The seeds of many plant species are often impervious to water and gases, thus preventing or delaying germination. Any process of breaking, scratching, or altering the testa, seed coat, through chemical or thermal methods to make it permeable to water and gases is known as scarification.
Types of Scarification
The main type of scarification is mechanical scarification. In mechanical scarification, the testa is physically opened to allow moisture and air to penetrate. Seed coats can be filed with a metal file, rubbed with sandpaper, nicked with a knife, cracked gently with a hammer, or any other possible form of physical abrasion to weaken and open the seed coat. Another type of scarification is chemical scarification. This is achieved by imbibing, soaking, seeds in concentrated acidic solutions at appropriate concentrations and durations of treatment. Chemicals such as sulfuric acid or even household bleach can be used to undergo this process. Another scarification method involves the use of hot water for brief periods. This is known as the hot water treatment. Scarification can also be achieved through the use of nutrient salts such as potassium nitrate. Regardless of the method, scarified seeds should be planted as soon as possible after treatment as they do not store well.
Scarification emulates the natural processes that over extended periods, and aided by microbial processes, lead to the permeability of seed coats. It is a standard technique in horticulture to facilitate the controlled and uniform germination of seed lots.  In the case of chaparral plant communities, many species' seeds require fire scarification 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.