Daily light integral and/or photoperiod during the young plant and finishing stages influence floral initiation and quality of witchgrass and marigold cut flowers

Abstract

To produce consistent and high-quality specialty cut flowers throughout the year, growers in temperate climates must utilize controlled environment greenhouses. Research-based information on photoperiod management and supplemental lighting for specialty cut flowers is limiting. Therefore, our objectives were (1) to determine the effect of photoperiod during the young-plant and finishing stages on floral initiation and morphology of witchgrass 'Frosted Explosion' (Panicum capillare) and marigold 'Xochi' (Tagetes erecta) and (2) to quantify the effect of daily light integral (DLI) on floral initiation and morphology of witchgrass during the finishing stage. Seeds of marigold and multi-seed pellets of witchgrass were sown and placed under 9-, 11- (marigold only), 12-, 13-, 14-, 15-, 16-, 18-, or 24-h photoperiods or a 9-h short day with a 4-h night interruption (NI) from 2200 to 0200 h. Plugs were distributed among 10-, 11-, 12-, 13-, 14-, 15-, or 16-h photoperiods or a 4-h NI, for finishing. Witchgrass was finished under a very low or moderate DLI of ≈3 or 10 mol^⋅m^-2⋅d^-1, respectively, while marigold was finished under a DLI of ≈10 mol^⋅m^-2⋅d^-1. Marigold grown under a photoperiod ≥ 11 h or a 4-h NI during the young-plant stage and finished under an 11- or 12-h photoperiod had thick stems and consistently met the marketable stem length of ≥ 65 cm. Up to 29% and 107% more stems were harvestable under 11- and 12-h finishing photoperiods, respectively, compared to a 10-h finishing photoperiod. Marigold visible buds were delayed, and stems were not harvestable under photoperiods ≥ 13 h or a 4-h NI after 8 weeks. Young witchgrass plants grown under a photoperiod between 14- and 24-h or a 4-h NI and finished under photoperiods ≥ 14 h or a 4-h NI, and at least a moderate DLI, were reliably harvestable (≥ 50 cm long with a fully developed panicle). Witchgrass finished under day lengths < 13 h (rep. 1) or < 14 h (rep. 2) flowered prematurely and were roughly one-sixth the length of harvestable stems at an open flower. All witchgrass stems grown under a very low DLI were shorter and thinner than those grown under a moderate DLI, and none were harvestable. Therefore, we recommend growing marigold 'Xochi' young plants under a photoperiod ≥ 11 h or a 4-h NI and finishing under a 12-h photoperiod. Additionally, witchgrass 'Frosted Explosion' young plants should be grown under a photoperiod ≥ 14 h or a 4-h NI and finished under photoperiods ≥ 14 h or a 4-h NI to prevent premature flowering. Witchgrass and marigold cut flowers should be finished under a DLI of ≥ 10 mol^⋅m^-2⋅d^-1 for consistent production of high-quality stems.

Keywords: Panicum; Tagetes; light-emitting diodes; specialty cut flowers; supplemental lighting; young plants.

FIGURE 1

Effect of photoperiod [9, 11, 12, 13, 14, 15, 16, 18, and 24 h, or a 4-h night interruption (NI)] on the height (A,D), root dry mass (B,E), and shoot dry mass (C,F) of marigold ‘Xochi’ (Tagetes erecta) and witchgrass ‘Frosted Explosion’ (Panicum capillare) young plants. Panel (F) presents data from replication 2 as trends from replication 1 were not significant. Coefficients are presented in Table 7.

FIGURE 2

Effects of young-plant photoperiod (9, 12, 13, 14, 16, 18, and 24 h, or a 4-h NI) and finishing photoperiod (10, 11, 12, 13, 14, 15, and 16 h, or a 4-h NI) on time to visible bud of witchgrass ‘Frosted Explosion’ (Panicum capillare). Figures represent (A) moderate-DLI-grown (≈10 mol^⋅m^{– 2⋅}d^{– 1}) cut flowers from replication 1, (B) very-low-DLI-grown cut flowers from replication 1, (C) moderate-DLI-grown cut flowers from replication 2, and (D) very-low-DLI-grown (≈3 mol^⋅m^{– 2⋅}d^{– 1}) cut flowers from replication 2. Black circles represent individual data points for sequential photoperiods; red circles represent averages from NI treatments. Model predictions are represented by response surfaces; coefficients are presented in Table 8.

FIGURE 3

Effect of young-plant photoperiod (11, 13, 14, 15, 16, and 24 h, or a 4-h NI) and/or finishing photoperiod (10, 11, 12, 13, 14, 15, and 16 h, or a 4-h NI) on time to visible bud (TVB) of marigold ‘Xochi’ (Tagetes erecta). Figures represent (A) the interaction between young-plant and finishing photoperiod on TVB of plants from replication 1, (B) the effect of young-plant photoperiod on TVB of plants from replication 2, and (C) the effect of finishing photoperiod on TVB of plants from replication 2. In panel (A), black circles represent individual data points for sequential photoperiods; red circles represent averages from NI treatments. Coefficients are presented in Table 9.

FIGURE 4

Effects of young-plant photoperiod [9, 12, 13, 14, 16, 18, and 24 h, or a 4-h night interruption (NI)] and finishing photoperiod (10, 11, 12, 13, 14, 15, and 16 h, or a 4-h NI) on time to open flower of witchgrass ‘Frosted Explosion’ (Panicum capillare). Figures represent (A) moderate-DLI-grown (≈10 mol^⋅m^{– 2⋅}d^{– 1}) cut flowers from replication 1, (B) very-low-DLI-grown (≈3 mol^⋅m^{– 2⋅}d^{– 1}) cut flowers from replication 1, (C) moderate-DLI-grown cut flowers from replication 2, and (D) very-low-DLI-grown cut flowers from replication 2. Black circles represent individual data points for sequential photoperiods; red circles represent averages from NI treatments. Model predictions are represented by response surfaces; coefficients are presented in Table 8.

FIGURE 5

Effects of young-plant photoperiod [9, 12, 13, 14, 16, 18, and 24 h, or a 4-h night interruption (NI)] and finishing photoperiod (10, 11, 12, 13, 14, 15, and 16 h, or a 4-h NI) on stem length of witchgrass ‘Frosted Explosion’ (Panicum capillare) at the open flower. Figures represent (A) moderate-DLI-grown (≈10 mol^⋅m^{– 2⋅}d^{– 1}) cut flowers from replication 1, (B) very-low-DLI-grown (≈3 mol^⋅m^{– 2⋅}d^{– 1}) cut flowers from replication 1, (C) moderate-DLI-grown cut flowers from replication 2, and (D) very-low-DLI-grown cut flowers from replication 2. Black circles represent individual data points for sequential photoperiods; red circles represent averages from NI treatments. Model predictions are represented by response surfaces; coefficients are presented in Table 8.