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Chapter III. Techniques of nursery operations in arid zones
1. Introduction
2. Choice of site for the nursery
3. Design of the nursery
4. Collection, handling, storage and pre-treatment of seeds
5. Seedling production
6. Highlights of section
Nurseries are places where seedlings are raised for planting purposes. In the nursery the young seedlings are tended from sowing to develop in such a way as to be able to endure the hard field conditions. Whether local or introduced species, nursery seedlings are found to have better survival than seeds sown directly in the field or through natural regeneration. So nursery seedlings become the planting material for plantations, whether these plantations are for production, protection or amenity.
Nurseries are of two types, i.e.:
Temporary nurseries: These are established in or near the planting site. Once the seedlings for planting are raised, the nursery becomes part of the planted site. There are sometimes called "flying nurseries" (Figure 3.1).
Permanent nurseries: These can be large or small depending on the objective and the number of seedlings raised annually. Small nurseries contain less than 100,000 seedlings at a time while large nurseries contain more than this number. In all cases permanent nurseries must be well-designed, properly sited and with adequate water supply (Figure 3.2).
Seedling production is a major expense of afforestation and every effort should be made to produce good quality seedlings at a reasonable cost. To this end mastering the techniques of nursery operations is essential. This chapter will review the various operations involved in the production of seedlings.
2. Choice of site for the nursery
When the site of the nursery is to be selected, four questions arise:
A. What is the type of the nursery?
Is it temporary or permanent?
B. What is the size of the nursery?
Is it large with 100,000 seedlings per year and more, or is it small with 50,000 seedling capacity per year or less?
Figure 3.1 A temporary nursery.
Figure 3.2 A permanent nursery.
C. Seedling demand
How big is the seedling demand? For example, a nursery surrounded by several development projects may demand huge amounts of different seedlings every year, whereas a nursery for small community woodlots may have a low annual seedling production.
D. Transport or distance from the nursery to places of seedling demand.
When these questions are answered, the nursery is sited where:
- Good water supply source is available, e.g. near a river or a well. Because water is very crucial to the nursery, this is a determining factor.
- Good soil source is available; as soil is bulky, it is needed in great quantities. Site soil must be at least free from salinity and alkalinity.
- Also the site must be well drained to avoid waterlogging and be fairly safe from flood hazards.
- Shelter against prevailing winds: sites which have a natural shelter by vegetation or any other formation are preferred to exposed sites. If the site is exposed then it must be sheltered artificially.
- The site must have good access roads to places of seedling demand. This will ensure that seedlings can reach the site in good condition. Bad roads and long journeys reduce seedling survival to a great extent.
- The nursery must be sited where labour is available or can be easily obtained and accommodated. Nursery work is labor-intensive and placing nurseries far away from habitation centres will be very costly.
Having decided on the site and size of the nursery, the site is carefully levelled, fenced, and a shelter from the prevailing wind is established.
The nursery must be well designed. The nursery is divided into a suitable number of blocks. These blocks contain adequate roads among them. Blocks are normally labelled by letters, e.g. A, B. C, etc. or by Roman numbers: block I, block II, block III, etc. Roads between the blocks should be wide enough to provide space for on-loading and offloading and contain turning space with a minimum width of 5 meters.
Each block is further divided into 4-8 sections with paths among them. Sections are labelled by their respective block label followed by a small letter, e.g. Section Ia denotes the first section from the left hand corner of block I (Figure 3.3).
Each section is further divided into beds. The bed is the smallest unit in the nursery design. Beds are normally one meter wide and their length may vary from 6-10 meters. Beds may be sunk in the ground at a depth of 30-35 cm below general ground level. In this case they may be laid with concrete, stone or bricks.
Also beds may be designed slightly higher than the general ground surface. In this case, the beds are surrounded by stakes, bricks or stones. In every case drainage in these beds is very important for seedling development and for nursery hygiene.
Beds are labelled by their blocks and section followed by Arabic figures, e.g. bed No. Ia1 denotes the first bed in section (a) of block I. Beds are separated by paths one meter wide to facilitate work and transport of seedlings by hand or wheelbarrow, watering and tending of seedlings.
In addition to these, the nursery design should contain adequate space for soil mixing (at least 5 x 5 meters). It should also contain a separate area for making compost. This is better placed slightly away from the nursery beds.
3.1 Size of the Nursery
The size of the nursery area stacked with containers (when containers are employed) and the total nursery area will vary with the diameter of the containers. The relationship between the diameter of containers (from 5 to 15 centimers) and the surface of the nursery area (in square meters) for the production of 100,000 potted plants is illustrated in Figure 3.4.
From Figure 3.4, one can see that, for containers with a diameter of 5 centimeters, 240 square meters of beds are required. To estimate the total nursery area, the area of seedbeds is multiplied by 2.5, to include road and service areas, and 100 square meters are added (for paths), based on the production of 2,000 seedlings per square meter of seedbed. Therefore, in general:
the total
nursery area = (2.5 x area of seedbed) + 100 square meters and,
for this example:
the total nursery area =
(2.5 x 240) + 100 square meters
Figure 3.4 Relationship between the diameter of the containers and the surface of the nursery area.
Not all nursery operations involve the use of containers. When bare-rooted planting stock is produced, the size of a nursery will depend, in large part, upon the "average" size of the planting stock and the level of production to be maintained.
3.2 Nursery water supply
Two aspects should be emphasized: (a) water quality; and (b) daily water requirement.
Water quality: It must be slightly acidic with a pH less than 7, with dissolved salts less than 550 parts/million, and with a conductivity less than 0.8 mho/cm. Generally fairly sweet and clear.
Water quantity: Adequate water of the above description should be supplied daily to the nursery.
The amount of water applied (at any one time) will vary with the weather conditions, the soil infiltration rate, and the size of the plant. During the period of germination, frequent light" watering is required to keep the seedbeds moist, but not saturated. As plants become larger, the total quantity of water applied is increased and the frequency of application is reduced.
As a guide to estimate the quantity of water to apply in one month, the following calculation can be made:
water quantity = water loss factor x E x area of seedbed where: water loss factor = values between 1.2 and 1.4, averaging 1.3
E = monthly evaporation
For example, assuming a water loss factor of 1.3, for a monthly evapotranspiration (E) of 0.2 meter and a seedbed area of 10,000 square meters, the water requirement for one month is:
water quantity = 1.3 x 0.2 x 10,000 = 2,600 cubic meters
Watering can be either by hand or through irrigation. Hand watering with cans, hoses fitted with spray-nozzles, or knapsack mist sprayers are methods used by small nurseries. For watering containers or seedbeds in which seeds have been sown, a fine droplet size is essential. Otherwise, the seeds can be washed out of the ground or the seed covering material can be washed away and the soil surface will be consolidated. Therefore, hand watering of the seedbeds is commonly done with a gardener's watering can or a knapsack pressure sprayer fitted with a fine mist-producing nozzle.
4. Collection, handling, storage and pre-treatment of seeds
4.1 Seed quality
Seeds are either collected by the forester or obtained from a known seed source in the country or abroad. In the latter case, the seed must be of good quality:
- it must be clean from dirt, debris and chaff;
- it must be free from pests and pathogens;
- it must have a high percentage of germination;
- it must be accompanied by a note, carrying the scientific name of the species, place of collection, date of collection, number of seeds/unit weight and whether any treatment has been applied.
4.2 Seed collection
To ensure good seed quality, fruit collection must be made from trees having the desirable characters. Such trees are labelled and their locality recorded on a map (Figure 3.5).
The phonology of these trees should be observed as to when they would flower, set fruit, and have mature fruits. Does fruiting take place every year, every two years? Are there any factors affecting fruit production? e.g. drought, defoliation by insects, etc.
Nature of fruit: dehiscent or intact. Does it remain on the tree or fall to the ground?
Hazards to the fruits: collected by humans, animals, insects, pathogens, blown by wind?
Collection time and method: well developed and mature fruits contain good seeds. So the collection time is when fruits are fully matured.
Fruits are either collected from the tree by beating the tree with a stick, or shaking the crown with a long hook, or by climbing.
Some fruits fall to the ground and they are collected. In such a case, the place of collection is cleaned beforehand.
Treatment of fruits: Collected fruits are cleaned, sprayed against insects and spread on a clean sheet to dry.
Figure 3.5 Collection of seeds from Acacia victoriae
4.3 Seed extraction
This is the process of separating the seeds from the fruit. Therefore, the method of extraction varies with the type of fruit. For example, Acacia seyal and A. senegal legumes dehisce once they are completely dry and a gentle shaking is sufficient to extract the seeds, while Prosopis spp. seeds are difficult to extract. The fruit is first pounded to remove the pulpy material, then the remaining part of the fruit is treated with dilute hot hydrochloric acid for 30 minutes; then washed and dried and then pounded again to get rid of the thin cover over the seed.
Eucalyptus seeds are extracted very easily when fruits become brown on top; they are collected and put in clean open tins to dry, once dried the fruits open, shedding the seeds and chaff.
Hyphaene thebaica seeds are extracted by sawing-off the shell.
4.4 Seed drying
Once seeds are extracted, they are cleaned of chaff and dirt and dried in the sun or in an oven. If seeds are stored wet, moulds and pathogens may spoil them.
4.5 Seed storage
Seeds, whether bought or collected, must be stored in a proper way until needed. Dry seeds can be safely stored in air-tight polythene bags at room temperature.
When seeds are stored they are normally labelled, given a number and placed in an air-tight bag inside a closed tin. A single tin may contain several bags and a card register system is used to indicate in which tin seeds are stored and how much is left after using a given quantity.
4.6 Seed viability
Some seeds lose their viability in a short period, e.g. Azadirachta indica seeds lose viability in about 6 months. Therefore it is important to test seeds which are stored to determine their germination percentage and it is useless to store any seeds that fall below 40% germination unless they are very rare or very expensive. The viability can be tested by:
Germination test: Filter paper method - where seeds are small, about 100 seeds are germinated in a petri-dish over a filter paper. Silt test - 100 seeds are sown in a container with silt soil.
Tetrazonium chloride test: This is a chemical that imparts colour to living tissue. The seed is cut and the liquid is smeared onto the cut surface to find whether the embryo is alive.
4.7 Number of seeds per unit weight
It is very important to know the number of seeds per gram or kilogram. Because seeds are ordered by weight, unless one knows how many seeds there are per unit weight, one may order too few or too many seeds.
The number of seeds/unit weight for any species is determined by taking about ten random samples of seeds having the same weight, counting the number of each sample and obtaining the mean.
There are many operations involved in seedling production. The most essential ones are described below:
5.1 Nursery soil mixtures
Nursery potting soil should have the following characteristics:
- it must be light;
- it must be cohesive;
- it must have good water retention capacity;
- it must have high organic matter;
- it must be fairly fertile or made so by the addition of 2 kg NPK/M3 of soil.
In the majority of countries with arid conditions, a mixture of one part sand, one part clay, and one part animal manure would be adequate. This is called 1:1:1 mixture. In the Sahel region, the mixture is formed of one part sand, one part manure and two parts soil. If river alluvium is available, it can be used directly.
5.2 Nursery soil treatment
Potting soil must be acidic (i.e. pH6). If it happens to be alkaline, it can be acidified by a solution of 2% sulphuric acid. Sometimes nursery soil has to be sterilized against pathogens by use of a 40% solution of formaldehyde applied as 80 cc per 5 litres of water and applied to the soil 7 to 10 days before sowing the seeds. Soil fumigation is also a treatment against fungi by methyl bromide gas.
5.3 Filling the pots/pot size
Polythene pots of different sizes are now used for raising nursery plants. This does not preclude the use of other containers like boxes, half tins, earth pots, etc (Figure 3.6). The pots are filled with nursery soil, taking care to have no voids by shaking and knocking regularly. The pots are filled, leaving a small space at the top, and stacked side by side on nursery beds.
It is very important to determine the pot size because: large pots require more soil, take a lot of labour to fill and transport; they occupy a large nursery space and require more water in contrast to small pots. But they produce large plants in a short time. The general rule is that "the harsher the planting site, the larger the pot should be".
The quantity of soil needed in a containerized nursery operation is directly related to the size of the containers used. The relationships between the diameter of the containers (ranging from 5 to 15 centimers) and their heights (15, 20 and 25 centimeters) and the soil volume (in square meters) is shown in Figure 3.7. A comparison between the smallest containers (diameter 5 centimeters, height 15 centimeters) and the largest (diameter 15 centimers, height 25 centimeters) is quite eloquent. To fill 100,000 small containers, 28 cubic meters of soil are needed; whereas 442 cubic meters of soil are needed for filling 100,000 of the largest containers (16 times more). Figure 3.6 can be used as a rapid method for estimating the amount of soil needed to fill containers with diameters between 5 and 15 centimeters, and heights ranging from 15 to 25 centimeters.
5.4 Pretreatment of seed
Some tree and shrub seeds are ready for sowing as soon as they are collected; others pass through a dormant stage, during which time the embryo completes its development. Often, a pre-treatment is used to hasten germination or to obtain a more even germination. The methods of pre-treatment vary with the different types of dormancy of tree and shrub seeds. The main types of dormancy are:
- Exogenous dormancy - associated with the properties of the pericarp or the seed coat (mechanical, physical, or chemical).
- Endogenous dormancy - determined by the properties of the embryo or the endosperm (morphological or physiological).
- Combined exogenous and endogenous dormancy.
Figure 3.6 Different kind of containers used for raising nursery stock.
Figure 3.7 Relationship between the diameter of the containers and their heights and the soil volume.
In general, the most frequently encountered type of dormancy in arid zones is exogenous dormancy. Some of the more commonly used methods of attempting to overcome this type of dormancy are described below.
Mechanical treatment - A small number of seeds can be scarified by scratching each seed with sandpaper, by cutting each seed with a knife, or by sandpapering the end of the seed that is opposite the radicle until the cotyledon is seen. With large quantities of seed, mechanical scarification can be achieved by pounding the seeds with sand, or by rubbing the seeds over an abrasive slab. A variety of other methods of scarification are also available.
Soaking in cold water - For a number of tree and shrub species soaking their seeds in cold water for from one to several days is sufficient to ensure germination. The improvement in germination is caused by the softening of the seed coat and the ensuring of adequate water absorption by the living tissues. When long soaking periods are used, it is recommended that the water be changed at intervals. Usually, it is important to sow the seed immediately after soaking without drying, because drying generally reduces the viability of the seed.
Soaking in hot or boiling water - The seeds of many leguminous species have extremely tough outer coats, which can delay germination for months or years after sowing, unless subjected to pre-treatment by immersion in hot or boiling water. The seed is immersed in two to three times its volume of boiling water, and allowed to soak from 1 to 10 minutes, or until the water is cold. The gummy mucilaginous exudations from the seed coat are then washed off by stirring in several lots of clean water.
Acid treatments - Soaking in solutions of acid is frequently used in the case of seeds with hard seed coats. Concentrated sulphuric acid (98 per cent) is the chemical used most generally. Most commonly, soaking times vary from 15 to 30 minutes. After soaking, the seed must be washed immediately in clean water. Tests should be made to determine the optimum period of treatment for each tree or shrub species, and even for different provenances, since overexposure to solutions of acid can easily damage the seed.
Seed inoculation - Legume trees have root nodules which harbor nitrogen-fixing bacteria. When seeds are planted outside their natural range, the soil should be inoculated with crushed nodules from natural stands. Some inoculum are available on the market which can be mixed with the seeds before germination.
Other treatments - For a number of salt bushes and shrubs such as Atriplex, washing seeds in cold water for one to two hours is sufficient to remove salt from the seeds and improve germination.
5.5 Sowing of seeds
Having determined the soil mixture, kind and size of container, one would proceed to sow the seeds.
Type of sowing: When the containers are beds or boxes, seeds can be sown by broadcasting or in lines. When the containers are pots, then it is pit sowing.
Depth of sowing: Seeds are sown at a depth of 1-3 times their diameter. When seeds are sown at this depth adequate moisture and optimum temperature will hasten their germination. Excessively deep sowing will impair seedling emergence. Small seeds like those of Eucalyptus are mixed with fine soil before sowing to facilitate uniform distribution of seeds and to avoid seed waste by dense sowing. To economize in sowing Eucalyptus seeds, the seeds are mixed with fine sand in the ratio of 2 sand: 1 seed. This mixture is placed in a container while a small brush is first dipped in water, then dipped in the sand/seed mixture and then brushed gently onto 4-5 nursery pots containing soil. This was found to give a maximum number of 4-5 seedlings per pot.
Ideal sowing time: This is determined by the period required to raise a plantable seedling of the desired size. For example, if it takes four months in the nursery to raise plantable seedlings of E. microtheca, to be planted in June; then the ideal sowing date for that species and locality is the first of February. Similarly, for planting in October, the ideal sowing date is the first of June.
5.6 Watering plants in the Nursery
After sowing, seed beds should be watered using a fine nozzle spray, producing almost a mist. This will-guard against removing and washing away fine seeds. Hand watering, whether by a container or with a hose, is the best method of watering. Watering is done frequently until seeds germinate.
5.7 Pricking out of seedlings
When seedlings raised in beds and boxes reach the 2-leaf stage, they are carefully picked up using a sharp stick and carefully replanted in pots or other beds. This is a very delicate process which is now avoided by sowing the seeds directly in pots and thinning the excess seedlings leaving only one good seedling per pot.
5.8 Care of Nursery Stock
The production of good quality seedlings will depend on how well the following activities have been executed in its nursery:
Weeding: Weeds compete for water and soil nutrients. They also block the circulation of air and may harbor insects and disease organisms. Where weeds are permitted to grow in the seedbeds, seedlings will be of poor quality; therefore weed competition must be eliminated.
The methods of ensuring a minimum of weeds in the nursery are: prevention, eradication and control.
Prevention is the practical method. It is accomplished by making sure that weeds are not carelessly introduced in the nursery. Eradication is the complete removal of weeds and their seeds from the nursery.
Control is the process of limiting weed dissemination. Eradication and control are generally carried out as one operation in the nursery.
Root pruning: Some of the tree and shrub species best adapted to arid zone environments are characterized by a strong taproot. However, when raised in a container, the development of the taproot becomes constricted; it can emerge from the bottom and will grow into the soil of the bed beneath if it is not cut.
The purpose of root pruning is not only to prevent the development of a long taproot, but to encourage the growth of a fibrous lateral root system in the pot or bed.
Root pruning can be done by drawing a piano wire between the base of the containers and the bed surface so as to cut through the descending roots. Alternatively, it can be done by lifting the pots and snapping off the roots. The timing and frequency of the pruning must be adjusted to the speed with which the roots grow and emerge from the bottom of the containers.
Control of Damping-off: Damping-off is a common and serious disease in many forest nurseries. It can occur either in seed beds or in containers after transplanting. Damping-off is a pre-emergent and seedling disease caused by various fungi. Some of these fungi attack the seed just as germination starts, whereas others infect the newly germinated seedlings. Affected seedlings topple over, as though broken at the ground line, or remain erect and dry up. A watery-appearing constriction of the stem at the ground line is generally visible evidence of the disease. Damping-off is favored by high humidity, damp soil surface, heavy soil, cloudy weather, an excess of shade, a dense stand of seedlings, and alkaline conditions.
One of the best preventive measures for damping-off is to maintain a dry soil surface through cultivation, to reduce the sowing density, and to thin the seedlings to create better aeration at the ground line. The need for soil fumigation is minimized in nurseries where fresh soil mixtures are prepared annually.
Hardening-off: Seedlings continue under nursery care while they develop for 2-3 months. Then the good ones will be selected and placed in separate beds. They are given less water and exposed to the sun gradually to condition them for planting in the site. This hard treatment is called hardening-off. Seedlings will develop a dark green colour and look healthier in the open than under nursery shade.
5.9 Vegetative propagation
Not all trees and shrubs used in planting programmes are produced from seed. Species whose propagation by seed is difficult can often be reproduced by vegetative propagation. Nursery stock that is obtained by vegetative propagation includes stumps, cuttings, and sets.
"Stump" is a term applied to nursery stock of broad-leaved species which has been subjected to drastic pruning of both the roots and the shoot. The top is generally cut back to 2 centimeters and the root to about 22 centimeters. Stump planting is suitable for "taproot-dominated" species. Frequently, stumped plants are used in sand dune stabilization plantations. Stumps are normally covered with wet sacks or layers of large leaves during transit to the planting site.
Cuttings and sets are also commonly used as planting stock. A "cutting" is a short length cut from a young living stem or branch for propagating; a cutting produces a whole plant when planted in the field. A rooted cutting is one that has been rooted in the nursery prior to field planting. "Sets" are long, relatively thin, stem cuttings or whole branches.
5.10 Size and quality of planting stock
There is a considerable range in what is considered the desired size of tree or shrub seedlings for planting. The optimum size varies, depending on whether the seedlings are bare-rooted or containerized, on the tree or shrub species to be planted, and on the characteristics of the planting site.
In general, it is agreed that plants with a well-proportioned root-to-shoot ratio represent good planting stock, but it is difficult to define an "optimum" root-to-shoot ratio. A root-to-shoot ratio based on weight might give a more accurate measure of balance. Stem diameter and height are other criteria for evaluating planting stock that might allow the setting of minimum acceptable limits. Experience indicates that medium-sized stock, between 15 and 40 centimeters, with a woody root collar, have a better survival rate that do smaller plants.
The maximum size for planting potted stock is largely determined by the size of the container. The larger the container, the larger the plant that can be grown in it; but the period of growth is limited to that free of harmful root restriction. Excessively tall plants can be lessened in the ground or blown over, and root development might be restricted or inadequate to cope with the high transpiration demand of a large top.
5.11 Preparation of seedlings for the planting site
Seedlings of plantable size are first graded. The grading of planting stock depends, to a large extent, on local experience and the establishment of local standards. The main objectives of a grading system for planting stock are:
- To eliminate culls, seedlings with damaged or diseased tops or roots.
- To eliminate seedlings below minimum standards of size and root development.
- To segregate the seedlings that exceed the minimum standards into two or more quality classes.
5.12 Transport of seedlings to the planting site
Packing of container-raised plants for transport presents few problems. They are put in trays and loaded into vehicles. The tins which have been used for seedling trays can be used for transporting container plants. Sometimes wooden trays are used, but these are heavy.
Often, plants are damaged during transport to the planting site. Therefore, adequate care must be taken to avoid mishandling of plants during loading and unloading from vehicles. Something that is often forgotten is that plants require protection during transportation, as the air-flow can cause drying. It also is important that the containers are packed tightly, so that they cannot move. Special shelves for stacking pots or trays can be added to the vehicle platform (each layer of trays being placed on a shelf, with one shelf about 50 centimeters above the other). When possible, plants should be transported in the planting season on cool, cloudy, or even rainy days to prevent desiccation during transport.
Shipping schedules should be planned to avoid delays and to allow proper disposition of the plants immediately upon arrival. Normally, plants should arrive one day ahead of planting; where shade and watering facilities are available, supplies can be brought several days in advance. As soon as the plants arrive at the planting site, they must be watered and, if necessary, heeled-in in a cool, moist, shaded place until they are needed for planting.
5.13 Organization of seedling production
Seedling production must be organized in such a way that plantable seedlings of good quality are produced in time. As time of planting is critical in arid countries - except when irrigation is applied - the organization becomes very important. All the processes which have been described earlier must be done perfectly and in time. These include a) seeds and their treatment; b) soil mixture; c) filling of pots; d) sowing; e) watering; f) pricking out; g) weeding; h) root pruning; i) provision of shade and shelter; j) cutting; k) hardening off; and l) transport to the planting site.
Only the number which can be planted in one day should be removed from the nursery to the site. According to the planting programme seedlings are hardened off and transported. The number of plants raised originally in the nursery is about 20% more than that planted in the field. This is to make up for culling and a reserve for replacing dead plants.
Administration is also very important in nursery work to ensure that:
a) nursery activities (jobs) are done correctly;
b) these activities are done in time;
c) labour requirement is available (man-days) for performing the work; and
d) materials/tools and equipment required to do the work are suitable.
This requires a nurseryman having a fair knowledge of labour productivity, nursery technique and prices of materials. Records of nursery seedling production as well as costs of materials and labour are kept to show the economics of nursery work.
Labour and material requirements depend on the size of the nursery.
Forms showing cost of tasks, e.g. seed collection, filling of pots with soil, sieving, mixing and preparing nursery soil, should be designed and filled in regularly.
