Water conservation and quality are high priority issues in agriculture and society as a whole. Irrigation management issues, specifically access to high quality water, irrigation scheduling, salinity, runoff water quality, and urban surface- and stormwater management are topics of major concern to the green industry and specialty crop producers. The green industry includes ornamental plant producers, landscape and ecosystem service providers, urban farmers, and green infrastructure managers. Production systems in this specialty crop sector includes ornamental and edible crop production in soilless substrates in containers, hydroponics, engineered substrates (for example roof-top plantations), and field nurseries. Climate change will likely influence rainfall patterns, fresh water reserves, and the frequency and severity of drought events. Drought and flood events, competition for water resources, urban expansion into previously rural production areas, production of crops within urban boundaries, demand for lower environmental impacts, and increasing legislation at state and county levels increase the need for these sectors to conserve water, manage stormwater runoff, and use alternative water sources with lesser quality. Water quality aspects relevant to irrigation include chemical components (such as alkalinity, salinity, nitrogen and phosphorus, pesticide residues), biological components (including plant pathogens, algae, biofilm, and human food safety pathogens), and physical components (such as suspended particulates, and turbidity).

Specialty crop producers are highly productive per unit area and require intensive inputs including water, fertilizer, pesticides, energy, and other resources. The waste stream from this production, which can include particulates, agrichemicals, heat, and plant diseases, could be transported by irrigation and storm runoff into containment ponds and/or off-site into groundwater or surface water (Camper et al., 1994; Hong and Moorman, 2005; Warsaw et al., 2009b; Wilson and Foos, 2006; Wilson and Boman, 2011). Irrigation water management affects nutrient uptake by crop plants and runoff of leached nutrients into local water systems (Pershey et al., 2015; Ross et al., 2002; Tyler et al., 1996; Warsaw et al., 2009b). Emerging constraints on water use and quality mean that the green industry needs to identify ways to manage water without negatively impacting marketable yields per area and per year.

A multistate, multidisciplinary research and extension group is therefore necessary to address the broad range of water quantity, quality and plant production issues in the green industry. To help solve these research and extension needs, this project has identified five principal areas of concern. These include (1) the quality of irrigation water sources, (2) irrigation management and water conservation, (3) crop production runoff management, (4) urban stormwater, and (5) soilless culture.

1. Water quality of irrigation sources

Water sources can have quality issues that require treatment and management before use for irrigation (Cabrera et al., 2018). Primary water sources available for specialty crops include groundwater, surface water, municipal reclaimed water, and recycled tailwater from runoff and drainage (Duncan et al., 2009).Water resources can become contaminated by infiltration of pollutants from nearby industrial, urban (Bale et al., 2017), and agricultural operations (Majsztrik et al., 2017). In some regions of the U.S., groundwater is impaired by natural geological features. Surface water is even more vulnerable to contamination and significant changes in its chemical, physical, and biological quality because it has no protective over-layer of soil. Municipal reclaimed water, or highly treated wastewater, is an additional source of irrigation water, but it can sometimes contain problematic levels of organic and chemical contaminants (Tanji et al., 2007). Plant producers must therefore use tools and techniques to systematically monitor key chemical parameters, employ technologies to remove harmful contaminants when necessary, modify horticultural and irrigation management practices, and/or select crops which are tolerant of lower quality water sources.

2. Irrigation management

Water inputs (total water volume required for crop production) and waste outputs (runoff) can be substantially reduced by precision irrigation management, and improving water use efficiency is the first step to reduce waste of this resource. Irrigation use accounts for 62% of surface and groundwater use in the United States (Kenny et al., 2009) while only supplying around 10% of plant needs, with rainfall supplying the remaining 90% (Assouline et al., 2015). Nursery and greenhouse producers vary greatly in their application practices (Majsztrik et al., 2018a). Supplemental irrigation (in addition to rainfall) is beneficial in nursery production in field soils. In contrast, supplemental irrigation is essential for production in soilless substrates, as they are typically characterized by high levels of porosity, low water holding capacity and container-produced plants have smaller root and soil volumes, compared to field-grown plants. The majority (75%) of nursery crops in 17 of the major nursery-producing states are grown in containers (USDA, 2007), and most floriculture crops are grown in greenhouses under rain cover (USDA, 2019). Improved irrigation efficiency is necessary because containerized systems provide little buffering capacity to reduce leaching of nutrients or pesticides, which in turn can become a source of surface or ground-water contamination.

A wide variety of irrigation systems are used in intensive plant and crop production systems, including recirculating sub-irrigation systems, drip irrigation / low-volume spray stakes, and overhead sprinklers. In general, the more efficient irrigation systems are more expensive to install and maintain. There is little information available on differences in water use among these different irrigation approaches for container production, even less information about differences on the economy of their use, or the compatibility of these systems with the quality of water sources.

Options for precision irrigation management to improve irrigation efficiency include controlling the timing, volume, and delivery of water through sensor-based measurement of soil moisture status, climate-based evapotranspiration modeling, and micro-irrigation. However, successful adoption of these methods requires science-based knowledge, cost-effective tools and the training of practitioners.

3. Crop production runoff management

In many areas of the country, specialty crop producers have begun to recycle tailwater and stormwater runoff from their facilities. This process can potentially reduce production input costs, because fertilizers and water in the runoff are re-used. However, recycled runoff water contain agrichemical residues that can have phytotoxic effects (Briggs et al., 2002; Riley et al., 1994; Warsaw et al., 2012; Willis, 1982; Wilson et al., 2006, 2010). Phytotoxicity problems may result when recycled water contains agrichemicals with high water solubility, or when a persistent agrichemical is extensively used and the recycled water is applied to plants sensitive to that compound (Bhandary et al., 1997). Herbicide effects can occur at 1 to 10 parts per million (ppm) and the plant growth regulator paclobutrazol at 5 parts per billion (ppb) have been found to be detrimental to growth and quality of several ornamental crops (Baz and Fernandez, 2002; Bhandary et al., 1997; Fernandez et al., 1999; Million et al., 1999). Other drawbacks of recycling include the need to invest in infrastructure to collect, capture and treat irrigation water runoff (Pitton et al., 2018). However in areas such as California, recycling of tailwater has been practiced since the 1970s (Skimina, 1986), and growers throughout the US have successfully adopted containment as a strategy for reducing water and nutrient runoff (Lea-Cox and Ross, 2018).

Managing runoff is a challenge for container producers. The volume of water applied and the amount of runoff generated is much greater than from field production. However, issues associated with runoff from field or container production areas are similar and can have a major impact on water resources. Runoff of rain and irrigation water is an important avenue for the movement of agrichemicals from production sites into nearby receiving water bodies (Bjorneberg et al., 2002; Taylor et al., 2006; White, 2013). Assessment and management of runoff plays a critical role in minimizing the environmental impacts of specialty crop production operations (White, 2013). Runoff can be substantially reduced by effective irrigation management (Pershey et al., 2015; Warsaw et al., 2009b). Capturing and treating or recycling are other options (Grant et al., 2018; Majsztrik et al., 2017; White, 2013). Many larger greenhouses have addressed the issue of runoff by using closed irrigation systems. Although subirrigation systems can substantially reduce runoff from greenhouses, they are cost-prohibitive and impractical for many operations, especially nurseries. 

Plant pathogens in irrigation water were recognized early in the last century as a significant crop health issue (Bewley and Buddin, 1921). Plant pathogens threaten the sustainability and profitability of the ornamental plant industries as much as water shortages. Recycling irrigation conserves water but it may also spread pathogens from a single point to an entire enterprise and from a single farm to other facilities sharing the same water resource (Hong et al., 2008a, 2008b, 2008c; Nyberg et al. 2014). This could result in severe crop losses. At least 26 species of Phytophthora, 26 species of Pythium, 5 species of Phytopythium, 27 genera of fungi, 8 species of bacteria, 10 viruses, and 13 nematode species have been identified in water sources (Hong and Moorman, 2005; Redekar et al., 2019). Among those pathogens are Phytophthora ramorum, the causal agent of sudden oak death (SOD), and Ralstonia solanacearum, one of the USDA select agents under the Agricultural Bioterrorism Protection Act of 2002. Therefore, there is an urgent need to assess the risk posed by recycling waterborne pathogens, to evaluate pathogen mitigation strategies, and to develop extension programs disseminating this information to growers (Lamm et al., 2017). A wide range of water treatment technologies can be effective for biological contaminants, including chlorination, chlorine dioxide, copper ionization, hydrogen peroxide, ozone, surfactants, and ultraviolet radiation (Raudales et al., 2014a), but there are many economic, crop management, and other factors involved in their successful adoption in addition to plant pathology (Raudales et al., (2014b).

4. Urban stormwater

Urban horticulture and production of specialty crops in controlled environments are significant emerging trends. Plant production within cities using roof-top farms and greenhouses, vertical farms, and other urban growing systems can provide local fresh food production; green roofs and green infrastructure can reduce energy inputs for heating and cooling, reduce stormwater runoff from rainfall events; and provide additional social, economic and environmental benefits. The need to conserve water and limit runoff is even more acute in urban environments, where runoff water volume and quality can be highly regulated and can have major economic impacts.  The subject of recent investigations, thermal pollution in runoff may be particularly detrimental to natural receiving waters (LeBleu et al., 2019). While typically associated with urban systems, Green Infrastructure Technologies and Low Impact Development (LID) systems have direct application to production systems as well as they can guide recommendations for producers (Morash et al., 2019) The management of urban growing systems would benefit from the application of science and technologies that have been successfully developed and implemented by rural and large-scale plant production systems. There is a need to further develop and improve existing methods and technologies that facilitate the reduction, infiltration, remediation, and reuse of tail-and stormwater runoff, targeting both urban and rural specialty crop producers.

5. Substrates and nutrients

Selection and design of container and urban crop production soilless substrates, with physical and chemical properties that are well-matched to plant needs and irrigation practices, can further reduce fertilizer and pesticide inputs and runoff. Nursery and greenhouse and food producers typically purchase or blend soilless substrates by screening single components or combining two or more inorganic or organic components. Substrates provide anchorage for stabilizing the plant in the container and provide a reservoir for water, mineral nutrients, and oxygen. Scientists have often approached substrate development or evaluation by focusing on either substrate chemical properties (pH, electrical conductivity) or static physical properties (water, oxygen, and anchorage). However, less attention has been applied to dynamic changes in the complete root zone system, where physical and chemical properties change over time during the crop cycle. These changes affect plant health, growth and crop time, root development, profitability, and resource efficiency (i.e., the amount of water and mineral nutrients required to produce a marketable crop).

Scientists are beginning to delve into dynamic physical properties of containerized substrates, in addition to conventionally-measured static substrate properties. Similarly, nutrient management has evolved beyond evaluation of "the best fertilizer" for a particular crop to studying and supplying crop nutrient needs through engineering substrate blends, components and amendments in order to increase nutrient use efficiency and reduce nutrient leaching. Furthermore, there is an increasing need to understand the economic and environmental sustainability of new and traditional substrates, especially in regards to source availability and quality for urban and specialty food crop production.