As can be read in germinating treeseeds, many seeds require pre-treatment before they will germinate. Although not comprehensive by any standard, below a start of a guide to breaking dormancy of commonly used bonsai species.
Stratification means that seeds are placed in a moist but not soaking wet medium, such as spaghnum mos. The container in which this happens ideally prevents excessive evaporation of water, in order to keep the medium moist throughout. Yet it should allow for gass exchange. A zip-lock back which has not been closed completely is often used. Cold stratification takes place at temperatures ideally below 10 degrees. The meat drawer of a fridge is a good place. Warm stratification is roughly at 18-22 degrees.
Hot scarification is done by pooring hot (Not boiling, but rather at about 80 degrees) water over the seeds. Leave the water to cool and llet the seeds sit in the water for 12-24 hours. Seeds should have swollen considerably if scarification was succesfull. Repeat if so required.
Pinus Sylvestris (Scots Pine). May benefit from 2 weeks cold stratification
Quercus Robur (English Oak).
2 month cold stratification or less
lagerstroemia indica (Crape Mytrle); 4 weeks.
Larix decidua (European Larch)
Larix kaempferi (Japanese Larch)
Pinus Mugo (Mountain pine), 4-6 weeks
3 month cold stratification
Acer buergerianum (Trident Maple).
Warm followed by cold stratification
Acer campestre (Field Maple). 4 weeks warm stratification followed by 12 weeks cold stratification.
Acer palmatum (Japanese Maple). 4 weeks warm stratification followed by 8-12 weeks cold stratification.
carpinus betulus (Hornbeam) 4 weeks warm stratification followed by 24 weeks cold stratification.
Ginko Biloba. Use fresh seeds, collected, cleaned and stored that year. Warm stratify (18-22c) for 4 weeks, followed by cold stratification for 8 weeks.
Malus Sylvestris (Crab apple). 2 weeks warm stratification followed by 12 weeks cold stratification.
Taxus Baccata6 months (!) warm stratification, followed by 7 months cold stratification.
Tilia cordata (Small leaved Lime). 16 weeks warm stratification, followed by 16 weeks cold stratification
laburnum alpinum Laburnum.