Photoperiod and temperature conditioning of 'Sweet Charlie' strawberry (Fragaria × ananassa Duch.) plugs enhances off-season production

•7, 9 h short days (SD) at 21/21 °C followed by 7, 9 h SD at 21/12 °C enhanced greenhouse production.•Conditioned plugs produced more fruit (41-152%) than non-conditioned controls every year.•Conditioning enhanced spring and total fruit production but had no effect on winter production.•Yield enhancement was not immediate but rather was expressed 5-6 months into production.

Experiments were conducted over four years evaluating photoperiod and temperature conditioning of three to four week old 'Sweet Charlie' strawberry (Fragaria × ananassa Duch.) plug plants for off-season production in a vertical hydroponic greenhouse system in New Jersey, USA. The critical photoperiod for 'Sweet Charlie' flowering was evaluated by exposing plugs to 4 different photoperiods (7, 9, 11 and 12 h) for 0, 7, 14 or 21 days followed by exposure to 0, 7 or 14 nights of chilling (<12 °C). Plugs conditioned with a 9 h photoperiod produced 1.2 kg m−2 during December and January while all other photoperiods averaged 0.8 kg m−2. Plugs exposed to seven photoperiod cycles exhibited enhanced production compared to those exposed to 0, 14 or 21 cycles. Chilling for 7 cycles after photoperiod treatment significantly enhanced production compared to 0 or 14 cycles of chilling. A conditioning protocol of seven 9 h short days (SD) at 21/21 °C (day/night) followed by seven 9 h SD at 21/12 °C (day/night) was established and productivity of conditioned plugs was compared to non-conditioned controls (fourteen 12 h days at 21/21 °C) in years 2, 3 and 4. While productivity varied considerably from year to year, conditioned plugs produced more fruit (41-152%) than non-conditioned controls every year. Conditioning significantly enhanced spring and total fruit production (weight and number) but had no effect on winter production. Yield enhancement due to conditioning treatment was not realized as greater early yield, but rather was expressed 5-6 months into production.