Now that you have planted your seeds in the nursery, let us focus on the nursery operations that are often needed when the seedling is growing. Many people assume that seedlings would only need watering at this point. They are right, to some extent. But, for most tree species in the tropics (especially in sub-Saharan Africa), other operations such as pricking out, root pruning, and hardening off would be necessary.

Pricking Out: one of the most traditional nursery operations in forestry 

Pricking out refers to a nursery operation where seedlings are transferred from the seedbed to the containers. The correct time to do pricking out is when the seedlings start to show true leaves. You should know that the first leaves from seed are the germination leaves. True leaves develop after the germination leaves.

The pricking out process involves:

• Before starting to prick out seedlings from the seedbed, you should ensure that your pots in the nursery bed are ready for seedlings.
• Watering the seedbed and containers with enough water before starting
• Prepare shade to reduce the exposure of the seedlings to direct sunlight.
• Fill a container with water.
• Use a dibble or pencil-sized stick to loosen the soil.
• Gently pull out the seedlings and place them in the container with water. The roots shouldn’t be exposed to the sun as they may dry out
• Do not take very many seedlings from the seedbed at once.

Now it is time to plant the seedlings to the nursery bed.

• Make a hole at the center of the planting pot using a stick or dibble.
• Cut the roots if they are too long for the pot.
• Avoid holding the stem as they are too feeble and tender.
• Ensure that the air pockets around the roots are closed by pushing the soil towards the seedling
• Water the pots well after completing the planting process. It is a good idea to arrange seedlings pricked out at the same time in the same nursery bed.

The use of seedbeds to establish seedlings is becoming an outdated nursery operation in forestry. Modern foresters recommend nursery operations that involve planting seeds directly to pots after pretreatment. However, seedbeds are still useful for some species and circumstances. For instance, species with very low germination rates are still best raised under a seedbed. Additionally, one can plant seeds in a seedbed in case he/she has the seeds, but the soil and or pots are not ready yet.

Pricking out is mainly done when an individual wants to transfer seedlings from a seedbed to the nursery bed or pots. But even for seedlings raised in pots, the pricking out process is necessary when you want to transfer seedlings from pots that have two or more seedlings to those that have none. 

Shading as a tree nursery operation 

The whole point of a tree nursery is protecting seedlings from extreme environmental influences until they can better withstand the conditions. Germinating and recently transplanted seedlings need some protection from the hot sun (especially in the tropics), heavy rain, and hard frost (in the temperate regions). Some form of shading can assist in protecting the seedlings from the environmental factors. Often, shading would help in reducing the losses through the leaves (transpiration), and it also reduces the temperatures of the plant and soil.

You should remember that plants require sunlight for good growth. As such, the shading needs to be regulated. Indeed, we do not recommend 100% shading at all! A poor, but unfortunately common, strategy in a nursery is to maintain the plants under shade for the entire nursery period. Plants with too much shade have the following characteristics:

• They are stunted or will grow slowly. Tall and skinny stems are among the common indicators of seedlings in too much shade.
• They are dark green or will be yellowish (which is more common)
• They would be inevitably susceptible to disease and insect attack.

Silvica recommends that 40-50% shading is sufficient for most species. Also, as the plant ages, the shading is reduced. The hardening-off section of this article describes the process in detail. Shade reduction should be a gradual process. The sudden removal of the shade may cause burning of the seedlings. You see, plants put under too much shade will adapt to the dark condition. Sudden exposure to sunlight makes it difficult for the chemical processes in the leaves to adapt fast. 

Nursery managers prefer natural shade for obvious reasons. They are cheap and mimic the natural conditions for the tree seedling. Regardless, nursery owners should ensure the trees do not provide too much shade. The shade trees should also allow sunlight to penetrate. An ideal shade tree would retain its leaves during the dry season and lose them during the rainy season. A tree that pollards well is also advantageous as it can be pollarded during the wet seasons. Shading using natural trees is a disadvantage for bare-rooted seedlings as the trees would directly compete for nutrients with the shade trees.

Construct a shade to protect the seedlings from direct sunlight for two to three weeks after pricking out.

You can use locally available materials such as grass or shade nets for shade construction.

Watering- an apparent nursery operation

Did you know that 90% of most plants is water? It goes without saying, the watering of seedlings is necessary nursery operation for the excellent growth of seedlings in a nursery. When grown in containers, nursery plants have only a limited volume of the substrate and do not have the ability of mature trees to search for water from below the soil surface. The amount of water seedlings require depends on:-

• Seedling age. More water is needed after germination when the seedling is young and at pricking out, but this requirement reduces as the seedling grows in age. The amount of water should reduce at the time when the seedling is nearing the planting stage. Like in shading, the reduction of water is part of hardening off (read about it in the later sections of the blog)
• Amount of sunlight. The sun causes the leaves to lose water through the transpiration process. As such, seedlings in sunny areas lose more water compared to the ones grown in shady areas. But, do not raise your seedlings in overly shaded areas to avoid water loss. Remember, sunlight is also a requirement for raising healthy seedlings. After all, plants need sunlight to make food in the process of photosynthesis.
• Soil type. A sandy soil loses water faster than soil with high clay content hence needs more frequent watering. The disadvantage of clay soil is that it becomes hard cracks when dry. Ensure that the soil used in the nursery is collected and mixed correctly. Watering the seedlings should be directed at the earth and not the leaves. A watering can or a hosepipe with a nozzle should be used to ensure uniform distribution water, and one should water the whole bed and not just the plants in the center of the bed. Also, you should ensure that water gets to the bottom of the container.

Too much water can damage the plants just as much as not enough water because of waterlogging, which makes it difficult for the roots to ‘breath.’

Ideally, watering should be done in the morning and evening when the sun is not intense. If this is not possible, then water once a day in the evening.

You can water the seedlings once or not all during rainy seasons. Just to reiterate, you should avoid under-watering and overwatering. Use an adequate amount of water, e.g., 20 liters for 1,000 seedlings.

Weeding

Weeds are a threat to healthy seedlings development as they compete with seedlings for nutrients, water, and light. Hence they must be controlled. As one of the nursery operations, tree nursery managers sometimes neglect to weed. They assume that weeds do not affect the growth of tree seedling. A good nursery operation ensures that seedlings are free from weeds.

Process

With your hands or a dibble, gently pull out unwanted growth (rouging) where weeds are observed.

Remove all the weeds around the beds with a jembe ( garden hoe).  Do not leave any rubbish around unless you want to turn it into compost.

Application of additional fertilizers (Nutrition)

Fertilizers or manure is applied when the seedlings show a sign of weakness. The most common fertilizers are NPK and DAP.

It’s important to note that you can also use manure from livestock. However, pest and diseases can also cause weakness in seedlings (not nutrients alone). The application of fertilizer is, however, not very common in most tree nursery operations because it is expensive. Also, most nurseries want to be eco-friendly, and inorganic fertilizers have several ecological issues. 

Root pruning tree seedlings

Root pruning is the tree nursery operation that involves cutting the roots of seedlings that have overgrown the containers or pots in the nursery bed. Many people I train have a concern about root pruning, but researchers have established that it is one of the beneficial nursery operations.

Why root prune? When seedlings have reached a certain size, and their roots become longer than the depth of the pots. The problem is that when the roots are left to grow, they will develop root systems in the nursery. You see, if a nursery foreman/manager allows this, it would be hard to move the seedlings when the time for planting outcome. Moreover, if the roots are cut when the seedling is old, the procedure is likely to result in a weakened plant. As such, periodic root pruning is recommended to avoid the problems associated with seedlings developing roots beyond their beds’ height. The period and interval of root pruning tree seedlings depend on different species and other conditions. Nevertheless, root pruning is done when seedlings’ roots have started to penetrate under the surface. 

Procedure

• Water the seedlings properly before root pruning.
• Cut the long roots underneath the container using a sharp knife, scissors or available cutting material.
• Water the seedlings well after root pruning. This watering helps the plant withstand moisture stress.
• Some nursery managers choose to place the seedlings on stones or polythene sheets to reduce the need for root pruning.
• Seedings on a raised bed can be pruned by using a sharp object underneath the bed after watering.

Hardening off

The process of hardening-off aims at acclimatizing tree seedlings to the harsh conditions of the field. The nursery operation is vital for the survival of tree seedling during planting out. While in the nursery, the seedlings are pampered as they are protected against sunlight, are watered twice a day, etc. The life in the field for a tree seedling is not as rosy. Often, seedlings have to encounter challenges ranging from excess sunlight to wildlife to livestock to human interference.

Although nursery managers cannot extensively prepare seedlings for the challenges in the field, they can gradually prepare the seedlings to the harsh conditions in the field using the hardening off nursery operations technique.

Hardening up process:

• The shade should be removed for some period before the seedling is planted out.
• Reduction in watering intensity (quantity) and frequency e.g., water twice a week and later once a week
• Before planting out, root pruning should be carried out frequently or re-arrangement of pots to allow more adoption to stress.
• Good preparation for out planting results in good field survival, therefore hardening off should be done 2 – 4 weeks before out planting time.

Postponing planting

Sometimes, it is not possible to plant out tree seedlings when they reach the right size (about 1.5 feet or 50 cm) because of various reasons. For the case of a nursery selling tree seedlings, the tree species may not have been bought. Some foresters recommend that the postponing planting nursery operations procedure is necessary. It involves cutting the tips of the plant to avoid overgrowing. However, cutting the tip for some species causes stunted growth or slow vertical development. Ideally, you should plan to avoid the situation where seedlings overgrow in your nursery.

Other nursery operations in tree nursery management include the ones that deal with establishing a nursery, soil collection and mixing, and seed collection and pretreatments. 

Seed Collection, Handling and storage

Like many crops, trees are grown from seed. As such, tree seed collection and handling are critical stages in tree growing and nursery establishment. The seeds must be collected from the right source and be of good quality. However, many people tend to collect seeds which are easy to collect and less attention is given to the quality of the seeds. A very wrong strategy!

What is a seed?

Seeds usually occur in fruits and they are capable of developing into another plant. There are different types of seeds including:

1. Follicle: A dry fruit that splits along one edge e.g Grevillea (Grevillea robusta is a common exotic tree in East Africa, most examples in this post will include cases and species in East Africa, particularly Kenya).
2. Pod: A dry fruit that splits along two edges e.g. Acacia species
3. Samara: A dry fruit with wings e.g. Combretum molle
4. Nut:g. podocarpus
5. Berry: A fleshy fruit with several seeds e.g. Warbugia ugandensis (East African greenheart)
6. Drupe: Fleshy fruit with one endocarp surrounding the seed e.g. Africa olives
7. Cone: Dry fruit with woody scales e.g. cypress species

Mother/ parent trees

Make all attempts to use good quality seeds for your nursery. Good quality seeds can be bought from certified suppliers or they can be collected. If you decide to collect the seeds, ensure that you identify good mother/parent trees. These are the trees which have good quality seeds and are used continuously to provide tree seeds. A mother tree for good seed collection has the following characteristics:

• Healthy, vigorous and well-formed
• Near the maturity stage
• Are good producers of the desired product
• Grows in areas surrounded by a healthy stand of the same species

Very young and very old trees should not be selected because they will not produce viable seeds

Before collecting seeds it is recommended that one does a flower and fruit survey. Basically, the survey means checking out the stages of the tree in terms of flower and fruit production.

The seeds can be collected from the trees in two ways:

• Collection on the ground: this is where one collects fallen seeds. One can use plastic sheets of paper to collect clean seeds.
• Collection from the crown:

After seed collection, handling process follows. The first handling activity is extracting the seeds. Pulpy fruits such as drupe and berries might require some soaking while dry fruits such as pods and follicle might require some drying and splitting.

After extracting seeds from fruits, it is time to store them, but not all seeds can store well as they have different behaviours. The different seed behaviour includes:

1. Recalcitrant seeds: these are the types of seeds that cannot store for a long time without losing viability e.g. East African Green Heart. Once they are collected and extracted, they should be planted immediately.
2. Orthodox seeds: these are seeds that can store for a very long time without losing their viability e.g. Acacia seeds can store for up to ten years
3. Intermediate seeds are between orthodox and recalcitrant

Pre-treatment of seeds

In natural conditions, seeds of many tree species take a long time to germinate or have very low germination percentage. Tree seeds are often in the dormant stage while they await the occurrence of favourable conditions. Pre-treatment of seeds is undertaken to speed up germination time and get good germination percentages. There are various pre-treatment methods including soaking, nicking, and cracking.

Soaking seed in hot water

Soak the seed in hot water until the seeds look swollen. This is used for seeds of trees such as Calliandra, most Acacias, and Albizia.

Procedure: It is a simple procedure that involves boiling water and placing seeds inside the water. The seeds remain until they are swollen (In most cases, the seeds will be soaked overnight).

Soaking seed in cold/cool water

This method is recommended for seeds that have soft seed coats such as Sesbania, Tephrosia, Dalbergia

species, Gmelina, Gliricidia, etc. Soaking in cold water usually take 24 hours, but some species could need 2 hours.

Procedure:

• Soak the seeds in cold water which is 2 times its volume
• Remove all floating seeds
• Sow the remaining seeds at the bottom in containers in the nursery or direct sow directly in the field after

Cracking the seed shell method

This method is used for tree species with a hard coat like Melia, Podo Species, Croton megalocarpus, etc. The idea for cracking is for water to penetrate through the hard coat for easier germination. Cracking is done using a sharp knife, a stone or a cracking machine

Nicking

This is by cutting the seed coat to enable moisture to pass through to the seed. It can be done by a razor blade, sandpaper, knife or fine pliers

The point of nicking is at the place furthest from where the seed was attached to the fruit/ pod. You should be careful that you may not damage the inside of the seed.

Seeds with hard seed coat such as Acacias can be pretreated in this way

No pretreatment

Seeds such as Croton megalocarpus, Neem, Cassia spp. and Kei apple can be sown directly into the container. Fine and light seeds such as Eucalyptus, Casuarina are sown in transplant beds and later pricked out into containers. It is important that fine seed is mixed with sand and uniformly broadcast on the seedbed to avoid overcrowding that can lead to damping off (a disease in the nursery).

Planting the seeds

After applying the correct pretreatment methods, it is time to sow the seeds. The seeds can be planted into a seedbed or directly into pots. Do you know what a seedbed is?

When sowing seeds, do not sow the seed too deep in the soil; the depth of holes should depend on the size of the seeds (usually 5 mm to 1 cm). Sowing too deep is likely to prolong seed germination period or the seeds may rot. As a rough guide, cover the seed with the soil equivalent to the size of the seed.  

Cover the pots or the seedbed after sowing with dry grass and if possible make a shade.

Ideally, the sown seeds should be watered twice daily (in the morning and evening). If this is not possible then water in the evening only since most of the water at this time is taken up by the plant because there is very little evaporation at night.

You need to plan the sowing time so that the seedlings could be ready for planting out at the right time.  is important the seeds are sown in time to enable the seedlings to attain the recommended size (at least 30-45 cm in height). This should be attained before the onset of rains (April or November for most parts in Kenya).

For the propagation of many tree species, there are specific procedures for seed collection, handling, storage, and pretreatment. The availability of the information is common on many forestry websites and local organizations. Make sure you understand the seed management of the species you choose to raise in your nursery.