Seed handling

What is the most important thing about handling seeds?

Remembering that they need to be looked after if they are to remain alive and able to germinate.

But aren't seeds resting stages that don't need the care that seedlings do?

some tree seeds are not really resting organs, but start to germinate immediately (B 13); Other kinds of seeds are resting organs, that can be dried and stored (B 33); but
Most seeds are liable to die, if they are subjected to:

1. rough handling;
2. extremes of temperature;
3. very rapid rates of drying; or
4. being left in wet conditions without being cleaned (B 32).

So it's better to think of them as delicate?

Yes; a potential seed crop needs looking after throughout the flowering, fruiting and collection periods; and then right up until the seeds are sown.

Why starting as early as that?

Because there may be steps that can be taken to increase the chance of collecting viable seeds. For example:

1. increasing pollination and the setting of fruits (B 11–12, B 23);
2. discouraging fruit-eating animals from taking most or all of the crop; and
3. reducing attack by seed-boring insects and infection by seed diseases.

But how can I do anything about animals eating the fruits?

Well, one can concentrate on reducing the number of fruits taken, for instance by:

1. scaring off wild animals like monkeys, apes or other herbivores by human watchers or noisy gadgets; or hunting them if they are not protected species;
2. using fences or hedges (C 46 in Manual 3) to keep wild or domesticated animals away, or tethering goats; or
3. collecting the fruits promptly (B 31), and sometimes slightly early.

How about seed pests?

Substantial losses of potential seed supplies, and seedlings with missing parts and poor growth, can sometimes be caused by:

1. insects, particularly those which bore into the seed before collection, or chew them afterwards;
2. rodents like mice, rats and squirrels eating seeds after collection, or even sowing.

If unchecked, some pests can spread within the tree nursery, giving continuing trouble.

Can anything be done about insects?

Ways of reducing insect attack can usually be developed (C 45 in Manual 3), especially when their life-cycle has been studied. For example, you could:

1. remove places where the resting stages of the insects occur; or
2. collect the fruits and clean the seeds very promptly;
3. keep a look-out for the numbers of an insect building up; and if necessary
4. spray with a pesticide.

And what about rodents?

They can usually be kept out by:

1. keeping fruits and moist seeds within wire mesh containers (diameter less than 1 cm);
2. storing thoroughly dried seeds in closed tins (B 33).

Setting traps, or keeping a cat, can also help keep the numbers down.

How about seed diseases?

There are three main types:

1. A very few kinds of fungi attack the flower buds, preventing seed formation. An example is the smut fungus Mycosyrinx nonveilleri which can sometimes destroy a high percentage on trees of Triplochiton scleroxylon in West Africa.
2. Many kinds of fungi and bacteria will start to rot seeds left in wet, decaying fruits, but this can be largely avoided by good practice (B 32).
3. A few fungi attack germinating seeds and very young seedlings (B 47).

What kinds of rough handling should be avoided?

Things like:

1. treading on fruits or seeds when they are lying on a hard surface;
2. throwing sacks of seeds about, or sitting on them; or
3. putting heavy things on top of seed bags in a carrying tray or a vehicle.

Moist seeds (B 12–13), and especially those that have already started to germinate (B 44) need particular care.

How about high temperatures?

Dry seeds are often more tolerant of heat than their seedlings are, and some can stand temperatures of 40–50°C, but hot conditions like this should generally be avoided.
Most moist seeds could be damaged or killed by such temperatures, even if they occurred for less than an hour. For instance, cocoa beans for consumption are often dried in full sun on the edge of a road, but those to be sown need to be kept much cooler and moister or planted immediately.

So how should seeds be dried, then?

See sheet B 33 for the general principles for dry seeds, and remember that moist seeds may need to be sown at once (B 13), or dried reasonably slowly. Rapid drying can impose too much stress on the cells (C 10 in Manual 3).

A method suitable for one tree species will not necessarily be appropriate for all the others.

But mightn't the seeds go mouldy if they are not dried quickly?

Sometimes yes, if drying is too slow, or if large piles of fruits or seeds are left without being turned over periodically; but
Usually no, if the air can circulate freely.

Aren't low temperatures good for storing seeds?

Not necessarily, because of chilling injury (D 11 in Manual 4) to moist tissues of many tropical species at the temperatures in a refrigerator (about 4–14°C).
However, after careful drying, some kinds are indeed best stored cold, and some can even stand being kept in a deep-freeze at temperatures of about -10°C to -20°C.

Where is the best place to keep those where low temperatures are harmful?

In a dry and airy place (B 33), protected from rodents. If an air-conditioned room is available (around 18–23°C), this could be ideal, but not if the electricity supply is liable to go off for long periods at a time.

How often should I check seeds?

Every day for recent collections which have not yet been dealt with;
Twice a week for seeds that are drying and may need turning;
Once a week for batches of species which are prone to attack; and
Once in 3 months for dried, protected seed in longer-term storage.

What do I need to look for when checking?

Any signs of a bad smell or mould developing, discolouring of some seeds, or the droppings of insects or mice.

What are the most important points about collecting seeds?

1. Collecting from a sufficient number of suitable parent trees (B 22);
2. Handling the fruits and seeds carefully, to maintain viability (B 13, B 30); and
3. Labelling the seed-lot and keeping a record of where and when it was collected (C 54, C 64 in Manual 3).

If seeds are being collected from experiments, assisted pollinations or controlled crosses (B 23), it is important to keep each batch separate.

When should fruits be collected?

At or close to the time they become fully ripe (B 12). For each species, one needs to judge how to strike a balance between collecting:
too early, when seeds might not be completely developed, while those the tree sheds first may be empty; and
too late, when most fruits might already have been eaten or the seeds dispersed.

Harvesting fleshy fruits for human consumption is often best done a little earlier, while they are still firm, if they will finish ripening after picking.

Supposing they don't all ripen together?

In some species, fruiting is spread over weeks or months, while in others all the fruits ripen at about the same time. In the former case, several collections may need to be made, but that could mean greater flexibility in arranging the collection to fit with the time for sowing the seeds (B 43).

What are the different ways of collecting seeds?

1. Picking fruits directly from the trees;
2. Gathering them up from the ground; or
3. Spreading sheets out under fruiting trees to collect falling seeds.

Which is the best?

It is often preferable to collect fruits (or cones) before they fall or release their seeds (B 12), because:

1. On the ground, they are even more likely to be eaten or removed by animals, and could get too dry or too wet;
2. As they fall, fruits and seeds can easily become scattered, especially if they are small or winged; and
3. it will be much easier to know which seed tree they come from (B 22).

But picking the fruits will involve climbing, won't it?

Yes, it usually does, except for:

1. species or mature clones (B 23) with a strong tendency to flower early (B 10);
2. miniaturised seed orchards that use effective flower stimulation techniques (B 14);or
3. situations where a fruit-picking pole can be used.

How do they work?

The simplest kind is just a long pole which is used to knock down the fruits. Alternatively an open bag, basket or tin is attached to a long piece of bamboo, so that fruits or seeds can be collected by pushing at the fruit when it has been manoeuvred inside. A somewhat more sophisticated version is the ‘fruit-picker’ which also incorporates a blade to cut the fruit stalk, and sometimes a pole in sections that can be screwed into one another.

What climbing methods can be used?

1. Step-ladders or small bamboo ladders for picking fruits at heights of 2.5–4.5 m;
2. Tall ladders for up to 10 m; or
3. Climbing up the main stem, with or without various climbing aids, for taller trees.

How can the trunk of a tree be climbed directly?

1. Simply swarming up a smooth bole, in species where the lower branches or large leaf-bases fall off cleanly;
2. Using available branches, cracks in furrowed bark and woody climbers;
3. Passing a loop of rope around the tree and the climber, and moving it up in stages; and reversing the procedure to come down;
4. Cutting notches or driving wooden or metal pitons into the trunk, and climbing on them;
5. Using a ‘tree bicycle’, which consists of a section supporting the climber's body, and two bands around the trunk which are moved up one by one; and
6. Various motorised lifting vehicles.

In some parts of the tropics, ladders are attached to the trunks of trees in order to make successive collections of honey from wild bees. Where appropriate, these could also be used for seed collection.

But the fruits are usually out in the crown, away from the trunk!

Yes indeed, except for those species where the fruits are borne on the trunk (B 11). So there is generally a second stage to fruit collection, in which the climber has to reach some of the fruits and detach them, or push at their stalks with a short pole.

Are there some safety guide-lines?

1. Always have at least two people working together, one helping the other.
2. Never hurry a person who is going to climb high up.
3. Avoid unnecessarily disturbing the attention of the person climbing.
4. Use ropes to tie ladders and the tops of step-ladders to the tree, and follow the instructions for the correct use of extension ladders and the fixing catches on step-ladders.
5. Arrange extra precautions for both people if sharp tools are to be carried;
6. Take up another rope to raise and lower a basket, rather than trying to carry more than a few fruits;
7. Consider other safety measures that might be sensible in the particular situation, such as a safety rope.
8. Feel free to abandon the attempt to climb if the risks turn out to be too great.

How about using a collecting sheet on the ground?

This is more likely to be successful if the:

1. ground is fairly flat, and there are no large stones or bushes under the seed tree;
2. site is likely to be relatively windless;
3. fruits or seeds are not winged, so will fall straight down;
4. stage of ripeness is similar for many fruits on the one tree.

What materials would be suitable?

Anything that is large enough, such as cloth, plastic shadecloth (A 24 in Manual 1), closely woven mats, roofing panels or old pieces of plywood.

Polythene sheeting can sometimes be used, but you may find problems with:

1. rain or dew accumulating on it: or
2. small seeds adhering to it, through static electricity.

Do seeds always need extracting?

No; because sometimes they are:

1. shed as separate seeds when the fruits ripen (B 12);
2. formed within single-seeded fruits (or those containing a very few seeds), that are distributed, and can be sown, as a single unit; or
3. best collected as wildings or germinating seeds (B 44).

Aren't all the other kinds too diverse for one method to apply?

Well, fruits and cones do differ greatly in their nature, size and the number of seeds they contain, and so the appropriate ways of handling them vary in detail. However, they can be broadly divided into:

1. fleshy fruits, in which the cells remain alive, and which are often eaten by animals, distributing many of the seeds in their droppings; and
2. dry fruits, that dry out and may split open on the tree, scattering some or all of the seeds. These seeds can usually be further dried, and stored for some time (B 33).

Note: try and avoid damaging the seeds during extraction and cleaning! Seeds with hard coats (B 13, B 34) are generally less delicate than those that are soft.

Sometimes seeds are hard to extract.

How can seeds best be extracted from fleshy fruits?

Firstly, they should be pulled open or chopped up, without damaging too many seeds;
Secondly, the seeds need to be separated from the fruit pulp, which can usually be done by washing, sieving and rinsing, which also cleans them; and
Thirdly, the water should be allowed to drain away from the seeds, so that they become surface-dry and less likely to start rotting.

Can they be left to steep in water?

This can often help to soften the fruit tissues, provided that:

1. the water is not too warm, which might reduce seed viability (B 13); and
2. they are not left under water for more than a few hours, or decay may begin and the seeds suffer from the breakdown products, or from lack of oxygen (B 40).

Are there some other general points about fleshy fruits?

1. The fruit juices often contain inhibitors of germination, making some or all the seeds dormant (B 14) until these are washed away or break down naturally.
2. The seeds are moist, and quite often cannot be dried and stored (B 33).

Sometimes seeds just fall out.

And are there some general methods for dry fruits?

1. With cones or fruits of sorts that will open, collect them straight into a cloth or strong paper bag, or a finely-woven basket, to catch the seeds as they fall out;
2. Put the kinds that open suddenly inside a closed cardboard box in a dry place, so that the seeds are not lost when they are flipped out;
3. Spread the other kinds out on a suitable surface where they can open up as they dry, releasing some or all of the seeds; and then

If necessary, shake or sieve to extract the rest of the seeds.

What about dry fruits that don't open up?

These are more difficult. For instance:

1. the dry seeds of teak remain ‘locked up’ inside the strong fruit wall. Many ways have been tried to extract the seeds without damaging them, including hot water and strong acids. Careful squeezing of individual fruits in a vice can release a flap in the wall above the seeds;
2. the closed fruits of some savanna and dry forest species release their seeds naturally after grass fires have occurred. If cutting them, shaking with small stones in a tin, or repeated soaking and drying in the sun, does not make them open, you might try holding a few briefly over a fire, but the timing will be critical or the seeds will be killed.

Teak fruits.

Supposing I want to try strong chemicals?

You could do a small experiment (B 48), remembering that:

1. they should be used with care, wearing protective clothing (C 45 in Manual 3);
2. some organic solvents could enter the seeds rapidly, damaging or killing them; and
3. all such chemicals need to be removed by rinsing several times in water afterwards.

And what about fruits with a single seed?

Usually they do not need to be extracted, but can be handled like seeds. Examples include Terminalia, which can usually be dried and stored, and the 2-seeded fruits of many diptercarps, which cannot.

Do all kinds of seeds need to be cleaned?

No, although most of them do. As well as removing most fruit remnants, twigs and leaves, cleaning may involve:

1. washing off soil, unless this is needed to inoculate the seedlings with closely associated micro-organisms (B 42; and C 30–32 in Manual 3);
2. removing the wings from seeds or fruits;
3. sieving or blowing away dust; or
4. picking out remaining items by hand.

Supposing the cleaned seeds quickly start to rot?

There are four possible explanations:

1. you need to change the techniques (B 50), for example by:
   1. shortening any periods of steeping;
   2. washing more thoroughly; or

   3. spreading the washed seeds out more thinly for better drying;

2. the species is particularly liable to rotting, in which case you might try:
   1. immediate sowing;
   2. rinsing the seeds in a 2% solution of domestic bleach; or

   3. dusting them with a fungicide such as propamocarb (B 47) before storing or sowing;

3. the seed-lot is infested with the resting stages of fungi or bacteria, when you could treat them as under (2); or

4. the seeds were empty or dead in the first place (B 13).

Where should I keep the cleaned seeds?

Immediately they are clean, make a choice between:

1. sowing non-storing seeds at once, or within 1–2 days; and
2. starting storable seeds on a suitable drying and storage regime (B 33), if they are to be kept for sowing later on.

Which kinds of tree seeds can be dried for storing?

1. Most species that have dry fruits (B 12, B 32). These seeds have usually already lost some of the water they contained by the time they are collected, though they often need further drying;
2. Some of those which produce single-seeded fruits; but
3. Only a few kinds with fleshy fruits, where the seeds are still moist when the fruits ripen.

So what needs to be done with those that cannot be dried and stored?

Usually they need to be sown at once, or within 1–2 days.

Isn't that sometimes rather inconvenient?

Yes, especially when:

1. flowering does not occur regularly (B 11); or
2. fruits ripen at the wrong time for sowing (B 12, B 43).

Can anything be done about it?

Here are some suggestions:

1. in species where fruiting is spread over a period of months (B 32), collection could be done at a somewhat more suitable time for sowing;
2. some kinds of fleshy fruits can be kept for a few weeks, possibly in a refrigerator, and so the seeds could be extracted and sown a little later;
3. out-of-season seedlings could be grown in larger containers (C 6 in Manual 3), and used as stockplants to supply cuttings (Manual 1) that could be rooted and ready for planting at the right time;
4. seedlings could be grown more slowly, or more rapidly, so that they might still be the right size for planting (C 33–34); or
5. you could consider whether they could be planted as stumps (C 47), where larger plants are cut back and so their height is not so critical.

What is the best way to handle the kinds of seeds which can be dried?

By extracting and cleaning (B 32) the batch promptly, and then:

1. spreading them out in an airy but protected place to dry, turning them over every few days;
2. making a record (C 54, C 64) and labelling them; and
3. putting them either in a seed bag or closed container, and storing them in a cool, dry place.

Where would be a good place to dry them?

Somewhere under cover where they are protected from rain and strong winds, and also from animals (B 30), but not in still, moist air that might encourage moulds to grow.

Air-conditioned rooms can provide ideal conditions for drying many tropical seeds, as the air is dry, cool but not too cold, and is moved about and replenished by the fan.

Special seed kilns are used in seed testing laboratories.

How fast can I dry seeds?

Aim for a moderate rate of drying:

1. not rapidly on a hot surface in full sun, unless you know the species can stand this without loss of viability (B 13); but
2. not so slowly that they start to go mouldy.

Tests suggested that Triplochiton scleroxylon fruits are best dried at a moderately slow rate of about 1% of their original weight each hour (see Longman et al., 1978 in B 52).

How dry do they need to be?

This tropical species stored best at 8–14% moisture content.
Experience in the USA suggests that a wide range of seeds stored best at 5–12% moisture content, although there were examples below 1% and as high as 30%.

But how can I tell how dry my seeds are?

An accurate determination is not usually practicable for the smaller grower. However, it is a reasonably straightforward job in a seed-testing laboratory, where different drying regimes can be tested using a large seed-lot, and then recommended procedures worked out for those who do not have facilities for accurate weighing of seed samples.

How is moisture content determined?

1. The batch of seeds is well mixed, and then a number of equivalent samples of seeds are taken and weighed;
2. Some are dried at once in an oven at 80–105°C until they give a constant dry weight (C 55 in Manual 3), because the moisture content is best expressed as a percentage of this;
3. The other samples are dried normally for various periods of time, then weighed again, and their dry weights are determined in the same way.
4. Suppose one batch weighed 84.5 g at the beginning, 48.3 g after normal drying for a month, and 41.1 g when it had been oven-dried to a constant weight. Then:
   1. the amount of water that was lost in the month is 84.5 – 48.3 = 36.2 g.
   2. the moisture content at the beginning was
   3. The moisture content after drying for a month was

How do you get the samples to be equivalent?

For larger seeds, you could count out a standard number, for instance between 15 and 30.
For smaller seeds, you could take a standard fresh weight, say between 50 g and 150 g.

Why not use the fresh weights of the living seeds as the standard, so that it was not necessary to destroy any of them by strong heating and drying?

Yes, that would be much easier, but fresh weights are so changeable that they would make too unreliable a standard for accurate studies.
For the grower, however, fresh weights can be give a rough guide of the rate of drying, and also for instance to:

1. estimate the approximate numbers of seeds being stored (by weighing for instance 5 samples of ten seeds, finding the average weight, and dividing the weight of the whole batch by that);
2. do a rough test of the germination percentage of small seeds (B 48); and
3. know how thickly or thinly to sow them on a seed bed or in a seed tray (B 43; C 63 in Manual 3).

What is a seed kiln?

A drying ‘oven’ in which the air humidity and temperature can be controlled, so that pre-set drying regimes can be routinely applied to seed-lots.

So it's no use to smaller growers, then!

It could be if you:

1. were buying seed of good quality from a larger agency (B 24);
2. could have a difficult species handled for you by a nursery co-operative; or
3. had the facilities to learn how best to dry and store your own seeds.

What kinds of containers can be used to store seeds?

There are basically two approaches:

1. Bags made of strong paper, cloth or other woven material, which allow air to pass freely through them; or
2. Tins or jars that can be firmly closed, preventing moisture from entering or leaving.

Which is best?

Bags are the most suitable in many cases, provided that they can be kept in cool, dry conditions, and protected from pests (B 30). The seeds can then dry further in storage, rather than going mouldy.
Sealed containers are preferable when already dry seeds need protection from a humid atmosphere. Note: when a sealed container is cooled in a refrigerator, the relative humidity of the air inside increases, and the moisture content of the seeds may rise.

And what kind of practical storage conditions are usually best?

These vary quite a lot from species to species, but in general seeds remain viable longer in dry, cool conditions that do not fluctuate greatly.

How cold is suitable for tropical trees?

Seeds can be stored:

1. in bags inside a dry, shaded, airy building;
2. in closed containers in a refrigerator; or sometimes
3. in closed containers in a deep-freeze.

For instance, Triplochiton scleroxylon fruits that had been dried in the way described above kept for 18 months at - 18°C with little loss of viability.

How long will my seeds keep alive in storage?

This can vary from a few weeks to several years.
If every seedling will be needed, it might be better to sow the whole batch soon, before any viability is lost.
If you have plenty of seeds, viability can be checked (B 48) from time to time by:

1. sowing a small, representative sample under standard nursery conditions,
2. cutting a few sample seeds across, and counting how many have firm, whitish contents;
3. applying the tetrazolium test, which can indicate which seeds are still viable; or
4. doing standardised germination tests, preferably in controlled environments.

NOTE: bear in mind that seeds that do not germinate may be dormant rather than dead (B 13).

What about longer-term storage?

This is particularly needed when:

1. plenty of seed is available only irregularly, maybe once or twice a decade; or
2. most or all the natural stands of the species are under threat of extinction.

Long-term storage of seed (and sometimes of pollen) is a job for a specialised centre, where:

1. the very best conditions can be discovered and used for each species;
2. storage regimes can be reliably maintained and records preserved over long periods of time; and
3. the material can be made available for use in conservation plantings (B 20) and crossing programmes (B 23).

Are there any unusual ways of keeping seeds alive longer?

Yes. For example:

1. the seeds of some colonising tree species stored much better when they were kept in moist cool conditions than when they had been dried first (Vázques-Yanes in B 52);
2. citrus seeds generally do not store well, but if the seed coats are carefully removed, they can be stored at low temperature for a longer time.

What are pre-sowing treatments?

In the narrow sense, they are ways of breaking any seed dormancy (B 13) that may be present;
In the wide sense, they also include anything else that needs to be done to the seeds before they are sown, in order for them to germinate successfully, such as:

1. Bringing seeds that have been stored at low temperatures gradually back to the propagation temperatures;
2. Soaking dry seeds in water for a few hours, so that their cells can become moist and be able to divide and grow (C 10 in Manual 3); or
3. Treating the kinds of seeds particularly liable to diseases or pests with a fungicide, insecticide or other poisonous substance (B 47; and safety advice in C 45 in Manual 3).

Which kinds of pre-treatments do dormant seeds require?

These are described in sheet B 13 for the commonest types of dormancy.

When should they be done?

Dry abrasion of ‘hard’ coats is generally best done 1–2 days before sowing, because it may let in moisture and resting stages of moulds that could shorten the storage life;
Treatment in liquids needs to be done shortly before sowing, as the seeds will be wet.

Other types of dormancy often involve providing specific germination conditions.

Why should seed temperatures be altered gradually?

Because sudden changes impose an extra and unnecessary stress on the living cells in the seed, which have been remaining alive at a very low rate of activity (C 14 in Manual 3).
For example dry, unmilled rice grains can remain viable when stored in a deep-freeze, but none will germinate if they are put straight into water at 25–30 C.

Is there a simple way of avoiding sudden temperature change?

You could let seeds warm up slowly by wrapping the unopened seed containers in a thick layer of newspaper, and leaving them in a shady place for 6–24 hours.

But I might want to open the containers to take out some seeds and keep the others in store!

Yes, you might; but opening a container while it is still cold from the refrigerator or deep-freeze will mean that water is immediately deposited on the seeds inside, just as happens on the outside of a cold drinks bottle or can. This cold water:

1. is an unfavourable way of moistening those seeds being taken to grow; and
2. will mean that the remainder are no longer dry, and will rapidly lose viability (B 13).

One way of getting over this problem is to store the dried seeds in several smaller containers, rather than a single large one.

Is soaking in water always necessary for dry seeds.

No. Sometimes it is better to let them take up water from the moist germination medium (B 42) after sowing (B 43).

Which kinds do need soaking first?

Those in which the entry of entry of water is slow, because:

1. the surface of the seed is not easily wetted, so water does not enter freely through the small hole in it (the micropyle); or
2. the seed coat (or combined fruit wall and seed coat in single-seeded fruits) restricts water movement. This can also apply to ‘hard’ seeds that have been scarified (B 13).

Pre-soaking can also lead to more uniform germination of all the viable seeds, which can be an advantage, particularly when growing seedlings for research (C 7 in Manual 3).

How can seeds be pre-treated against diseases and pests?

If these are liable to cause problems, seeds can be coated with small quantities of an appropriate powder, or soaked briefly in a diluted solution (B 47). Alternatively, in difficult cases a small amount of fungicide could be incorporated in the germination medium.

NOTE: always wash your hands well after using any of these poisonous chemicals!

For most species, however, proper seed handling and protection (B 30–34), coupled with good germination conditions (B 40–47), mean that such pre-treatment may be unnecessary.