The wholesale value of bedding and garden plants was $1.86 billion in 2015, which was 44% of the total wholesale value of floriculture crops in the United States, thus making bedding and garden plants the largest crop segment in this industry. Annual bedding plants generated $1.3 billion, representing 69% of the total bedding and garden plant division. Although the wholesale value of bedding and garden plants had decreased 1% from the previous year, those sold in hanging baskets and containers 5 inches or larger increased, and those sold in smaller containers or flats were reduced compared with the previous year (USDA, 2016).
Considering the economic value of potted bedding plants and the economic and environmental needs to reduce irrigation water usage, many studies have been conducted to determine bedding plants’ responses to water deficits using one of two methods: holding the substrate moisture content (SMC) at a constant level or repeatedly drying down from container capacity (CC) to a lower SMC level. Lowering SMC to a constant 20% from 40% combined with lower fertilization increased flowering of petunia ‘Dreams White’, which increased the visual quality while lowering production costs (Alem et al., 2015). Conversely, it was reported that petunia ‘Hurrah White’ grown at a constant 10% SMC and impatiens (Impatiens walleriana ‘Cajun Violet’) grown at 30% SMC using the dry-down method reduced flower number (Blanusa et al., 2009; Chyliński et al., 2007). Decreased irrigation water usage during greenhouse production increased water use efficiency in American alumroot (Heuchera americana), gaura (Gaura lindheimeri), impatiens (Impatiens walleriana), salvia (Salvia splendens), and vinca (Catharanthus roseus) (Burnett and van Iersel, 2008; Garland et al., 2012; Jaleel et al., 2008; Nemali and van Iersel, 2008).
A greenhouse production challenge to growing bedding plants in smaller containers and flats is height control, which typically has been managed by withholding irrigation combined with several plant growth regulator applications to produce compact plants with shorter internodes (Dole and Wilkins, 1999). Reduced irrigation inhibited petunia (Petunia ×hybrida) shoot growth, which implied that lower SMC could be used as an effective growth control method during greenhouse production of petunias (Blanusa et al., 2009; Niu et al., 2006). Water deficits induced by amending the substrate with osmotic compounds produced more compact salvia (Salvia splendens ‘Bonfire’) with greater leaf area:height (Burnett et al., 2005).
Sensor-based irrigation systems are being used to reduce irrigation water usage and produce more compact plants with lower costs while increasing or maintaining their aesthetic quality. The sensor-based irrigation system was first developed by Nemali and van Iersal in 2006 to reduce water consumption by maintaining a distinct and constant SMC (Nemali and van Iersel, 2006). Previous research showed 40% SMC to be similar to the traditional well-irrigated treatment commonly used by growers for potted plant production, whereas 20% SMC was an alternative irrigation treatment that reduced water usage without detrimental effects on plant quality (Alem et al., 2015; Bayer et al., 2015; Guo et al., 2018; Jacobson et al., 2015).
Using a sensor-controlled dry-down method, the responses of two bedding plant species to wide-range (WR) SMC (CC dried down to 20% SMC) varied compared with narrow-range (NR) SMC (CC dried down to 40% SMC). WR-SMC produced more compact angelonia (Angelonia angustifolia ‘Angelface Blue’), but it did not impact heliotrope (Heliotropium arborescens ‘Simply Scentsational’) shoot growth; however, both species had more root growth with WR-SMC. WR-SMC increased the visual quality of both angelonia and heliotrope postproduction by increasing the percentage of plant shoots covered with inflorescences and decreased water input during production, thereby decreasing production costs (Guo et al., 2018).
Even though studies have been performed to determine plant responses to water deficits during greenhouse production, the effects of water deficits on postproduction quality remain unclear (Islam and Joyce, 2015). The objective of this study was to ascertain the effects of NR-SMC compared to WR-SMC irrigation methods on growth, development, and physiological parameters during the production of coleus, petunia, lantana, impatiens, and salvia grown in 1.67-L containers. The new trend of growing bedding plant species in larger containers (USDA, 2016), much like potted flowering and foliage plants are produced, was further justification to study irrigation of bedding plants grown in pots rather than flats. We used the same dry-down method as described previously (Guo et al., 2018) to apply SMC treatments, and we irrigated plants to CC (54%) after target SMC levels were indicated by sensor readings. The root substrate was allowed to dry down to the target SMC, and it was re-watered to CC repeatedly as needed. We determined whether plants produced with WR-SMC acclimated to infrequent irrigation during simulated shelf life and analyzed the economics of these irrigation methods by considering production inputs and shrinkage through the market channels. Finally, we quantified plant quality before and after simulated shelf life. Our hypothesis was that WR-SMC during production would lower irrigation-associated costs, control growth, better-acclimate plants to the postproduction environment, and allow plants to maintain higher visual quality during postproduction.
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