Materials and methods

Ten spinach cultivars (Table 1) were evaluated for spring and fall greenhouse production. Seeds were soaked in distilled water for 48 h at 8 °C and then drained and rinsed with distilled water. Rinsed seeds were placed in petri dishes containing filter paper to absorb excess water and incubated at 8 °C for 48 to 72 h until radicles reached 3 to 5 mm. Seeds with visible radicles were planted into 18-cell (100 cm², 400 cm³ per cell) polystyrene trays (Speedling, Sun City, Fla.) using BM1 commercial root substrate (Berger, St. Modedste, Que., Canada) on 18 Oct. 2005 and 9 Mar. 2006. Seedlings were hand-irrigated with tap water as needed until roots were visible in 50% of the drainage holes of tray cells. Trays were then floated in a solution of fertilizer and water contained in a 5 × 6-ft watertight bench top (Midwest Gromaster, St. Charles, Ill.). Fertilizer concentration in the solution was based on 100 mg·L⁻¹of nitrogen from a liquid fertilizer containing 20N–8.7P–16.6K (The Scotts Co., Marysville, Ohio). The nutrient solution was replenished as needed to maintain a 4-inch depth in the watertight bench as a result of evaporation or plant uptake. Plants were grown for 53 and 37 d in a polycarbonate-covered greenhouse with average daily maximum and minimum temperatures of 25 (±2), 17 (±2), 28 (±3), and 20 (±1) °C for fall and spring, respectively, and then harvested. Plants were harvested once their height reached 4 to 6 inches by cutting at the substrate surface, and then counted, weighed, and evaluated for bolting. Color was analyzed using a chromometer (CR-200; Minolta Corp., Ramsey, N.J.) by randomly selecting three leaves from each tray. The color analysis with the chromometer included lightness, hue, and vividness. Three representative plants from each cultivar were evaluated by three panelists for consumer preference based on appearance using a 1 to 5 rating scale with “1” having pale sparse foliage with little plant mass and “5” having dark, densely packed foliage with large foliage mass.

Table 1.Yield and quality of spinach cultivars for greenhouse production in Oklahoma: marketable yields, visual ratings, and leaf color analysis.

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Results and discussion

The highest producing cultivars in Fall 2005 were ‘Olympia’, ‘Samish’, ‘Padre’, ‘Bolero’, and ‘F91-415’ with yields of 2093, 1996, 1956, 1920, and 1884 g·m⁻², respectively (Table 1). The six cultivars that produced highest in Spring 2006 were ‘F91-415’, ‘Bolero’, ‘Baker’, ‘Space’, ‘Padre’, and ‘F-380’, which had yields of 1649, 1560, 1471, 1453, 1444, and 1396 g·m⁻², respectively. Of these six cultivars, ‘F91-415’, ‘Bolero’, and ‘Padre’ also did well in Fall 2005. Three cultivars, ‘Baker’, ‘Catalina’, and ‘F91-415’, were consistent in yield for both Fall 2005 and Spring 2006 exhibiting no differences within a cultivar between seasons. Production was generally higher in these studies compared with that reported by Joshi et al. (1992) and although their yields were recorded from studies in Antarctica, similar greenhouse growing conditions were maintained in both studies.

‘Samish’, ‘Padre’, and ‘Cypress’ had the highest consumer preference ratings in Spring 2006 (Table 1) indicating a darker foliage color and more plant mass than other cultivars in the study. Ratings for these three cultivars were 5.0 for ‘Samish’, 4.3 for ‘Padre’, and 4.0 for ‘Cypress’. Although other studies did not have ratings that combined color and plant mass, Brandenberger et al. (2004) reported ‘Padre’ and ‘Cypress’ as having significantly higher color ratings than a majority of cultivars included in field trials.

Bolting was not observed in the Fall 2005 study, but in Spring 2006, ‘Samish’ had 6.5% bolting, whereas all other cultivars had none. In 2 years of spring field trials in Oklahoma, ‘Samish’ had the highest amount of bolting of all cultivars (91%) (Brandenberger et al., 2004). Although bolting was less in the greenhouse than in field trials, premature bolting of ‘Samish’ occurred in both. It appears that the much shorter growth cycle in the greenhouse resulted in bolting being less of an issue than in field production.

Lightness readings for cultivars differed in Fall 2005 (Table 1). ‘Bolero’ was the lightest being similar to ‘Samish’, ‘Catalina’, ‘Baker’, and ‘F-380’. ‘Samish’, ‘Catalina’, ‘Baker’, and ‘F-380’ were also similar in lightness to ‘Olympia’ and ‘Padre’. ‘Cypress’ was darker than ‘Bolero’, ‘Samish’, ‘Catalina’, ‘Baker’, ‘F-380’, ‘Olympia’, and ‘Padre’. ‘F91-415’ was the darkest of all cultivars. Lightness values for ‘Space’ were not recorded as a result of crop failure from poor germination. During Spring 2006, no differences were observed among cultivars for lightness values.

In Fall 2005, the hue of spinach cultivars varied such that ‘F-380’ received the highest value (Table 1). ‘Baker’, ‘Padre’, ‘Olympia’, and ‘Catalina’ received lower values but were similar to one another. ‘Olympia’ and ‘Catalina’ were also similar to ‘Cypress’, which was similar to ‘Bolero’. ‘Bolero’ and ‘Samish’ were similar but received higher values than ‘F91-415’, which had the lowest values of all. Hue refers to the color family ranging from green to red. Although these differences were perceived by the chromometer, human evaluators would likely consider all the spinach cultivars to be green. During Spring 2006, no differences were observed for hue.

Vividness was highest for ‘F-380’ in Fall 2005 (Table 1). ‘Baker’, ‘Olympia’, ‘Padre’, and ‘Catalina’ were similar to one another and less saturated than ‘F-380’. ‘Olympia’, ‘Padre’, and ‘Catalina’ were also similar to ‘Cypress’, which was similar to ‘Bolero’. ‘Bolero’ and ‘Samish’ were similar and ‘F91-415’ was the least saturated (most gray) and similar to ‘Samish’. During the Spring 2006 production, no differences were observed among cultivars for vividness values.

Pearson correlation coefficients were developed to relate chromometer readings with the visual ratings. However, no significant (P = 0.05) correlations were observed (data not shown).

Selection of cultivars for production is a complex decision based on many factors, of which yield is critical. ‘F91-415’ and ‘Bolero’ consistently yielded well in Fall 2005 and Spring 2006, whereas other cultivars yielded well in one season or the other. ‘Olympia’, ‘Samish’, and ‘Padre had high yields in Fall 2005, whereas top-yielding cultivars in Spring 2006 also included ‘Baker’, ‘F-380’, ‘Padre’, and ‘Space’. Although visual ratings were only recorded in Spring 2006, they are an indicator of what consumers desire in fresh spinach and help complete the information needed for cultivar selection. ‘Samish’, ‘Padre’, and ‘Olympia’ had some of the highest visual ratings while still yielding well, particularly for fall production. ‘F91-415’, the highest average yielding cultivar for both seasons, had visual ratings near the middle of the cultivar list. Color readings varied in fall production but not the spring. Although not recorded, this could be incited by a difference in light quality during the production periods that resulted in different plant pigment production.

Number of days required for production in these studies was comparable to those reported by Ikeda et al. (1995). Overall, the cropping period was shorter in the spring than in the fall. This was likely the result of variation in temperatures and longer daylengths in the spring. When considering crop production cost, spring production would be less expensive as a result of shorter cropping cycles.

This study demonstrates that greenhouse spinach production has potential to become a viable enterprise regardless of time of year. The float bed system performed well in these studies. This system was chosen because of its potential for labor savings, uniform plant densities, and ease of harvest (Pearce et al., 1999). Based on yield and quality factors, the authors recommend further commercial trials of ‘Olympia’, ‘Samish’, ‘Padre’, and ’F91-415’ for fall greenhouse production and ’F91-415’ and ‘Padre’ for spring production.