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Scarification of hard seeds

Scarification of hard seeds

A treatment for hard and woody seeds of many species in the families Caesalpinaceae (Cassia, Petalostylis, Caesalpina), Fabaceae and Mimosaceae (Acacias).
A hard waxy testa in many of these species prevents gas exchange and permeation by water, keeping the seed in a dormant state. This testa is purely a physical barrier and if it is broken and conditions are favourable the seed germinates rapidly. There are several methods of breaking through the waxy barrier:

1. Boiling Water: this is the method most commonly used. The seeds are placed in a cup or mug and boiling hot water is poured on them. Use about 5 times the volume of water that would just cover the seeds. Let soak for 12-24 hours as the water cools. The softened seeds will swell up, usually to several times their original size, and they will become soft. The successfully treated seeds can be separated from those that didn't swell up and should be sown straight away. The hard seeds that were left can be treated again the same way several times until soft.

2. Abrasion: seed can be rubbed between sheets of sandpaper, or a jar or drum can be lined with sandpaper and the seed is then shaken or tumbled until the seedcoat shows signs of abrasion. Seed should be sown straight away. To see if the abrasion was successful, seed can be soaked in cold water overnight. If it has swollen up and softened it will also most likely germinate. If it stays hard, then it should be dried and treated again.

3. Filing & Cutting (Nicking): the outer coating of seeds large enough to handle with some ease, can be broken by sing a small, three cornered file or sharp knife. Care must be taken to not damage the embryo of the seed and to avoid this it is a good idea to determine the exact layout of the seed before cutting. Seeds with woody coats are readily handled this way, a shallow slice being taken off the woody coat to just expose the seed inside. In my personal experience, this method has the highest success rate if done carefully.

4. Sulphuric Acid: This method should only be attempted by persons familiar with the handling of concentrated sulphuric acid and the related safety precautions - serious injury is possible. Acacia seeds responds very well to soaking in concentrated sulphuric acid for a period between 10 and 20 minutes. Use only a glass or earthenware container, and wear protective clothing, gloves, and glasses. Other hard coated seed should also respond to this method, but little is known regarding the lenght of soaking time. Light stirring is needed to ensure good contact between the acid and the seed coat. After treatment the acid must be drained off as thoroughly as possble before washing the seed with water to remove any remaining acid. Adding water to concentrated acid can cause splattering through sudden heat release. Seed should be rinsed under running water for 5 minutes and sown immediately.

5. Hydrogen peroxide: recent experiment have shown that this chemical is useful for improving the germination of many species, but no specific data for australian species is available. The process is simlar to the sulphuric acid treatment.

6. Dry heat: Different species have different threshold temperatures where the testa is penetrated. This may range from 50 to 80 degrees Celsius. Similarly the thermal death temperature also varies from species to species and may be anywhere above 50 degrees Celsius. Acacias respond well to a treatment of 70-80 degrees Celsius for 30-60 seconds. This is an unreliable method unless the exact parameters are known for a particular species.