Extended Greenhouse Nursery Periods Reduce the Field Growth Duration of Rice: Implications for Double Cropping Systems in Cool Temperate Regions

The primary constraint for rice-rice double cropping in cool temperate regions (38°–39°N) is the limited growing season. This study quantifies the effects of extended greenhouse seedling periods on reducing field growth duration of rice (Oryza sativa L.) using the DPRK variety ‘Kapsan 1’. Three experiments were conducted over two years (2022–2023) at 38°30′ N. Experiment 1 examined seedling periods (35, 40, 45 days) on field growth duration. Experiment 2 quantified Growing Degree Days (GDD) savings from greenhouse seedling periods (30–45 days). Experiment 3 determined optimal seeding densities (60–120 g per 3.3 m2). Additionally, 30-year climate data were analyzed and DSSAT v4.7 simulated double cropping success probability. A meta-analysis of 15 Chinese studies was conducted. Forty-five-day seedlings reduced field growth duration by 23.7 days compared with 35-day seedlings (P < 0.001). GDD savings increased proportionally: 198°C (30 d), 235°C (35 d), 268°C (40 d), and 298°C (45 d), yielding the regression: GDD saving = 6.8 × seedling days – 8.5 (R2 = 0.98). Optimal seeding density for 45-day seedlings was 65 g per 3.3 m2, with recommended field density of 72–78 g per 3.3 m2. The study site provided 3724 ± 118°C annually over a 194-day growing season. DSSAT simulation showed that 45-day seedlings increased double cropping success probability from 73.3% to 90.0% (16.7 percentage point improvement). Meta-analysis confirmed that growth duration reduction in this study (23.7 days) was significantly greater than the Chinese mean (17.6 days), with a positive latitude-reduction relationship (β = 0.82, R2 = 0.68). The 45-day greenhouse seedling period reduces field growth duration by 24–30 days, saves 298°C of GDD, and increases double cropping success probability by 16.7 percentage points, providing fundamental information for designing rice-rice double cropping systems in cool temperate regions.