Maple seeds are samaras about 1-1.5 cm (0.4-0.6 in) long, ripening in different seasons. Most species, even under optimal conditions, have a natural germination rate of 20-50%. Simply remaining in the soil, they may germinate only years later.
Scarification significantly increases seed germination and swelling; without disrupting the upper layer, germination will be minimal. The outer coat is carefully filed or scratched with a hard tool without damaging the seed, after which they are soaked for 12-24 hours in warm water with the addition of stimulants (SUPERthrive, HB-101, Aminozol, Epin, or Zircon). Subsequent pre-sowing preparation is conditionally divided into 3 scenarios depending on the species.
1. Acer BUERGERIANUM, Acer GINNALA, Acer ELEGANTULUM, Acer MACROPHYLLUM, Acer PALMATUM, Acer PLATANOIDES, Acer SACCHARUM, Acer TATARICUM, Acer TRUNCATUM (and most species not in the list)
require cold stratification for successful germination. Seeds are placed in a small plastic zip-lock bag, inside of which lightly moistened peat moss or vermiculite is added beforehand, from which water should not be released when squeezed. Air is removed with a palm motion from the bottom to the zipper, the bag is tightly closed and placed in the refrigerator on the bottom shelf. Stratification is carried out for 2-4 months at a temperature of 3-5 °C (37-41 °F). Once every 1-2 weeks, the contents are checked. If condensation appears, excess moisture is removed by gentle pressing, after which the bag is turned over. In case the substrate dries out, 1-2 drops of water are added. Once germination begins, seeds are removed from the refrigerator and planted in prepared soil.
2. Acer CAMPESTRE, Acer GLABRUM, Acer GRISEUM, Acer JAPONICUM, Acer PSEUDOSIEBOLDIANUM, Acer SHIRASAWANUM (and challenging rare/mountain/Asian species)
due to germination difficulty have an additional stage of warm stratification. After scarification and soaking, seeds are left in a warm room at a temperature of 20-30 °C (68-86 °F) for 2-4 months. Next, the seeds are placed in a plastic bag and undergo cold stratification (as described in point 1), sometimes with a repeated cycle. One should not be surprised by low germination; realistic expectations are only about 20-30%. After germination begins, seeds are planted in prepared soil.
3. Acer RUBRUM, Acer SACCHARINUM (and hybrids such as Acer × FREEMANII)
do not require stratification and are sown immediately after scarification and soaking.
After any of the preparatory stages, seeds are sown in a light, air-permeable substrate to a depth equal to the seed size or buried 0.5-1 cm (0.2-0.4 in), moistened from a spray bottle, placed in a mini-greenhouse or covered with film, and germinated in light, but not in direct sun at a temperature of 18-22 °C (64-72 °F). Seedlings require ventilation. It is necessary to avoid waterlogging and drying out of the crops.
Seedlings emerge in 2-4 weeks; the maximum germination period can exceed 6-8 weeks, and some seeds may germinate only in the 2nd-3rd year. After the first true leaves unfold, pricking out into individual pots is carried out. Cultivar forms are also propagated by grafting onto hardy rootstocks to preserve ornamental coloring.
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