Some seeds are contained in outer shells that are almost impenetrable. These hard outer coatings provide plenty of protection from the elements or potential pests, but they may keep the seed from germinating as quickly as you'd like. Seeds with hard outer coatings include moonflower (Ipomoea alba) and members of the legume family. Others produce their own chemical germination inhibitors and may also be frustratingly slow to germinate. These include the seeds of mint and mustard.
In nature, such seeds are exposed to natural processes that help them germinate. These include microbial action, which can take place in the soil wherever the seed may lodge, and weathering, the actions of freezing and thawing, and abrasion from grit in the soil. All of these may take some months to affect the seed, however gardeners can use the processes of scarification and stratification to help seeds germinate by mimicking nature's abrading phenomena.
Scarification is the abrasion of the seed coat by hand, water, or chemicals. To scarify a seed such as the rock-hard Kentucky coffee tree (Gymnocladus dioica) seed, try nicking the seed with a knife, scraping it with a nail file, or rubbing it with sandpaper. Soaking a seed in water for 12 to 24 hours may also soften its outer coating. This works well for morning-glory (Ipomoea tricolor) seeds. As soon as the seed swells, remove it from the water and plant it immediately. The seed may require a soaking of more than 24 hours; make sure to change the water every 24 hours as it soaks. And if your efforts to break the hard outer covering by hand are unsuccessful, try soaking the seed in sulfuric acid for 2 to 4 hours.
Stratification is the process of unlocking the dormant seed embryo by applying either heat or cold, depending on what the seed requires. Most stratification is done in the refrigerator in a moist medium, but if the seed likes warmth, it will most likely thrive in an environment that is heated at a steady 77 degrees. This can be accomplished in a greenhouse setting or with heat applied directly beneath the seed.