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Home HortScience Fertilizers, Irrigation Methods Prove Lucky for Shamrocks
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ITHACA, NY—Roses may be at peak demand on Valentine's Day, but they grace bouquets year-round. Shamrocks have a single day to shine. Shamrock growers strive to produce the healthiest and most attractive shamrock plants possible for the popular plant’s sales surge around St. Patrick's Day. Chad T. Miller of Cornell University's Department of Horticulture led a study comparing two types of irrigation (overhead vs. subirrigation) and five fertilizer concentrations for growing shamrocks in greenhouse environments. Details of the study were published in HortScience.

Various forms of irrigation in greenhouses have pros and cons. Overhead, or "drip" irrigation systems can reduce labor costs and improve the efficiency of water use. But the systems have drawbacks, including the risk of residue on foliage from fertilizer applications.

Benefits of subirrigation, also known as "ebb and flow", include the ability to water many plants at once, thus reducing labor costs and improving the efficiency of fertilizer. Because water doesn't come in contact with the leaves, foliar diseases are limited with subirrigation, but soilborne pathogens can be a problem, especially if the water is being recirculated. Another problem is the accumulation of salt, which does not get leached out with this irrigation method, and can lead to reduced plant development.

Not only is the irrigation method important, the correct selection of fertilizer is critical for optimal plant growth.

According to Miller, few studies have investigated the optimal irrigation and fertilization of greenhouse shamrocks. The Cornell team developed an experiment to determine the effects of fertilizer concentration and irrigation methods on the growth of shamrock variety Oxalis regnellii, and to determine the effect of fertilizer on the development of varieties Oxalis regnellii and Oxalis triangularis.

For the first experiment, individual pots of Oxalis regnellii were watered either with subirrigation or overhead irrigation systems and fertilized with one of five concentrations of fertilizer ranging from 50, 100, 200, 300, or 500 milligrams of nitrogen per liter.

For the second trial, Oxalis regnellii and Oxalis triangularis were planted in pots and watered with an overhead irrigation system. The plants were also fertilized with one of seven varieties typically used in greenhouses, some containing additional nutrients to enhance plant growth. Each fertilizer comprised a different combination of nitrogen, phosphorous, and potassium.

After 10 weeks the plants were measured and their leaves were assessed for color. The study found that overhead irrigation produced the best plants in terms of stature and root mass in Oxalis regnellii. The greenest plants were those fertilized with the 300 level. Nitrogen concentrations below 100 or above 500 milligrams per liter were detrimental to plant growth.

"For subirrigated plants, green coloring increased linearly with nitrogen concentration," Miller noted. "However, lower concentrations of nitrogen fertilizer should be used with subirrigation to avoid excessive salt accumulation. Furthermore, the highest concentrations of nitrogen decreased plant heights in both irrigation methods."

Plant heights were also reduced in the second experiment when phosphorus-free fertilizer was used. The second experiment found that the tallest, widest, fullest Oxalis regnellii were fertilized with the 15N–2.2P–12.5K composition. This and another composition, 20N–1.3P–15.8K, were considered the "best performing" fertilizers by the study.

Though the researchers were able to determine beneficial watering and fertilizer compositions, they noted that other factors, including temperature, light intensity, and growing medium, must also be considered for the health of shamrocks.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at


Original Article:

Fertilizer Composition, Concentration, and Irrigation Method Affect Growth and Development of Oxalis regnellii and O. triangularis
Chad T. Miller, Neil S. Mattson, and William B. Miller
HortScience 46:1110–1115. [Abstract][Full Text][PDF]

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