Subirrigation for Greenhouse Crops

Subirrigation is becoming an increasingly common way of watering and fertilizing greenhouse crops in Massachusetts. This article is for growers considering a subirrigation system or just starting out with a new system.

Advantages to subirrigation

There are three major economic advantages to subirrigation. The most commonly cited advantage is the savings in labor needed for watering the plants: a single person can water thousands of plants by operating the flooding system manually or with the help of a computer. Additionally, there is a potential savings in water and fertilizer with subirrigation since both are recirculated and not lost by leaching or runoff. Also, depending on the system and how it is installed, a grower can expect an increase greenhouse space efficiency (percentage of total floor area in use for growing plants).

Many growers report more uniform plant growth and less foliar disease with subirrigation. The increase in plant uniformity may be the result of more even and complete moistening of the growth medium and better distribution of nutrients absorbed by capillary flow. The absence of water on the leaves with subirrigation probably results in less foliar disease.

The elimination of fertilizer and pesticide leaching and runoff from the greenhouse is a very important reason for using subirrigation. In order to achieve the goal of reduced leaching and runoff the system must be maintained as a truly closed system. The immediate practical value of preventing irrigation effluent from escaping the greenhouse is not always apparent, but protection of water used for drinking and recreation from contamination is probably the most important long-term benefit of subirrigation. 

Challenges to using subirrigation

Like any other new way of growing greenhouse crops there are a number of challenges to overcome to use subirrigation successfully. The two greatest challenges for most growers is the initial cost of the system and the ability to retrofit the system in an existing greenhouse. A conservative estimate of payback time is 5-10 years, but the period could be as short as 2-3 years depending on the system chosen, whether existing bench frames can be retrofitted, and whether productivity of the system is maintained at a high level.

An excellent economic analysis of subirrigation systems was recently published by Wen-fei Uva and her colleagues of Cornell University (Uva, W. L. et al., 2001). Some readers may have heard Wen-fei speak on her work at the New England Greenhouse Conference in October 2000. Her article is very detailed, but concise, and would help growers in choosing a subirrigation system.

A grower beginning to use subirrigation will have to learn some new ways of irrigating and fertilizing to use the system successfully. Growth medium and irrigation solution testing for pH and EC is one important skill to acquire. Since the growth medium tends to accumulate salts with subirrigation it is critical to be able to test for EC on a regular basis without having to wait for results from a commercial lab. Also, growers who maintain nutrient and pH levels in the irrigation solution by adding fertilizer or water to stock tanks manually rather than with automatic equipment need to carefully monitor EC and pH to maintain the proper ranges.

Successful use of subirrigation requires extra attention to cleanliness to avoid disease and insect problems. The use of pesticides and other chemicals, particularly as drenches, can be problematic with subirrigation so adoption of IPM techniques, especially pest population monitoring, is very important. Cleanliness will be discussed a little more later in the article.

Subirrigation systems

There are three basic closed, recirculating subirrigation systems currently in use in New England: ebb-and-flow benches, trough benches, and flooded floor systems. There are some variants on these, for example, the "Dutch movable tray system" is very similar to ebb-and-flow, but a complete system is highly mechanized for a number of tasks. Capillary mats and collection trays are also a form of subirrigation, but they are not normally closed systems.

Ebb-and-flow. The ebb-and-flow system is very common and is quite familiar to most growers. The system consists of a shallow, molded plastic bench top which is flooded to water and fertilize the plants; when irrigation is complete the remaining solution drains from the bench and is pumped back to a storage tank.

Ebb-and-flow is very versatile because the bench tops can accommodate all sizes of pots and bedding plant flats (although not on the same bench or irrigation zone at the same time because of the differences in water absorption rates between container sizes). The bench tops can be installed on existing frames and, with the rolling feature, ebb-and-flow benching can be 80-90% space efficient. Ebb-and-flow benches are easy to retrofit in clearspan greenhouses, but not in greenhouses with many internal supports. This system has the highest initial cost, $4 to 6/ft², installed on existing bench frames and including tanks, delivery and return pumps, plumbing, and installation. A major portion of the cost comes from the specially molded plastic bench tops which cost about $2.50/ft².

Troughs. This system works by running a film of irrigation solution down a slightly inclined, shallow metal trough holding the plants. The troughs empty in a return channel for recirculation. The pots or flats in the trough have plenty of opportunity to absorb solution as it runs past.

The trough system is very easy to retrofit on existing bench frames. The troughs can be obtained in various lengths and widths from a commercial manufacturer or they can be fabricated by a local metalworking firm to the growers specs. A trough system is about 70-80% space efficient, less than ebb-and-flow, because normally spaces are left between the troughs. Most growers use this system mainly for potted crops, but it is possible to do bedding plant flats if the open mesh style of tray is used to hold the paks. However, because of the trough spacing, it isn't possible to space flat-to-flat except in an individual trough.

The initial cost of the trough system is about 2-6/ft². The cost of this system can be fairly low if the troughs are made locally or if they are installed on existing benches. Most of the plumbing is simple to put together and inexpensive.

Flooded floor. In this system the entire floor of the greenhouse is covered with a concrete carefully designed and installed to pitch toward openings in the floor. Through these openings the irrigation solution enters to flood the floor and, following flooding, the excess drains back to the storage tank. The floors can be installed with bottom heating and divided into zones for separate flooding and bottom heating.

Flooded floors can be used to grow plants in all container types and sizes as long as separate irrigation zones are provided for each type. Space efficiency is about 85-95%. Most greenhouses with flooded floors were built with them rather than retrofitted later. The bottom heating option an efficient way of providing the proper growing temperature for the plants because the air close to the plants is heated and the larger volume of the greenhouse does not have to be heated so much.

Some growers complain that in a flood floor plants close to the flood/drain openings tend to be overwatered, especially bedding plants. Also, as in the case of any floor growing system, all the bending and squatting needed to work with the plants can be tiring for workers.

Initial cost for a flooded floor is $3-5/ft², but costs can vary significantly depending on the amount of excavation required for the storage tanks and piping, whether or not bottom heat is installed, and whether the floor is divided into zones for separate irrigation. A very skilled concrete contractor is needed to get the pitch of the floor right to encourage proper drainage and to prevent puddling.

Fertilizing subirrigated plants

Since there is little or no nutrient leaching with subirrigation less fertilizer is needed compared to traditional overhead watering. The general rule for fertilizing subirrigated plants is to use one-half the rate (ppm) of fertilizer normally applied by overhead irrigation.

Several years ago I subirrigated poinsettias with solutions of 100, 175, 250, or 325 ppm N from peat-lite 20-10-20 fertilizer (Cox, 1998). The plants finished about the same size with nearly as large bracts as plants watered from overhead. Leaf analysis revealed normal levels of most nutrients at all fertilizer rates and no evidence of a serious nutrient deficiency or excess. EC (soluble salts) levels were higher with subirrigation than overhead watering. EC was highest near the top of the growth medium because of surface evaporation and deposition of nutrient residues. None of the treatments developed had excess EC.

The results of this study demonstrated that poinsettias grow well over a wide range of fertilizer concentrations in subirrigation including levels applied by traditional overhead watering. In fact, most growers I've visited in New England who subirrigate poinsettias on a large scale use fertilizer rates in the range of 200-250 ppm N. Use of fertilizer rates above 250 for subirrigated poinsettias increases the risk of excess EC leading to growth inhibition and plant injury. Learning to use an EC meter to monitor soluble salts on a regular basis is very important with subirrigation.

Chemicals and subirrigation

Many insect and disease problems can be prevented by adopting a new standard of greenhouse cleanliness and through the use of simple IPM practices to prevent infestations and infections from getting out of control.

To the author's knowledge no pesticides are currently labeled specifically for application through a subirrigation system. This means that for now growers must apply pesticides as they would to overhead watered plants only more carefully. Heavy or frequent foliar spraying, or use of growth medium drench treatments, are risky practices because enough chemical may enter the irrigation solution to cause undesirable effects to the plants in the long term. To avoid this problem, some growers divert irrigation water from their subirrigation system for conventional disposal following a pesticide application rather than letting it return it to the tank for recirculation. In the absence of definitive information on the extent of buildup and effects of recirculated chemicals, growers should try to limit pesticide treatments as much as possible especially growth medium drenches.

Zero Tolerance™ disinfectant is one chemical that can be recirculated in subirrigation with beneficial effects. Zero Tolerance™ can control algae and a wide variety of root disease organisms. The product label has specific directions on its use in subirrigation systems.

Interestingly, there is some interest in applying plant growth regulators (PGRs) through subirrigation. Currently A-Rest™ and Bonzi® are labeled for use in "chemigation" systems including subirrigation by ebb-and-flow and from saucers. Labels for both PGRs have detailed instructions on how to apply the chemical so as not to cause plant injury and to protect water supplies. In the author's opinion, it too early to draw conclusions about the efficacy and safety of PGR application this way but it is being studied in Florida (Barrett, 1999) and I will have some preliminary results to report soon.

Finally, cleanliness is very important. As a routine practice dead plant material and other large "stuff" should be removed from growing areas, inside tanks, and plumbing after each crop. Then the system should be disinfected with Zero Tolerance™ or Green-Shield™.

These sort of cleaning practices are not common in traditional growing (although they should be!) but they are essential for successful growing in subirrigation.

References

• Barrett, J. 1999. Bottoms up with growth regulators. Grnhse. Prod. News. 9(9):32-33. (September issue).
• Cox, D.A. 1998. Subirrigation vs. overhead watering for poinsettia. Floral Notes. 11(1):8-10. (July-August issue).
• Uva, W. L., T.C. Weiler, and R.A. Milligan. 2001. Economic analysis of adopting zero runoff subirrigation systems in greenhouse operations in the northeast and north central United States. HortScience 36(1):167-173.

Dr. Douglas A. Cox
Plant and Soil Sciences
University of Massachusetts, Amherst