M.A. Lemay, Agriculture Technical Information Service

As operation costs increase and the availability of fresh water decreases, the greenhouse and nursery industries are being compelled to improve both water irrigation and recycling efforts as part of their business practices. In the U.S., primarily Florida and California, these efforts are strongly in place in anticipation of imminent regulations to force advanced water management in greenhouses. Consequently, over the past five to 10 years these greenhouses have undergone strict evaluations of their water management in order to develop best practices.
     At present there are four major methods of water irrigation used in greenhouses and container nurseries: 1) overhead irrigation; 2) cyclic (pulse) irrigation: 3) capillary sandbed irrigation; and 4) micro (drip) irrigation. Overhead irrigation is by far the most common method used in greenhouses. Cyclic irrigation is a variation on the overhead approach, which involves watering in several stages. Capillary sandbed irrigation allows the roots to access a saturated substrate. Micro irrigation uses individual watering heads in each pot. For a comparison of the four irrigation methods using both small and large pots, see Tables 2 and 3.

Overhead Irrigation
Overhead irrigation is the most common method used by the nursery industry. It is the most practical because it is inexpensive to install; requires relatively low maintenance; and provides frost protection (Beeson Jr. and Knox , 1991). Although the precise mechanics of the system vary, the general setup includes a network of suspended pipes and sprinklers. The majority of systems irrigate once daily for one hour. The spray targets a general area and not individual plants. As a consequence, scattered pot placement can result in extreme water loss (approximately 75 per cent) from evaporation during application and water not reaching the pots (Haman & Yeager 1997). Installation costs (PVC pipe, sprinklers, ball valves, and labour) are extremely low in comparison to other methods. In order to recover costs for lost water, enhancements and/or recycling are essential.
     Overhead irrigation usually results in overwatering to ensure that everything gets sufficient water (centre waters faster than the edges) (Pearce 1994). It is especially ineffective for trees because foliage often blocks the path of water to the container.

Cyclic Irrigation
Cyclic irrigation is an enhancement of overhead irrigation. Water and fertilizer loss during conventional overhead application is significant. Therefore, by splitting the application time and inserting resting phases, the water loss due to leaching is reduced considerably (Karam & Niemiera 1994). Applications using this method usually comprise four to six 15-minute applications separated by 30-minute stationary phases. Karam & Niemiera (1994) reported that cyclic irrigation reduced water and fertilizer use by 30 per cent and 50 per cent respectively. Water runoff was decreased by 77per cent and NO3-N runoff was decreased by 90 per cent.

Capillary Sandbed
Capillary sandbed irrigation omits overhead application and instead relies on the "capillary" ability of roots to draw water from below. The major requirements for this system to operate are land leveling, ground cover, and large flow rates (Haman & Yeager 1997). It is the first requirement that keeps this system from becoming more popular. It is extremely difficult to level a large area (approx. 1-acre) even for relatively flat land. Therefore, capillary sandbeds are most often employed in greenhouses that can easily produce level benches or cement beds on a relatively small scale (Haman & Yeager 1997).
     To operate this system, a bed of sand is flooded with water. Plants are placed in containers that allow the roots to extend beyond the bottom of the container (or at least allow water to enter through the base).
     Benefits include an even distribution to all plants, water is delivered directly to the roots, and the plants can still be easily relocated. Svenson et al. (1997) reported a reduction in water usage of 80 per cent in comparison to conventional overhead irrigation. Evaporation is also reduced.
     There are several drawbacks to capillary sandbed irrigation. Svenson et al. (1997) reported three major problems: 1) weed growth; 2) roots extending from conand 3) fertilization. Perhaps cost could also be added as a fourth problem. Adams et al. (1997) reported a material cost of approximately $830 (CDN) for a capillary sandbed that offered 192m2 growing area (not including water costs).

Micro Irrigation
Micro irrigation has received considerable attention over the past few years, although it is the least employed system (less than 5 per cent of ornamental crop acreage) (Hodges et al. 1994). Micro irrigation offers the most substantial savings when compared to overhead irrigation (95 to 97 per cent), including leachate reduction of 97 per cent (Table 1) (Gonzalez et al. 1992). The major difference between this system and the previously mentioned three is that water is delivered directly to the pot, and therefore, evaporation and overspray are essentially not issues. Haydu and Beeson Jr. (1997) reported that micro-irrigated plants reached market size in significantly shorter time than those irrigated with conventional overhead systems (28 weeks vs. 82 weeks for live oak). Bir and Bilderback (1996) stated that crops will often be of higher quality, more uniform size, and will survive better in the first field season.
     Materials include emitters, spaghetti tubing (3.2 mm), polyethylene tubing, PVC pipe, electric valves, clocks, and labour (Haydu & Beeson Jr. 1997). Perhaps the most important difference between micro and overhead systems is that installation costs for overhead systems are independent of container size and dependent on total area whereas micro irrigation costs are directly related to the container size and the number of containers (Haydu & Beeson Jr. 1997).

Selecting the best irrigation system
Micro irrigation is the most efficient with respect to plant growth and water usage. The major drawback is that nurseries interested in production from small pots (#3) do not benefit as much from this system as would those growing larger products (#10 or #15).
     It is also necessary to evaluate the usefulness of aborting an already existing, albeit less efficient system. Those greenhouses/nurseries that are building new facilities should definitely consider installing micro irrigation systems. However, operations that already exist might consider alterations and enhancements to the conventional overhead approach. Cyclic irrigation is essential, along with efforts in water conservation and recycling.
     Such enhancements include Multiple pot boxes (Figure 1). Containers are grouped in tiered boxes (usually nine plants per box) instead of being placed on the floor. The box acts as a reservoir to trap excess water that falls between the containers and leachate. The trapped water can be drawn into the plant using capillary action through the roots. Individual plants can be moved from box to box, or the boxes themselves can be relocated. Haman et al. (1998) reported that Viburnum odoratissimum can be produced to market size in three months using multiple pot boxes in comparison to the conventional six months. The authors also noted that over four months, an overhead system applying 13 mm daily without tiered boxes would have used 84 per cent more water.
     Another enhancement is Grouping Pots. Rackley (1992) suggested that grouping crops according to water use would limit those areas that require high watering rates. Finally, Yeager et al. (1996) suggested that Pot Spacing could greatly reduce water loss from overhead irrigation (Table 4). Triangular arrangement is significantly more efficient than square arrangement. This efficiency is highest when the distance between containers is small.

Recycling
An increase in research on the feasibility of water recycling has most likely stemmed from 1) environmental concerns, and 2) the unwillingness to abandon traditional methods of irrigation. It is easy to understand why some greenhouse/nursery operators are hesitant to overhaul their irrigation system when enhancements can offer a comparable improvement.
     Most recycling systems obtain their water from irrigation runoff and rainwater. The systems usually consists of three major components: 1) runoff collection; 2) reservoir; and 3) pumping. Wastewater is greatly reduced in both micro irrigation and capillary sandbed irrigation, such that recycling is not necessary. Consequently, those systems still using overhead and cyclic irrigation should consider implementing a recycling program.
     Concerns such as pathogens, high iron and nitrogen levels and reduced nutrient levels affect growers when considering the use of recycled water. Several studies have shown that using recycled water is not detrimental to crop growth, although most growers filter the recycled water and mix it with a set proportion of fresh water (Behrens 1994; Fairweather 1994). However, Labous and Willis (1994) used 100 per cent recycled water on their test plants and found no significant difference in plant growth or colour in Pyacantha rogersiana and P. 'Orange Glow'. A review of the current literature identified only two detailed descriptions of recycling systems. Both include installation costs but do not report actual water savings. The two systems are the Cellugro (Gill et al. 1997) and the Rackley system (Rackley 1992). This does not mean that these are the only two systems in use today. Rather, it indicates that other system operators have not published their information. More research is needed on the costs and effectiveness of recycling systems. The following will compare the above-mentioned systems.
     Cellugro is a closed-loop system installed 8 in. (approximately 20 cm) below the soil grade (Gill et al. 1997). Runoff water and leached nutrients pass through the plant and through a porous polypropylene layer acting as a barrier between the growing and the water storage areas. The growth "cells" are usually 2.4 m X 6 m (14.4 m2) with reported costs of $980 (US$.) but also come in larger sizes.
     Since the system is below grade, it offers frost and winter protection and therefore, additional winter protection is unnecessary. Recycling in the Cellugro system takes place when sensors detect a drop in soil moisture; water from the storage area below is reapplied to the plants.
     Rackley's recycling system is used in conjunction with overhead/pulse irrigation. It incorporates the three recycling system requirements stated previously (runoff collection, storage and pumping). Rackley (1992) tested the system on 13.5 acres (approximately 8.1 ha) using growth beds on a 0.33 per cent slope that directed water to collection canals (also sloped). The water collected in the canals was diverted to a storage pond. Pumps installed in the storage pond reapplied the collected water to the crop. Rackley reported the system to cost $8,000 US to $10,000 US per acre (0.4 ha). It was estimated that the Rackley system could reduce water usage by 50 per cent.

Putting it all together
Even when considering different pot sizes, micro irrigation is consistently more efficient and cost effective, although costs are strongly dependent on container size (Tables 2 and 3). Those greenhouses and nurseries that market large plants, i.e. shrubs, bushes, etc. (#15 containers) should strongly consider micro irrigation even if overhead systems presently exist - the savings are significant.
     Greenhouses and nurseries that generally grow smaller product (#3 containers or smaller) might consider cyclic irrigation (with integrated enhancements, e.g. multiple-pot boxes, grouping pots, and pot spacing) in conjunction with a recycling system.

Mark J. Fitzpatrick is a graduate student at Brock University studying the evolution of mate choice in crickets. He plans to begin a Ph.D. in September 2000.

Amy Lemay, M.Sc. is a Research Analyst with AgTIS, the Agriculture Technical Information Service.

The above is one of five research reports prepared by AgTIS under contract to the Landscape Ontario Grower's Group.

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