However, 12 h photoperiod and 3 wk duration may be an adequate practice for nurseries that treat multiple crops in their SD facilities. The root and shoot regrowth after cold storage was highest in seedlings that had received 12 h photoperiod and 5 wk duration. The frost hardiness of the seedlings improved when the photoperiod was reduced from 12 to 8 h, and when the SD duration was increased from 3 weeks to 5 weeks, but reducing the photoperiod from 12 h to 8 h caused growth reductions. All the SD treatments yielded healthy seedlings that grew well after the cold storage. The seedlings were subjected to one of the following treatments: (1) 12 h photoperiod + 3 weeks duration (2) 8 h photoperiod + 3 wk duration (3) 12 h photoperiod + 5 wk duration and (4) 8 h photoperiod + 5 wk SD. The aim of the experiment was to define a short-day treatment (SD) that harden seedlings to sustain 3 months of cold storage, but does not have adverse effects on growth, morphology, and vitality. Mini-plug transplant seedlings of Norway spruce have been cultivated in closed growth systems, so-called plant factories, for few years. Our conclusions, being based on the short term effects of field storage, are that conifer seedlings can be stored in closed boxes for only three days in August and about a week in September, October, and spring. Mortality rates were high in the autumn experiment due to the exceptionally warm and dry weather. In the spring experiments, prolonged storage increased the mortality of seedlings. The frost hardiness was weakened in the Norway spruce seedlings that were stored in closed boxes for 21 days in August and October. In all of the experiments, increased closed-box storage reduced the maximum photochemical yield of photosystem II (Fv/Fm) in the needles, and reduced root growth after planting. After storage, the seedlings were planted in a field. The control seedlings were stored in open storage in the nursery. In each experiment, the seedlings were stored in closed cardboard boxes in a nursery for one, three, seven, 14, and 21 days. In the third Scots pine experiment, mini seedlings (30-cm3 peat plugs) were packed in May. In the first experiment, Norway spruce seedlings (85-cm3 peat plugs) were packed in cardboard boxes in August, September, or October, and, in the second experiment, in the middle of May. The aim of this study was to determine the maximum safe duration for the field storage of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) container seedlings in closed cardboard boxes for different planting times (dates) in Nordic boreal conditions. Additionally, seedlings are increasingly being stored in cardboard boxes in spring, summer, and autumn plantings in Finland. Based on our findings, small-sized Scots pine and silver birch seedlings can be planted safely at 6–8 cm planting depths, if at least 20% and 50% of their shoots, respectively, are above ground.įor spring plantings, conifer seedlings are usually packed in closed cardboard boxes and freezer stored over winter. Deeper planting decreased damage in Scots pine seedlings in the first growing season, which was not observed in silver birch. However, the height differences between the planting depths were not apparent until the end of the second growing season in both tree species. At the end of the first growing season, the deeper-planted birch seedlings were the tallest, as opposed to the deeper-planted Scots pine seedlings. Scots pine seedlings (mean height of 9 cm) were planted to depths of 2, 5 and 8 cm on inverted mounds in September 2018 and May 2019. Silver birch seedlings (mean height of 16 cm) were planted to depths of 3, 6 and 8 cm on spot mounds in May 2016. We studied the effects of planting depth on the early field performance of small-sized silver birch (Betula pendula Roth) and Scots pine (Pinus sylvestris L.) container seedlings. Its effects on smaller-sized seedlings are poorly understood. Deep planting is recommended in Nordic countries only for normal-sized container seedlings planted on mounds.
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