No doubt you've read a multitude of pre-germination treatment guidelines for various species and encountered garden jargon in describing some of the techniques required. In this article, I hope to make the universe of seed germination less intimidating and make it user friendly. Knowing what these terms mean is essential to all those wanting to start germinating seeds from the more challenging species.
Cold stratification: seeds must be given a 24 hour presoak at room temperature then place seed in moist medium (peat, sand, vermiculite, paper towel, muslin, or a combination of) and exposed to 4 C (34-38 F) for a predetermined period of time as needed by the species in question (from days to months).
Warm stratification: soak seeds 24 hours at room temperature then place seeds in moist medium (peat, sand, vermiculite, paper towel, muslin, or a combination of) and exposed to 18-25 C (65-80 F) for a predetermined period of time as needed by the species in question (from days to months). Please note that some species are very sensitive to warm stratification temps such as Eucommia where temperatures over 25 C is lethal, but with Gymnocladus the optimum temperature is 30 C.
Scarification: tougher seed coats, in nature, are abraded by sand and other soil particles to allow the entry of water, a vital element for waking the embryo. This can be done by adding the seed to a small quantity of coarse sand in the palm of your hand and gently rub back and forth between your hands. Use a catch plate in case any material escapes your grip. The amount of time you need to scarify is generally brief (15-20 seconds), but should not be aggressive. Should be about the same pressure as you use to break down peat moss lumps. Some people recommend putting seed in a jar with sand paper, but you end up shaking the jar for so long, in my opinion; you might as well be churning butter. Remember you only want to abrade the seed coat as if you go too far and wound the embryo, this could be a fatal injury as this is a mode for rot promoting pathogens such as bacteria and fungus to enter...never mind the bugs.
Serious scarification: this is for the large heavy-duty seeds that you must physically use a good steel file to abrade the outer coat. Gymnocladus is a good example with its two seed coats. Hot water soak: this is a good alternative for those who don't want to use sulphuric acid or other caustic materials in the home especially if you have kids and pets (and/or spousal units) underfoot. Sometimes you may have to scarify the seed coat to mimic the etching action of the acid.
Hot water: from the tap or really hot water (off the boil) is used. If boiling/boiled water is called for, you pour boiling water into a one cup (250 ml) measuring cup. Wait one minute then pour over your seeds and let them sit in the hot water, let cool and let sit for 24 hours. You know when your successful using this technique as the seeds will noticeably swell 2-3 times its original size. If your seeds don't swell, scarify and redo the hot water treatment. This should do the job. Word of caution: use this technique for seeds that specifically call for it such as Robinia and Cercis as using hot water on seeds that don't require a hot water soak is lethal.