Scarification describes the process of abrading or otherwise damaging a hard or tough seed coat in order for it to take up water and oxygen so that chemical changes can occur in the seed to transition to germination and the appearance of seed sprouts. Scarification is thought to occur naturally by animals nibbling seeds, intestinal abrasion, chemicals of digestion or microbial breakdown of the seed coat. It can also be affected by repeated freezing and thawing and wet and dry cycles and for some seeds by going through fire. Seeds such as sweet peas, ipomoea, lupins, strelitzia and thunbergia are often seen as candidates for this treatment. At home scarification can be done artificially by puncturing, splitting, softening or burning the seed coat taking care not to damage the embryo inside.
Many of our vegetable seeds do very well when planted without any scarification, including peas and beans. I tend to plant a few extra peas and beans to fill the gaps when planting outdoors. Planting in the right conditions and watering well is more important for beans such as runner and climbing french beans which need warmer temperatures to germinate.
Traditionally one of the most popular seeds for scarification is the sweet pea and opinions remain divided as to the usefulness of the process. I also have my reservations about how to safely scarify round and difficult to hold seeds with a scalpel but I am sure that those dedicated to this art have experimented with a variety of methods and hopefully haven’t damaged their fingers in the process. Sandpaper would be a safer but possibly more tedious method.
Scarification in small-scale growing is probably a matter of choice, however larger commercial enterprises need a higher more consistent germination rate. In areas such as agroforestry and agriculture specific seeds may be scarified to get a higher percentage germination rate. Good germination means fewer gaps between plants and therefore better establishment of a crop cover which in turn reduces the competition from weeds. Further information can be found on the Forest Research website for where scarification is used in tree seed germination. An interesting overview of research into the germination of nitrogen fixing fodder crops such as alfalfa can be found in Seed Scarification Methods and their Use in Forage Legumes by E. Kimura and M.A. Islam.
The Forest Research site shows how the use of both physical scarification and stratification are often needed to prepare, what is often a very deeply dormant seed, for germination. Some tree seeds may require a variety of seed preparations and cycles, sometimes over 2-3 years in order to germinate. In my first blog when I planted an acorn with my grandson there probably were rather more issues at play than I was aware of including possible scarification, moist stratification cycles at different temperatures, time factors and whether there was viable seed within the acorn as well as the time required before germination would take place.
Of interest on the website is the fact that some tree seeds are quite short lived and do not store well. Trees such as oak, horse chestnut and sweet chestnut particularly do better being sown as soon as possible after ripening. So if you are thinking of planting a tree with your children it could be worth checking out what is required and planting in the autumn when you collect the seed.