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5.2 Propagation and management techniques

Collecting and Storing Seeds

Seeds should be collected from healthy, vigorous trees that are middle-aged. In general, large seeds germinate better and produce larger seedlings than small seeds. However, all seeds should be clean, dry, and free from insects. They can be stored in baskets, gunny bags, or boxes if air is allowed to circulate freely around the container and should be checked periodically for insect infestation. Seeds vary in the length of time that they remain viable, or are able to germinate. Stored seed from some species may keep for several years. However, it is best to use fresh seed when planting.

Preparing Seeds for Planting

Many tree seeds must undergo a period of dormancy before they will germinate. There are 2 types of dormancy: physical and physiological. By pretreating seeds in various ways it is possible to overcome both forms of dormancy. Physical dormancy occurs in seeds with protective seed coats. It ends when the seed coat is somehow opened by a process of mechanical abrasion, nicking, soaking in hot water or acid, or by passing through the intestines of a bird or animal (scarification).

There are several methods of pretreating seed by scarification:

· use sand paper to scratch the hull (this can be time consuming);

· mix the seeds in a container with wet coarse sand and shake the container;

· use fingernail clippers to crack or nick the seed, being careful not to clip the seed germ;

· immerse the seeds in an acid bath for a few seconds - be careful to store acid solutions very securely (Weber and Stoney 1986).

Physiological dormancy takes place in seeds that have not yet digested the fats, proteins, and other substances stored in the seed. These substances must be broken down into sugars and amino acids that can be absorbed by the embryo before the seed will germinate. Seeds with physiological dormancy are still immature and unripe, but can be treated in a way that promotes respiration in the seed (stratification), such as exposing the seed to a change in light or temperature.

To pretreat seed by stratification:

· bring water to a boil in a suitable container;
· remove from heat and let stand for 5 minutes;
· add the seeds and let them soak overnight;
· plant the seeds the next day (Weber and Stoney 1986).

Little research has been done on the subject of physiological dormancy of trees indigenous to Tanzania. Table 5.2 lists expected dormancy for selected native species.

Propagation by Seed

Seedbeds or containers should be prepared by watering frequently in small amounts for about 2 weeks before planting. Weeds that sprout during this time should be removed. Spacing is determined by the expected germination rate. Spacing of seeds is closer if germination is expected to be low. Conversely, if seed is fresh and germination is expected to be high, plants should be spaced further apart. Where known, information on recommended spacing is provided in the species profiles.

Vegetative Propagation

Some trees do not produce viable seed and must be propagated vegetatively, such as certain kinds of bananas, figs, and oranges. Other species, such as Euphorbia, may be more successfully propagated by cuttings than by seed.

TABLE 5.2 Selected Tanzanian Species Posing Germination Problems and Their Expected Dormancy

Species

Physical Dormancy

Physiological Dormancy

Acacia albida

·


Acacia tortilis

·


Albizia schimperana


·

Borassus aethiopum

·


Catha edulis


·

Clutia abyssinica


·

Cordia africana

·


Kigelia africana

·


Maesopsis eminii


·

Melia volkensii

·

·

Sclerocarya caffra

·


Syzygium guineense


·

Trema guineense


·

Trichilia emetica


·

Vangueria infausta

·

·

Warburgia salutaris

·

·

Zizyphus mauritiana

·


Adapted from Shehaghilo 1990.

Vegetative propagation consists of several methods including cuttings, root cuttings, and grafting. In East Africa the most common form of vegetative propagation is by cuttings. Cuttings should be taken from young, vigorous shoots or suckers from a healthy, mature tree. Depending on the species, the cuttings can be placed either in a pot filled with water, or directly into a trench which is kept moist. After a period of time the shoot will produce roots, and it can then be transplanted to a permanent site.

Cuttings from roots is another method of vegetative propagation, whereby roots are dug up, removed from the plant, and cut into pieces. Buds will form and produce a shoot and new root system. However, this method is uncommon. For further information about vegetative propagation, see a textbook such as Hartmann and Kester (1983).

Seedling Nurseries

In a nursery many small seedlings can be sheltered and intensively cared for in a small space as nurseries can provide access to water, protection from grazing animals, and shade. Nurseries should be located near a reliable water source and on level ground. Seedlings can be grown either in beds or in containers such as cans, plastic pots or bags, or in pots made from local materials such as banana leaves. Seedlings should be weeded every 10 days. Different species will require various amounts of time in the nursery, depending on their rate of growth and other conditions. Before the seedlings are outplanted they should be hardened off by gradually reducing the amount of water and shade for a few weeks beforehand. Weak, diseased, undersized, or overgrown seedlings should be culled.

Tending of Trees and Stands

Seedlings should be outplanted at the beginning of the rainy season to ensure an adequate water supply, and watered immediately after they are planted. Thereafter, hand watering may be needed if rainfall is inadequate. Weeding is essential to give the seedlings adequate light and air circulation, and to prevent competition for nutrients from weeds. As the trees grow, thinning may be needed to reduce competition for light and nutrients, and to remove weak or diseased trees.

Pruning and pollarding are 2 other widespread management practices used by farmers in Tanzania. Pollarding is a harvesting technique by which selected branches and the top of the tree are cut, often for fuelwood. This technique allows control over the height of the tree, whereas pruning controls lateral growth. Pruning can also be used to control the release of nutrients and improve productivity, for example in fruit trees. It is preferable to undertake these operations during or at the end of the dry season in order to facilitate healing and encourage new shoots and branches to grow during the rainy season.


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