Irrigation and collection of pondwater

Irrigation pondwater samples were collected as described by Park et al. (2020). High-density polyethylene (HDPE) bottles (0.5 L) were used to collect pondwater samples (n = 1 nursery⁻¹). Each HDPE bottle was thoroughly rinsed with pondwater three times before the final collection. All pondwater samples were collected close to the submersible intake pump at a depth of 0.6 m. After collection, samples were placed in a cooler with cold packs and then placed in a freezer set to −2 °C until samples were shipped for analysis.

Samples were sent to a commercial laboratory (Brookside Laboratories Amplify; New Bremen, OH, USA). Each sample was analyzed to determine calcium (Ca; ppm), hardness (ppm), iron (Fe; ppm), sodium (Na; ppm), and total suspended solids (TSS) (Brookside Laboratories Amplify). Pondwater pH and electrical conductivity (EC) were measured in-house with a portable probe (GroLine HI 9814; Hanna Instruments, Woonsocket, RI, USA). Alkalinity (ppm) was also measured in-house with a handheld colorimeter (HI775; Hannah Instruments). These specific parameters were chosen to represent common challenges that producers have with irrigation pondwater quality.

Soilless substrate collection

Samples of aged pine bark were collected from bark piles that the nurseries used to produce most of their crops. Sampling procedures were based on modified methods described by Kaderabek (2017); at each sampling location, sub-samples were collected by removing approximately 3 L of bark at different horizontal depths (0.3, 0.6, 0.9, and 1.2 m; n = 1) and at a constant vertical depth (1.5 m). This sampling method was used to best represent and mimic how producers sample their pine bark materials. Samples were stored indoors in sealed 4-mm polyethylene bags to minimize moisture loss in a 20 °C temperature-controlled room. Within 30 d, all samples were analyzed to determine substrate physical properties.

The North Carolina State University porometer method was used to characterize substrate physical properties (n = 3) (Fonteno and Harden 2010). Substate physical properties included the container capacity (CC; maximum water storage under gravitational drainage), AFP (minimum air space under gravitational drainage), total porosity (total pore space), and bulk density (dry mass per volume).

Statistical analysis

All means and standard deviations (SDs) were calculated using JMP Pro (18.0.0; SAS Institute, Inc., Cary, NC, USA).

Irrigation

Irrigation pondwater from large nurseries in eastern Virginia generally exhibited suitable chemical properties for production, with most parameters within or near recommended ranges (Cabrera and Johnson 2015; Robbins 2018). While some values exceeded recommendations, results were comparable with irrigation water quality from other east coast specialty crop producers (Argo et al. 1997).

Irrigation water pH plays a critical role in determining the solubility and availability of mineral ions in the root zone (Peterson 1982). Recommended pondwater pH ranges between 5.2 and 8.0 (Cabrera and Johnson 2015). Pondwater pH among surveyed nurseries averaged 7.6 ± 0.9 (µ ± SD), with three nurseries exceeding 8.0 (Table 1). In principle, slightly alkaline pH levels are generally acceptable if alkalinity values are low (<60 ppm). Alkalinity indicates the capacity of water to resist changes in pH; therefore, low alkalinity can help manage high pH caused by low buffering capacity and rapid changes in pH (Lopez et al. 2010; Robbins 2018). However, alkalinity that is too low can result in inconsistent or declining pH values throughout production. Eastern Virginia nurseries generally exhibited low alkalinity (38.8 ± 23.3 ppm; recommended range, 60–120 ppm) (Cabrera and Johnson 2015) (Table 1), which highlighted that producers should be aware of potential challenges regarding maintaining pH levels or pH decline during cropping cycles. High alkalinity is a more common challenge among nurseries because high alkalinity resists acidification and can result in gradual increases in pH (Lopez et al. 2010).

Table 1. Irrigation pondwater quality of eight large nurseries within eastern Virginia.

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Water hardness measures the combined concentration of Ca and magnesium (Mg) ions and can provide additional context for alkalinity (Lopez et al. 2010). Recommended hardness values range from 20.0 to 150.0 ppm, while recommended Ca values range from 40.0 to 100.0 ppm (Robbins 2018). Average nursery pondwater hardness (43.0 ± 33.0 ppm) and Ca (12.2 ± 10.6 ppm) were within or below these recommended ranges (Table 1). While low Ca may limit nutrient availability, high hardness or Ca can produce residues on foliage (Boser 2023), clog or damage irrigation equipment (Cabrera and Johnson, 2015), or result in nutritional imbalances in the root zone. These measurements indicated that treatment for hardness is not needed (Bilderback et al. 2017).

The EC serves as a common proxy for soluble salt concentrations in irrigation pondwater and is a critical parameter in managing ψ_S in the root zone. Because most container producers rely on controlled-release fertilizers, recommended irrigation pondwater EC is typically low (0.0–0.3 mS·cm⁻¹) (Robbins 2018). Eastern Virginia nursery pondwater EC levels were within recommended levels (0.15 ± 0.1 mS·cm⁻¹) (Table 1). Nursery retention ponds generally collect recycled surface runoff from applied irrigation, which can inflate pondwater EC values. High pondwater EC (>1.0 mS·cm⁻¹) may require increased leaching fractions (volume of water leached ÷ volume of water applied) to prevent salt accumulation in the root zone (Owen et al. 2019); however, the measured values indicated that increased leaching fractions are not necessary.

The Na and chloride levels are common concerns for nursery producers because of the potential to induce imbalanced ψ_S and salt stress (Robbins 2018). Elevated Na levels can interfere with Ca, Mg, and potassium (K) uptake, modify K:Na ratios, and increase pH over time. However, sodium concentrations across surveyed nurseries averaged 9.8 ± 7.0 ppm and were within the recommended ranges (0.0–30.0 ppm).

Additionally, Fe is an important micronutrient for plant development. However, high Fe levels can intensify the growth of Fe-born bacteria populations and result in unesthetic foliar deposits (Bilderback et al. 2017). Pondwater Fe levels were consistently low (<0.1 ppm) (Table 1) across surveyed nurseries and caused little concern (1.0–3.0 ppm) (Cabrera and Johnson 2015).

Irrigation pondwater can have high TSS from organic particulates, sediments, plant debris, or chemical precipitates from recirculated irrigation or surface runoff. High TSS can clog emitters or transport pathogens or residual agrochemicals (Abdi and Fernandez 2019). Average TSS values across surveyed nurseries were low (19.3 ± 35.1 ppm) and within recommended limits (0.0–192.0 ppm). Filtration is often recommended for producers to mitigate potential setbacks and is necessary when TSS is high (Fischer et al. 2018).

Soilless substrates

Large eastern Virginia nursery producers primarily source their pine bark substrates from North Carolina because of cost and local availability (Virginia nursery producers). The substrates physical properties measured in this case study are comparable to those reported by Altland et al. (2018), who surveyed physical properties of pine bark from commercial bark suppliers in North Carolina.

Eastern Virginia nursery substrates were, on average, within the recommended CC (0.46 ± 0.08 cm³·cm⁻³) and total porosity values (0.81 ± 0.08 cm³·cm⁻³); the recommendations for these range from 0.45 to 0.65 cm³·cm⁻³ and from 0.65 to 0.85 cm³·cm⁻³, respectively (Bilderback et al. 2013). Outdoor nursery producers in the eastern US will typically use substrates with lower water storage to mitigate waterlogged containers and maintain optimal physical properties during potentially long periods of high precipitation (Raviv et al. 2019).

Many large eastern Virginia nursery producers create a single unique substrate composite or use a pine bark-based substrate to produce most of their crops (Virginia nursery producers). The nurseries surveyed primarily use pine bark (stand-alone) without mineral sand (Virginia nursery producers). Pine bark used in nursery 1 contained the greatest CC (0.59 cm³·cm⁻³), and the producer typically incorporated two bark substrates (finer particles and coarser particles) from different sources (Virginia nursery producer). The finer particles likely settle within pores space and contribute to the increased CC (Table 1) (Bilderback et al. 2005). Bark substrates from nursery 6 also contained greater CC (0.55 cm³·cm⁻³); however, this is likely attributable to finer bark particles used for 1-gallon container production (Virginia nursery producer). Nursery 3 produced trees in larger containers (>15 gallons) and typically sourced coarser pine bark to increase drainage.

The substrates surveyed were slightly above the recommended AFP values (µ ± SD; 0.36 ± 0.08 cm³·cm⁻³); recommended values ranged from 0.10 to 0.30 cm³·cm⁻³ (Table 2). Altland et al. (2018) also reported average AFP values >0.30 cm³·cm⁻³ for aged pine bark from North Carolina suppliers. This assessment suggested that Virginia nursery producers may need to irrigate at greater volumes or frequently apply irrigation throughout a singular day to maintain suitable moisture conditions or avoid water-related stress (Mathers et al. 2007). Cyclic irrigation is an effective irrigation management technique that can help producers manage water availability and moisture distribution in bark-based substrates (Criscione et al. 2024).

Table 2. Physical propertiesⁱ of pine bark substrates used in outdoor container production for >1-gal. containers within nine Eastern Virginia nurseries.

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