Knowing the meaning and application of these propagation terms will help take the pain out of erratic germination come the growing season.
In botany, this is a term used to describe the process used to treat seeds, usually with a cold spell, ahead of germination. Many herbs, shrubs and trees from temperate climates require cooler temperatures ahead of germination to mimic winter changing to spring.
This means that although the fruit/seed is produced in autumn the plant will not grow ahead of winter, but afterwards, just as spring breaks, increasing its likelihood of survival.
Cold stratification can be achieved by soaking seeds overnight, then sealing in a plastic container in a 50:50 sand/peat (or substitute) mix. Seeds generally require 10-14 days in the refrigerator, though some species may require longer, and you will know according to when you see the first shoots appearing. Examples: lavender, eucalyptus, echinacea.
Some seeds are protected by extremely tough seed coats (testa) which, in the wild, may protect them from bushfires or passing through an animal’s gut and will only germinate after exposure to similar conditions.
As gardeners, however, we like uniform germination and without treatment the seeds will germinate ‘erratically’, meaning there could be weeks or months between each seed.
Scarification is the process by which we abrade, chip or treat the seeds with a chemical to break down the tough testa and allow the seeding to germinate. For hard seed coats, then filing the outside works best, though be careful not to damage the embryonic plant (morning glory & canna are common examples).
Seed companies treat seeds then can coat them to make the sewing process easier
Above Ranunculus and here Dahlia
For heat treatment, then many legumes require soaking in hot or boiling water and Acacia species even benefit from an acid bath! Famously, Protea and many other South African and Australian species require smoke treatment to prompt germination, mimicking the bushfires of their homelands.