Large-scale container-grown nursery plant production began in the early 1950s and helped to diversify the nursery industry. Most of the clay, recycled metal, and wooden containers initially used in nurseries were replaced by plastic containers in the 1960s, and container production systems became more abundant because of ease of transplanting and year-round production and/or sales (Davidson et al., 1988). Plastic has remained as the most common material for nursery containers but due to economic and environmental considerations such as price fluctuations, high collection and sanitation costs, and chemical contamination concerns; used plastic containers are primarily disposed of in landfills (Garthe and Kowal, 1993; Hall et al., 2010; Helgeson et al., 2009) and given limited access to recycling centers enhanced interest in alternative containers (Fulcher et al., 2013). Green industry stakeholders (i.e., nursery, greenhouse, and landscape professionals) have identified the use of plantable, compostable, or recyclable containers as an alternative way to improve the sustainability of current production systems (Nambuthiri et al., 2015).
Container materials and colors can affect plant growth by modifying substrate temperature and the water budget (Markham et al., 2011). Heat energy enters the production system from direct and reflected solar radiation. Containers act as a solar collector in the system, as does the exposed substrate surface (Martin and Ingram, 1992). Excessive substrate heating occurs due to absorption of solar radiation and lack of heat dissipation from the nonporous black plastic containers (Beattie et al., 1987). These conditions will result in changes to the root environment and may influence plant growth and productivity. A major concern for growing plants under full sun in plastic containers is the substrate temperature, which can injure roots if it exceeds 40 °C for several hours (Mathers, 2003; Ruter and Ingram, 1990). Heat is transferred to the substrate in the container primarily through conduction and lost to the external environment via convection. Martin and Ingram (1992) developed simulation models to study substrate temperature and found that properties such as color, porosity, reflectivity of container surface (albedo), thermal conductivity of the container, thermal exchange between the container sidewall and the growing substrate, and the surrounding environment can influence substrate temperature.
Supraoptimal substrate temperatures that are sublethal cause indirect injuries such as loss of plant vigor, nutritional disorders, increased susceptibility to diseases and other anomalies with plant developmental characteristics (Ingram et al., 1986a). Root zone temperature greater than 40 °C suppresses plant growth and quality from direct and indirect temperature effects (Ingram et al., 1986b). Many researchers have observed the deleterious effects of high rhizosphere temperature on photosynthesis and assimilate partitioning (Gosselin and Trudel, 1986), water stress (Ingram et al., 1986a), nutrient deficiencies (Johnson and Ingram, 1984), or impaired plant growth and development (Martin and Ingram, 1988).
One way to deal with heat stress is to use containers with porous walls to allow evaporation of water from the outer surface, improving heat exchange between the substrate and environment (Bunt and Kulwiec, 1970). Container material composition has been found to influence substrate heat development because of the differing thermal properties of the components (Bunt and Kulwiec, 1971). Any material that reduces absorption of solar radiation and increases exchange of heat in the production system can contribute to reducing supraoptimal substrate temperature. Porous containers (clay, paper, peat, etc.) showed a slower increase in substrate temperature than nonporous (plastic, glass, paraffin-protected, etc.) containers due to the higher latent heat for vaporization of water (Jones, 1931).
Green industry stakeholders have identified the use of biodegradable container alternatives as a way to improve the sustainability of current production systems (Behe et al., 2013; Dennis et al., 2010; Hall et al., 2010). An online survey showed that consumers were willing to pay a premium for compostable, plantable, and recyclable containers due to their pro-environmental attributes (Khachatryan et al., 2014). Recently, several alternative containers made from WP, paper, coir, rice hulls, peat, bioplastic, recycled plastic have become commercially available (Hall et al., 2010; Nambuthiri et al., 2015) and have been recently evaluated for their suitability for a variety of species and environmental scenarios (Evans and Hensley, 2004; Lopez and Camberato, 2011; Nambuthiri and Ingram, 2014).
This study evaluated the impact of container material on substrate temperature dynamics and sidewall evaporative water loss under laboratory, controlled environment, and in aboveground nursery settings.
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