- introduction: favourable seed propagation environments
What happens when a seed germinates?
Is germination one of the critical stages in the life of a tree?
Yes, it is (C 40 in Manual 3), since it involves:
Under natural conditions, there are so many hazards that most viable seeds (B 13) never become seedlings (B 4). Even in a nursery, it is still quite easy for damage and losses to occur (B 47).
Can these be avoided?
Yes, many of them can (B 3, B 50), with favourable propagation conditions and careful checking. It is a great pity to lose a lot of plants at this stage, particularly when:
What sort of environment is best for germinating seeds?
For seeds to germinate and grow well, their propagation conditions need to be favourable in respect of temperature, light, moisture and aeration.
Generally, they also need to be sheltered from wind and heavy rainfall (B 41), and sometimes they may require extra protection from diseases and pests (B 47).
Does the temperature really matter much in the tropics?
Average temperatures in the tropics normally permit germination to occur without supplementary heating, unlike the situation in cooler parts of the world.
However, temperature is still an important factor because:
But what can I do to alter temperature?
How important is light at this early stage?
The germination and early growth of seedlings depend on substances stored in the seed. Only a little light is needed while there are no expanded leaves for photosynthesis.
Do they need any light at all?
A few species with light-sensitive seeds (B 13) will generally not germinate unless some light reaches the embryo, penetrating through the covering materials (B 43) and the seed coat.
All other kinds of trees will germinate in the dark, but do need some light as soon as they emerge, or the seedlings will become elongated and weak (C 41 in Manual 3).
So how much shading is usually needed?
For shade-bearing species (D 10 in Manual 4), moderately heavy shade at first (B 41); and
For light-demanding trees, moderate shade at first.
Then, when many seedlings in the batch are beginning to expand foliage leaves, start to reduce the shade slightly.
Small seeds contain less reserves than larger ones, and may need a little less shade.
Isn't it easy to provide seeds with moisture and air?
No, it is quite a skilled job to provide enough of both of these essentials, but not too much!
The main keys to success are:
How does the germination medium strike the right balance?
Because it is a special soil mix (B 42), like a very good potting mix (C 6 in Manual 3), which:
Why do seeds have to be watered very carefully?
too much water can knock them down or wash them away, waterlog them or make them liable to damping-off or other germination diseases (B 47); but
too little moisture can dry them up before the seedling root system has become established (B 3).
Where should seeds be germinated?
In seed trays, seed beds, soil-blocks or containers (Manual 3).
Which is the best?
They each have advantages and disadvantages:
Seed trays can easily be moved around to protect the young seedlings, to reduce the amount of shade, and also when the seedlings are to be potted up or transplanted (B 45)
Seed beds may be more convenient when sowing large numbers of seeds, and usually need less frequent watering;
Soil-blocks with regular root pruning can produce planting stock with very good root systems;
Containers can be useful for large seeds, and may avoid the need to pot up the young seedlings.
And what are their disadvantages?
Seed trays can more easily dry out, and may lead to bent or tangled root systems if they are
shallow, or when potting up is delayed;
Seed beds are harder to protect, and could be more liable to the rapid spread of invasive weeds, a disease or a pest;
Soil-blocks could cause wilting or uprooting if the seeds are not sown in a regular pattern, or the root pruning is done carelessly or infrequently;
Containers can be wasteful of nursery space if the germination percent is low, and may fall over and dry out.
When should seeds be sown?
For moist, non-storing seeds: sow as soon as they have been cleaned (B 32).
For dry, storable seeds: dry and keep them in good storage conditions (B 33) until a date when they will produce planting stock of the right size (B 43; and C 34 in Manual 3).
So I shouldn't just sow seeds at the start of the rainy season?
Generally no; because this is usually the time to plant seedlings out. Sowing dates often
vary between 2 and 6 months before planting dates, depending how whether the seeds are
slow or quick germinators, and how rapidly the young nursery trees grow to a suitable size;
Occasionally yes; for:
What else might affect success with germination?
In many other tree species, mycorrhizal inoculations can be done either in the germination medium (B 42) or at the stage that the very young seedlings are potted up or transplanted into beds (B 45).
- shelter and shading for germination
Why is a very sheltered place important for germinating seeds?
Because shelter will tend to reduce:
What can I do if the nursery site is rather exposed?
For research plants, you might build a greenhouse or shadehouse (C 48), or germinate seeds in controlled environment chambers, if available.
Are there some simpler ways of providing more shelter?
Note: do not use this method for light-requiring seeds (B 13, B 34, B 40).
What about shade for the germination environments?
The aim is to:
Which kinds of shading are best?
Many different materials are suitable, including palm leaves, coarse grasses, bamboo slats, matting and plastic shadecloth (A 24 in Manual 1). The ordinary type of nursery shade can generally be used, but it should be particularly well made.
There is some evidence that the light quality (D 11 in Manual 4) under green foliage is less suitable for seedling growth than when the leaves in the shading are dead.
How much shade is needed?
In general, use a heavier shade than for established nursery seedlings, because it is easier to
correct for too much shade than for too little.
Light-demanding species need slightly less shade than those that are shade-tolerant.
When should I start reducing the amount of shade?
This depends a great deal on the species, the season and how heavy the original shading was. The best time is generally 1–2 weeks after the seedlings start coming through, when the first leaves (or sometimes the green seed leaves) have expanded, and the young shoots are beginning to extend. The timing might need to be slightly:
earlier for light-demanding species and those with small seeds and small first leaves,
since they do not have a lot of stored materials; but
later for shade-bearing species and those with large seeds and first leaves, because their greater reserves and leaf area allow more flexibility, with no need to risk reducing the shading too quickly.
How much should I reduce it?
It is usually better to decrease the shading in frequent small steps, not all at once, to a the level normal for older nursery plants of the species at that season.
Are there ways of making changing the shading easier?
Is there anything else that might help?
Aren't there some kinds of seeds that don't need any shade?
Yes. For instance, colonising species might germinate perfectly well in the open. However, when their roots are disturbed for potting up or transplanting into a bed (B 45), they might need a good deal of temporary shade, especially if their shoots are already quite large.
What other shading problems might I have?
- the seed germination medium
Why shouldn't I sow the seeds in ordinary nursery soil?
Because they will usually germinate and thrive much better if you sow them in a specially prepared germination medium.
What are the chief features of a good medium?
One that provides:
What should the germination medium be made of?
A mixture of different components. There generally needs to be some:
coarse washed sand (to encourage aeration and free drainage); and sieved organic matter (for water retention, soil texture and some nutrients).
Many common materials can be used, mixed with sand in various proportions, such as:
Could I use an ordinary potting mix?
Good seed germination media are broadly similar to those for potting up older seedlings (C 6), but except for large seeds it is usually best to make a finer mix by:
How can I find out the best mixture?
Should the germination medium be rich in nutrients?
No; it should generally be less rich than a potting mixture, because:
Could I make it easier for those associations to form?
Yes, by including in the germination medium small amounts (say 2–10%) of inoculum of:
Where would I get the inoculum?
Won't adding those things make the seeds more likely to rot?
Definitely not, when pure cultures are used;
Generally no, because these are beneficial micro-organisms; but
Occasionally yes, if others that cause disease are also present (B 47).
Is it also possible to inoculate the seedlings later on?
Yes, small quantities of the relevant inoculum can be well mixed either into:
A few species like the ectomycorrhizal pines need the inoculum in the germination medium, or they grow very poorly.
What can seed trays be made of:
These are often made up from pieces of thin wood. Plastic seed trays of various kinds are also available (B 52), or they can be cut out of used containers and holes punched in them.
How big should they be?
The depth is the key measurement to choose, because:
If they are too shallow, they may restrict the roots and dry out rapidly; but
When they are too deep, trays may be heavy to carry and tend to break soon.
So what size might be convenient?
Depth: about 10–12 cm (minimum 6 cm; maximum 15 cm);
Length: around 40–50 cm; and
Width: about 30–40 cm.
Before making or ordering seed trays, think about choosing a size that will fit well on nursery beds, and on trolleys or benching if they are used.
What about sowing in nursery beds?
Prepare new seed beds of a size that is convenient to the nursery layout (C 22), making
sure that they will have good drainage, and then add the germination medium as a layer 5–10
cm thick on the top.
Renew existing beds by removing weeds (C 44), levelling and adding some new medium.
Are there some other things to watch out for?
- sowing and covering seeds
When should I sow the seeds?
For seeds that can be stored (B 33): about 2–6 months before they will be planted, so that by
then they will have germinated and reached a suitable size; but
For seeds that cannot be stored: as soon as they have been collected and cleaned (B 31–32).
What needs doing beforehand?
Before actually sowing the seeds:
How should I start, if I am sowing in seed trays?
Put the germination medium into the trays, and then firm it down gently with a flat board, so that the surface is smooth and even, and comes to around these distances below the top:
1 cm - for seeds less than 2 mm diameter;
1.5 cm - for seeds of 2–5 mm diameter;
2 cm - for seeds more than 5 mm across.
And what about seed beds?
Cultivate the surface of the bed, put 5–10 cm of germination medium on the top, rake it level and firm it down, but not too hard.
How should the seeds themselves be sown?
Smaller seeds: scatter them thinly and evenly over the surface of the germination medium. With practice, one can avoid having many seeds touching each other, which could:
Seeds large enough to pick up: it may be easier to sow these individually, spaced out:
What spacing might be suitable for these larger seeds?
No closer than 1 cm to each other, and often 5–25 cm apart, to allow each plant enough room. See C 63 in Manual 3 for calculations about the space needed.
Why do seeds need covering?
What sorts of covering would not be suitable?
Very fine soil particles, which could impede free drainage, holding water near the surface,
which encourages damping-off disease (B 47); or
Large particles or lumps of earth, which might restrict the emergence of the young shoots.
Which kind of covering is best?
Although this can vary from one species to another, one can often use:
And how should it be put on?
Smaller seeds: sieve the covering material over them until they are covered uniformly. The covering should come up to about 7–8 mm below the top edge of seed trays when it has been gently firmed down.
Larger seeds: these can either be:
How thick a covering would that be?
Here is a rough guide, which you may need to adapt for particular species:
Seeds less than 2 mm diameter: just enough to cover them;
Seeds 2–5 mm across: cover with about 4 mm; and
Seeds bigger than 5 mm: use about the same depth as the seed diameter.
What about seeds that are light-requiring?
In such cases (B 13, B 34), the covering should not be too thick, and should preferably be of a light colour, allowing light to penetrate further.
How about watering seeds?
Method 1: Do NOT water now if the medium is moist enough, and the seeds already
contain enough water for germination, either naturally (B 13) or from soaking (B 34).
Method 2: Water well with fine droplets (B 46) if the seeds are dry and/or the germination medium is not moist enough.
Afterwards, check regularly (B 47), and water sparingly as needed (B 46). With seed trays, you might consider restricting their drying by covering them loosely with clear or white polythene sheeting until the seedlings emerge.
Are there other ways of covering seeds?
Some growers cover seed trays with thick newspapers or black polythene and don't water again until the seeds come through. Two problems that can happen with this technique are:
What other difficulties might I have?
These and other problems are covered more fully in sheets B 3, B 50 and B 52.
And what are the signs of good germination?
Plenty of well-spaced, healthy seedlings emerging.
Seedlings often come through with the shoot bent over like a hook, which in the light soon straightens out.
How about the young leaves?
The seed leaves may remain below ground, or emerge and turn green. Often they contain
The first foliage leaves usually look different from those produced later on, so make sure that an unfamiliar species can be distinguished from a weed.
- using wildings and already germinating seeds
What are wildings?
Seedlings that are collected from:
Where might I find the droppings?
Some birds and climbing mammals have perches under which wildings might be found. Bats may pass droppings while on the wing, or at their day-time roosts in caves or hollow trees.
How about seeds that have already germinated?
Many wildings have to be dug out of the ground, but sometimes they can just be picked up, because:
|Collecting wildings and soil.|
What are the advantages of using them?
And how about their disadvantages?
How should they be handled?
Put them in moist polythene bags within boxes to minimise crushing, and keep them out of the sun and wind. Transport them as soon as possible, but without bumping them about.
Where should they be taken?
Usually to the nursery, where they can be:
Sometimes directly to the planting site, where wildings might establish successfully if:
Which species can be planted directly?
Trials at Kavwaya in Congo showed for instance that it was fairly straightforward to plant wildings of Gaetnera paniculata, Millettia laurentii and Ricinodendron heudelottii directly into farmers' fields after cassava and groundnuts had been planted.
How big can wildings be, to stand up to being moved?
Ideally, they need to be less than about 50 cm tall. However, if most of the wildings available are larger, and the root systems have been considerably disturbed, you might try:
- potting up and transplanting young seedlings
Is there anything special about potting up very young seedlings?
The general principles are the same whenever nursery root systems are disturbed (C 42 in Manual 3), but extra care is needed when the plants are very small (B 40; C 40).
Why should they be disturbed so young, anyway?
Quite often they do need to be moved, because the small seedlings are:
Sometimes digging them up can be avoided, since the young seedlings:
But isn't the best kind of root system one with a strong taproot?
No, only for the initial stage of germination, when it makes the vital link between seed and
After that, taproots are important in just a few tree species, and for direct sowing (B 4).
In general, long seedling roots need to be discouraged in nursery plants, so as to produce a bushy, compact root system (C 34, C 47) that can be successfully planted (Manual 5).
How can the long roots be checked?
When should seedlings be potted up?
The best time is usually at a fairly early stage, when:
Do this work early in the morning, or in the late afternoon.
How should it be done?
What differences are there when transplanting into a nursery bed?
When should I water?
Firstly, the potting medium should be moist or the transplant bed pre-watered if necessary.
Then (C 46), there are three alternatives:
Are there some general guide-lines?
- watering seeds and very young seedlings
Is there anything special about watering seeds and very young seedlings?
Yes, there are several important points, in addition to those described for watering older
plants in containers and nursery beds (C 43 in Manual 3).
During germination, the new seedlings:
What does this add up to in practical terms?
That it is very easy to give too much water, or too little.
So how can I avoid those extremes?
Dry seeds (B 12, B 33): by making sure that they can take up enough water to germinate (B
34), but do not then remain in waterlogged conditions; and
Moist seeds: by washing them free from remains of the fleshy fruits (B 32), but neither leaving them to soak for too long, nor dry out.
And then by using a good germination medium?
Yes, that's right - one that provides enough aeration as well as moisture (B 42–43). In addition, adequate shading (B 41) reduces the need for frequent watering.
Do I need to water seeds straight after sowing?
Choose Method 1 or 2 (B 43), depending on the circumstances.
Should I water the seeds again before they come up?
Generally no; as there will be little evaporation from them, and so the germination medium
may remain moist, except perhaps on the surface; but
Sometimes yes; if wind or high temperatures are drying out the medium more rapidly.
What about watering when the seedlings are just coming through?
This is the most difficult stage to judge correctly. Until the medium is beginning to get a little dry, it is usually best to avoid watering, because:
How can I check whether the medium is starting to get a bit dry?
On the other hand, avoid waiting until seedlings are beginning to wilt.
When watering is needed, how should it be done?
Start and finish the watering to one side of the seed bed or tray, allowing only a steady stream of droplets to fall on the germinating seeds.
What if I do not have access to a watering can?
You could make a lot of small, regularly spaced holes in the bottom of a tin, and then dip this
repeatedly into a bucket, using it to sprinkle water evenly.
Another way is to stand the trays briefly in shallow water, so that water enters from below, and then let the excess drain off (C 43).
And what should I do as the shoots expand?
The frequency of watering, and the amounts given, both need to increase as the leaf area (C 12) becomes larger.
What about when the root systems are disturbed?
The time when young seedlings are first potted or transplanted into beds (B 45) is a second key stage in their development. Stress can be minimised by:
But supposing some of them do start to wilt?
Take action as soon as you see this beginning to happen, or a lot of your trees might be damaged or die. You could for instance:
Note: do NOT water them if the soil is still moist! The aim is to reduce water loss from the shoots until the uptake of water by the roots can catch up (C 13 in Manual 3).
Does it matter how clean the water is?
Yes it does, if the water contains a lot of nutrients, dissolved pollutants that are toxic to plants (C 24), or resting stages of micro-organisms causing disease (B 47); but
No, it may not, if it is mixed with fine sediments, since these can usually be removed by:
What could that be made of?
You could for example put a layer of cloth or newspapers into a sieve, cover that with a thick layer of fine sand, and set it on a bucket. Pour on water a little at a time - the first that comes through may be a little dirty, but then it should become much cleaner.
Are there other problems I might run into?
See sheets B 3 and B 50 for this.
- checking and protecting very young seedlings
What sort of checks are needed during germination?
Those described in sheets C 40 and C 66 in Manual 3, but done more frequently, so that:
‘daily’ checks might occur 2–3 times each day; and
‘weekly’ checks could happen every two or three days.
Isn't it a waste of time to do it so often?
What do I need to look for?
Any signs of poor growth or unusual colour, or seedlings falling over.
Any problem with the shelter, shading or watering.
For further details, see sheets B 3 and B 50.
Which kinds of protection do very young trees need?
How can I keep off pests?
By following the advice in sheets B 30 and C 45, paying particular attention to:
What kind of diseases cause most problems?
The commonest attack is from damping-off disease. The symptoms of this are:
Is there anything I can do if my seedlings are attacked?
It is sometimes possible to save important seedlings by cutting above the infected part and rooting the top of the shoot as a leafy cutting (B 2; A 4, A 40 in Manual 1).
Can I prevent damping-off happening and spreading?
As well as considering points (B 1–3), you could:
Are there other diseases that might give problems?
See sheet B 24 and B 30–34 for avoiding or reducing problems with seeds before sowing, and C 40 and C 45 in Manual 3 for the protection of older seedlings.
Anyway, what does a thriving young seedling look like?
Some features are easy to see, such as the:
But other things may be hidden, such as the:
What else is important with a larger nursery?
- germination tests and seed experiments
Why should one bother about doing germination tests?
Often they are not needed; but
Sometimes they can be very useful, for instance:
But won't the tests themselves often be a waste of seeds and time?
There are certainly many occasions when germination tests are not appropriate, for example:
Can't I get a rough idea by just sowing the seeds and seeing what happens?
Yes; sometimes that's the best thing to do; but
The drawbacks are that:
if they come up, the spacing of the young seedlings may be unsuitable;
if they don't, you may not know whether the seeds were empty, dead or dormant (B 13), or if perhaps the seed propagation conditions were unfavourable (B 40–43).
How does one set up a proper germination test?
There are several kinds of test (B 33), including:
What are the most important things about seed testing?
How many seeds would be needed altogether?
With a large seed-lot, one could for instance use 10 replicates with 100 seeds in each, making
1000 seeds in all.
With a smaller seed-lot, one might perhaps test 5 replicates of 30 seeds, using 150 seeds.
What sort of controlled environments are needed for more accurate seed testing?
Don't the seeds dry up easily during the test?
Small seeds can be sown on filter paper in petri dishes with lids, using just enough water for
the papers to remain moist after dry seeds have absorbed water (B 34).
Larger seeds can be sown in shallow containers and put inside polythene bags.
If watering is needed, it should be done with water that has been kept at the same temperatures.
How long should a germination test continue?
Ideally until several weeks after the last seeds germinated. Sometimes, however, it may be more convenient to fix a shorter period, and then perhaps cut the non-germinating seeds across to assess whether they were viable or not.
What about the tetrazolium test?
This is an alternative to germination testing, which is based on a colour reaction on the surface of the cut seed. The chemical name of the substance used is 2,3,5- Triphenyltetrazolium chloride (xxxx in B 52). Fully viable embryos usually stain red, doubtful seeds pinkish red and dead seeds a paler pink. (Is this right?) The number of empty seeds can also be counted with this technique.
Supposing I wanted to do an experiment about seed germination?
In that case there would be two or more treatments applied, each with say 150 to 1000 seeds divided into 5–10 replicates.
What sort of treatments might be relevant?
Many different kinds, for example involving:
Another kind of experiment might compare different methods of testing seed germination.
Can you give some guide-lines?
Are there other kinds of experiments using seeds?
Yes, there are several kinds, such as:
Does ‘confounded’ mean ‘mixed up with’?
Yes, that's right - so that different causes cannot be separated. There are a number of reasons why it is easy to draw conclusions that are not valid from experiments (C 67-B (2), and C 67-O in Manual 4).
Apparent ‘treatment’ effects could also arise for example from:
So how can one avoid that kind of thing happening?
Partly by keeping the growing conditions as uniform as possible (C 48 in Manual 3);
Partly by ‘grading’ (B 45 in Manual 1) and randomising (C 15) the experimental seedlings; and
Partly by using experimental layouts such as randomised Blocks to make sure that each part of a variable experimental site contains all the treatments (D 55 in Manual 4).