Hard-shelled seeds need a little help
Seeds with hard shells can be forced into germination by scarification, instead of letting nature take its course. Many commercial plant growers scarify seeds by soaking them in concentrated sulfuric acid. The seeds are soaked in the sulfuric acid for varying periods (depending on the species) and then washed and planted.
But sulfuric acid is dangerous, and vinegar is much safer. It may be less effective than sulfuric acid, but then you won’t risk being maimed or harmed by a dangerous chemical.
You can also thin hard seed coats by using sandpaper, nicking them with a knife, or cracked gently with a hammer (is that an oxymoron?).
Another method is to use boiling water. Bring water to a boil, remove the pot from the heat, and put the seeds in the water. Allow the seeds to soak until the water returns to room temperature. Then sow them. Remember, though, that scarified seeds do not keep well, so sow them as soon as you complete the scarifying process.
