This a hard seed coat adaptation is particularly true for species that come from dry environments.
Some of the seeds that we sell have hard seed coats that allow them to sit in the ground for many years waiting for the proper conditions to come along for germination. This adaptation is particularly true for species that come from dry environments. These seeds will not germinate until the water can penetrate the seed coat.
To facilitate this the seed coat needs to be softened to allow water to inside. Our recommendations for scarification are based on the average requirement for each species. It is safest to start with the least harmful method and work up from there. A cold water soak for a short time duration is probably the least harmful. This would be followed by a longer time duration and then warmer water up to the boiling point. Whenever mention is made of soaking in hot water, etc. It means to initially subject the seed to the hot water and then to let it cool in that water. DO NOT keep the water hot for 24 hours, etc. After a treatment has been finished a few seeds should be cut to see if they have imbibed (taken up water). Continue the treatments until you notice that the seeds have indeed imbibed.
Repeat cycles may be required but always drain seed every 24 hours and start again with fresh water.
If boiling water has been tried and the seeds have still not imbibed, concentrated sulfuric acid or mechanical scarification must be used. For the acid, treatment seeds are placed in a glass container and covered with sulfuric acid. The seeds are gently stirred and allowed to soak for 10 minutes to several hours, depending on the species. When the seed coat has been modified (thinned), the seeds are removed, washed, and sown. Sulfuric acid can, however, be very dangerous for an inexperienced individual and should be used with extreme caution! Proper protective clothing and eye protection must be worn. Vinegar is safer (but less effective treatment) and can be used for species that do not have an extremely hard seed coat; the technique is the same as with sulfuric acid.
For mechanical scarification, seed coats can be filed with a metal file, rubbed with sandpaper, nicked with a knife, or cracked gently with a hammer to weaken the seed coat.
Following acid scarification, the seeds should be dull in appearance, but not deeply pitted or cracked as to damage the embryo. Scarified seeds do not store well and should be planted as soon as possible after treatment.