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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 chemicals 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. The thawing and freezing of water, fire and smoke and chemical reactions in nature are what allow seeds to germinate but we can speed the process up by using the various methods described thus far. The common objective is opening the testa and allow air and water into the seed. In horticulture, scarification is often used to facilitate the controlled and uniform germination of seed lots.
A paper was published by the New Zealand Journal of Experimental Agriculture and stated that the seeds they examined in their study germinated only 30% under the preferred conditions, yet when they were treated chemically with concentrated sulphuric acid or mechanically scarified, the germination rate increased to more than 80%.
Another study was done on four different types of Great Basin lupine species to see the effect different methods of scarification would have on said seeds. The Longspur lupine (L. arbustus), Silvery lupine (L. argenteus), Hairy bigleaf lupine (L. prunophilus), and Silky lupine (L.sericeus) were the four species experimented on throughout the study. To summarize the experiment produced various results, due to the difference in species. The Silky lupine’s highest germination rate was achieved via mechanical scarification at 66.4%, opposed to its 22% germination rate found in the control group. Using thermal and chemical scarification, germination increased to 48.8% and 44% respectively. Interestingly enough 68% of the Longspur lupine seeds germinated in the control group and all scarification methods decreased the success rate of germination. The silvery lupine had 52% of its control group germinate but through mechanical scarification it rose to 85.2%. Finally the hairy bigleaf lupine’s control group germination rate was 32% yet when treated with sulfuric acid it rose to 76.8% showing the varying results of the experiment.
Win Pe, M. J. Hill &Margot E. H. Johnston (1975) l. Acid Treatment and mechanical scarification, New Zealand Journal of Experimental Agriculture, from https://doi.org/10.1080/03015521.1975.10425778
Covy D Jones, Mikel R Stevens, Von D Jolly, Bryan G Hopkins, Scott L Jensen, Dave Turner, and Jason M Stettler, (2016). Evaluation of thermal, chemical, and mechanical seed scarification methods for 4 Great Basin lupine species, from https://www.fs.fed.us/rm/pubs_journals/2016/rmrs_2016_jones_c001.pdf
Na (2019) Stratification and Scarification, from https://www.botanicalinterests.com/product/Stratification-and-Scarification