10. SEEDS

10.9 Pre-sowing treatment

Seeds of many species do not germinate well unless they are exposed to certain conditions. This state of not germinating unless the required conditions are met is called dormancy. In the natural environment the conditions may be exposure to fire or being eaten by animals. When seeds are eaten they are exposed to the hydrochloric acid in the stomach of the animal, and this breaks the dormancy without damaging the seed.

Similar methods are used by man to treat seeds and break the dormancy of seeds he wishes to germinate. There are several methods of pre-treating tree seeds, but knowledge of a few simple techniques is sufficient to get reasonable germination of almost all species.

For pre-treatment purposes, seeds can be divided into five groups.

Group 1: Seeds requiring hot water treatment

This group includes most leguminous trees with pods and more-or-less flat seeds with a hard seed coat. Normally, such seeds germinate faster and better if treated with hot water. The procedure is as follows. Heat some water to near boiling point and then take it off the fire. Pour the hot water over the seed in another pot and leave the seed to soak for about 24 hours. (Do not boil the seed.) The seed will absorb water, swell and sink to the bottom of the pot. All swollen seeds should be removed and sown immediately, whilst those which are not swollen should be left in the pot for a further 24 hours. After they have swelled up, they should be sown too.

Figure 10.5 Hot-water treatment of seeds

Another efficient but time-consuming technique of pre-treatment for hard-coated seed is to nick the seed coat with a knife or fingernail clipper. Only a small nick is required. Due to the amount of labour required, nicking can only be recommended for small quantities of seed. Examples of species in this group are Acacia spp., Calliandra calothyrsus, Cassia spp., Leucaena leucocephala, Cassia spectabilis, and Parkinsonia aculeata.

Seeds in this group can usually be stored for some time, and hot-water treatment is normally more important if the seed has been stored than if it is fresh.

Group 2: Small light seeds with no need for pre-treatment

Many important species have small, dry seed, some of which can be stored for at least a few months without losing viability. Examples are Eucalyptus spp., Casuarina spp., Cupressus lusitanica, Iacaranda mimosifolia, Juniperus procera, Pinus spp., and Psidium guajava.

Other species also have small, dry seeds which do not require treatment but they cannot be stored for long. For these species the best germination is obtained from fresh seed. Examples are Grevillea robusta and Markhamia lutea.

Group 3: Large seeds with wings which should be removed before sowing

Species with large winged seeds normally do not require treatment, but removal of the wings makes the seed easier to handle and speeds up the penetration of water after sowing. Combretum fruit should be opened and the fleshy seed sown immediately. Seeds of many Terminalia species and of Tipuana tipu can be stored for some time, but Combretum seeds should be sown fresh. Examples of species are: Combretum spp., Tipuana tipu, Terminalia spp.

Group 4: Medium-sized or large seeds with no need for pre-treatment

This group includes many seeds with a high oil content, e.g. Croton spp. and Vitex spp. Some of these seeds are surrounded by fruit pulp, which should preferably be washed away before drying or sowing the seed. Seeds of Azadirachta indica, Bauhinia spp., Bridelia micrantha, Croton spp., Dovyalis caffra, Eriobotrya japonica, Erythrina abyssinica, Prunus africanus, Syzygium spp., and Vitex spp., for example, should be sown as fresh as possible for best results, whereas seeds of Afzelia quanzensis, Cordia spp., Melia azedarach, Schinus molle and Sesbania spp. can be stored for some time. Albizia seeds can also be stored for some time, but only if they can be kept free of insects. Seeds of Calodendrum capense should be floated in water before sowing to separate the floating non-viable seed from the sinking good seeds. Examples of species are Afzelia quanzensis, Albizia spp., Azadirachta indica, Bauhinia purpurea, Calodendrum capense, Cordia spp., Croton spp., Dovyalis caffra, Eriobotrya japonica, Melia azedarach, Prunus africanus, Schinus molle, Sesbania spp., Syzygium spp., and Vitex spp.

Group 5: Large seeds with hard seed coats which require cracking

Some seeds which have a very hard seed coat could be treated mechanically to break the seed coat and allow water to penetrate inside the seed. Cracking the seed coat must, however, be done with great care in order not to damage the seed. This type of seed can normally be stored. Examples of species are Adansonia digitata, Podocarpus spp. and Ziziphus spp.

What to do if the recommended seed treatment is not known?

If the recommended method for a desired species is not known you need to try various options and monitor the result. One option is to sow the seed without any treatment. Comparison of the seed's appearance and structure with the groups described above is recommended as similarities with other better-known seed may give hints as to what might be suitable treatment.

Further reading

The information provided here is a rough guideline. More detailed information can be found in other publications. A Selection of Useful Trees and Shrubs for Kenya and Multipurpose Tree and Shrub Seed Directory, both published by ICRAF, are particularly recommended.