Scarification in botany involves weakening, opening, or otherwise altering the coat of a seed to encourage germination. Scarification is often done mechanically, thermally, and chemically. The seeds of many plant species are often impervious to water and gases, thus preventing or delaying germination. Any process designed to make the testa (seed coat) more permeable to water and gases (and thus more likely to germinate) is known as scarification.
Scarification, regardless of type, works by speeding up the natural processes which normally make seed coats permeable to water and air.
The most common type of scarification is mechanical scarification.
In mechanical scarification, the testa is physically opened to allow moisture and air in. Seed coats may be filed with a metal file, rubbed with sandpaper, nicked with a knife, cracked gently with a hammer, or weakened or opened in any other way.
Another type of scarification is chemical scarification, which involves the use of one or more chemicals to promote germination. It can involve imbibing or soaking seeds in precisely concentrated acidic or basic solutions for varying amounts of time. Chemicals such as sulfuric acid or even household bleach can be used to affect this process. Chemical scarification can also be achieved through the use of nutrient salts such as potassium nitrate.
Thermal scarification can be achieved by briefly exposing seeds to hot water, which is also known as hot water treatment. In some chaparral plant communities, some species' seeds require fire and/or smoke to achieve germination. An exception to that phenomenon is Western poison oak, whose thick seed coatings provide a time delayed effect for germination, but do not require fire scarification.
Regardless of the method, scarified seeds do not store well and need to be planted quickly, lest the seeds become unviable.
Because scarified seeds tend to germinate more often and in less time than unaltered seeds, scarification finds use not just in industry but on the small scale. In home gardens, for example, the seeds of plants which are otherwise difficult to grow from seed may be made viable through scarification. In horticulture, scarification is often used to facilitate the controlled and uniform germination of seed lots.