- auxins and rooting
What are auxins?
They are one of the groups of plant growth regulators (hormones). Some auxins are artificial compounds, but many are produced naturally by plants, especially in young growing parts of the shoot. In nature, they move always from the tip towards the base of shoots, but when applied at the base of cuttings there can be some movement in the opposite direction.
What do they regulate?
Auxins control many different aspects of growth and development, as is typical of plant hormones. For instance, they are known to influence the elongation of stems and leaves, the setting and ripening of fruits, and the growth in thickness of trees. They also stimulate the formation of new root tips in stem cuttings.
-=no auxin; +=IBA and NAA mixture.
What kinds of auxins are best for rooting cuttings?
Two artificial auxins, known as IBA and NAA, are most commonly used, either singly or mixed together (see A 63 for the chemical names and sources for auxins). Several other auxins are available, but turn to them only if you draw a blank with IBA, NAA and IBA/NAA.
How are they applied to the cutting?
There are three main methods:
Method 1 - Quick dip: dissolve the auxin in alcohol (or another organic solvent), dip the basal few mm of the prepared cuttings for 1–5 secs into the solution and let the alcohol quickly dry off before setting them. (Note: If left in alcohol for more than a few seconds, the base will die!)
Method 2 - Powder (talc): Buy a commercially prepared auxin rooting powder, put the basal few mm of the prepared cuttings into the powder, gently tap to remove excess, and set at once.
Method 3 - Soak: dissolve the auxin in water, and leave the prepared cuttings standing with the basal 2–3 cm in the solution for 4–12 hours before setting them. (Note: Many auxins are slow to dissolve in water.)
With the ‘bubble-bath’ system, the cuttings remaining with their bases in water, which is kept aerated (as in a fish tank). Small quantities of auxins are dissolved in the water to assist the rooting.
Which is best?
Method 1 has been generally found to give the best results for commercial nurseries producing
woody ornamentals in the USA. Advantages include the correct amount of auxin actually
entering the cutting, dissolved in the alcohol, and roots forming higher up the stem.
(For research treatment, see A 45.)
Method 2 can be convenient:
Some rooting powders also contain a fungicide (A 52, A 63), which helps to discourage disease.
Method 3 is a stand-by in the few species that are intolerant to alcohol.
What are the disadvantages?
Method 1: the alcohol evaporates during use, making the solution more and more concentrated.
(Can be solved by using 50/50 alcohol/water, using only a little auxin solution at a time and closing
the container with the thumb, and having a good production line of prepared cuttings.)
Method 2: easy to get too much powder on the base of the cutting (which can sometimes stop the outgrowth of the new roots), and the powder loses its effectiveness if not kept dry and in a refrigerator.
Method 3: difficult to use with larger numbers of cuttings, and sometimes the leaves may wilt during treatment.
How strong should the auxin solution be?
A useful start can be to use IBA at a concentration of 2.5–5.0 grams/litre (= 2500–5000 parts per million, or a 0.25–0.5% solution), or alternatively a 50:50 mixture of IBA and NAA. The best auxin and concentration varies from one tree species to another, but can be found out by experiment (see A 45), or by trial and error. Research suggests:
2 g/l IBA for Triplochiton scleroxylon, Vochysia hondurensis and several other tropical tree
a range from 0.5–4.0 g/l IBA for Albizia guachapele;
4 g/l for Cordia alliodora and
10g/l IBA for Khaya ivorensis.
Note: too strong an auxin treatment can discourage the roots from emerging from the stem).
Do all tropical trees respond?
Most do so, but no clear differences in rooting were found in several experiments with Lovoa trichilioides.
Supposing I haven't any auxins?
You can still root cuttings of many kinds of tropical tree. Rooting will probably be slower, so concentrate on getting the propagation conditions as favourable as you can (A 30–35). See A63 for sources of auxins.
What do the auxins actually do?
Sometimes they can:
Where should I keep the auxins?
They don't keep well at room temperature in the light, so store all auxins in a refrigerator at
Provided they are kept dry as well as cool, pure auxins can be stored for several years, and powders for about a year. (Note: in the humid tropics, allow the container to warm up for 30 mins before opening it, to avoid getting the contents wet). Solutions of auxins keep only for a week or so, even in the refrigerator.
- trimming the leaves
Why should I trim the leaves?
It is important for leafy cuttings of species with leaves longer than about 8–10 cm. Experiments with Triplochiton have shown that rooting is better when the leaf-blade is neither too big nor too small.
Why is this so?
Large leaves may lead to more loss of water by the unrooted cutting. On the other hand, small leaves may not produce enough sugars and other substances needed for the cutting to survive. Trimming also makes the cuttings easier to handle, take up less propagation space and be less likely to rot.
What is the best size?
This varies from one tree species to another. Aim for a total area of about 50 cm2, except for the mahogany family (Meliaceae). The best size is also influenced by the type of cutting, auxin application used, propagation environment, etc, so it may be worth doing a trial (A 45).
Does the shape matter?
Not very much. With leaves shaped like a hand, it is easy to cut off the ‘fingers’ of the larger leaves, and with compound leaves to take off some of the leaflets. If insects have eaten part of the leaf, leave more of the rest. For small-leaved species try multi-node cuttings (A 42), with more than one leaf above the rooting medium.
What is the best way of trimming the leaves?
With a sharp pair of scissors, avoiding crushing the leaf tissue.
When should the trimming be done?
Trim before the shoots are detached from the stockplant. This will reduce water-loss (A 34 & A 42), and avoid extra bulk in the polythene bags used for carrying the cuttings. However, if a few leaves are missed, they can be trimmed while preparing the cuttings (A 43). If leaves can be trimmed the day before, this will allow the propagation to start early.
Should I trim all the leaves on the stockplants?
No, only those which are on wood suitable for cuttings (see A 6). In any case, at least two good basal leaves should be left on each shoot, except where it is to be heavily pruned (A 25).
What about non-leafy cuttings?
It is often best to take shoots that have already lost their leaves naturally. If there are leaves present, most should be cut off (best to do this at the top of the leaf-stalk), and the topmost one or two can be reduced to about 1–2 cm in length (A 4).
- taking leafy shoots
What needs to be ready before I start?
How big should the cuttings be?
As a rough guide, aim to take moderately vigorous shoots that will cut up into cuttings that are
2.5 – 12 cm long, with a stem diameter at the base of about 4 – 8 mm). If possible, avoid thin
shoots making little or no new growth; and thick very vigorous twigs, although rapidly growing
young coppice shoots are fine before they become too woody. (Non-leafy cuttings are often
larger - see A 4).
Also important is the number of nodes in a cutting, which will depend on the growth habit of the tree species, how the stockplants have been managed and whether you have plenty of material or not.
How many nodes should my cuttings have?
When should I take the shoots?
Early in the morning, preferably in misty weather or just after rain. This will keep drying out to a minimum (A 34).
How should I take them?
What sort of polythene bag is best?
Any kind will do, but the best are of a new type that are white on the outside and black on the inside. The shoots inside are kept much cooler because the white surface reflects heat, while the black one radiates it away. If these are not available, try making double bags yourself by cutting and stapling sheets of black and white polythene.
How long can cuttings be kept before setting them?
The sooner you can set the cuttings the better (see A 44). However, avoid opening polypropagators in the middle of the day, because the humidity inside immediately drops. So plan to set your cuttings before 10 am or after 4 pm. Leafy cuttings of Triplochiton scleroxylon will not keep in bags more than 2 nights.
- preparing leafy cuttings
Why shouldn't I set the cuttings immediately?
It is very important to avoid delay. However, the shoots first have to be divided up into cuttings, and there are usually other things to be done before they are set. These include:
How do I re-cut the base?
Make a new clean-cut base to the cutting, using a very sharp knife or scalpel blade; in order to
What about auxin treatment?
With a fresh-cut base, dip the bottom 5 – 10 mm of the cutting in rooting auxin (A 40) in order to:
What unwanted material needs to be removed?
How do I guard against pests and diseases?
If possible, avoid putting unhealthy cuttings into a poly-propagator. Where supplies are limited:
What if I want the cuttings for a rooting experiment?
Cuttings should be as uniform as possible for research (A 45). If you have plenty of shoots, choose a standard size and type of cutting. If there is a limited number, grade the cuttings into different size classes before allocating equal numbers of each class to the two or more techniques or rooting conditions you wish to compare.
- setting leafy cuttings
What should I do first?
How far apart should the cuttings be?
So that they don't touch each other, but with no wasted space.
How do I put them in?
Make a hole with a small stick (not too deep, and about the same diameter as the cuttings), put in the cutting and make the medium firm around it.
Won't the cuttings dry up while they are being set?
Yes, unless you are careful to:
Should the cuttings be in rows?
Rows are much easier, for several reasons:
What about labels?
At least two labels are needed, one before the first cutting in a batch or clone, one after the last. Write clearly and firmly with a good HB, B or 2B pencil, not a pen.
Essential information includes:
In research propagation (A 45), also:
This may require one label per cutting.
What else is important?
How can I speed things up?
- rooting experiments
What's different about rooting cuttings for research?
It needs greater uniformity, comparability and care at all stages, so that valid comparisons can be made.
Where should I start?
Multiply plenty of cuttings of a few clones, and grow them as well-managed stockplants (A 24–27). In time, you should have a good supply of uniform, clonal shoots.
But surely they will still produce better and poorer cuttings!
Very true. But in any case it is a good plan to standardise or grade the cuttings for experiments (A 43), whether you are comparing two or more types of cutting; techniques; rooting conditions; or clones.
How and why do I standardise or grade the cuttings?
But I thought everything had to be randomised!
Not everything. It is the individual cuttings within each group that need to be randomly allocated to treatments. Grading into groups is meant to be a conscious selection of unavoidable differences, just like choosing where the positions for the blocks should be in your experiment.
What about the propagating conditions?
Example of a simplified layout for rooting experiments
|Block total = 72 cuttings|
|Block total = 72 cuttings|
↓ continue with different randomisation for other Blocks
Can I improve the uniformity of auxin treatment?
Yes, the easiest way is to add one drop (about 50 microlitres) of an alcoholic auxin solution to
the re-cut base of each cutting, using a micro-pipette. If you are testing auxin type or amount
(A 40), make up solutions at different concentrations, so that you still add one drop to each
(Note: Alcohol evaporating from the stock solutions will make them more concentrated.)
Do research cuttings need extra day-to-day attention?
Yes. Unrooted leafy cuttings need to be kept in very humid conditions at all times, as they have no roots to take up water. This is especially vital in experiments, because variation in water stress can greatly confuse the results. Uniformity can be helped by the following hints:
What layout should I use for the experiment?
How about labels?
Before taking the shoots, write these clearly and firmly with a good HB pencil, not a pen. (Even the waterproof types can become indistinct in tropical conditions.) Labels will be needed for rows or for individual cuttings: abbreviate and condense the writing, but include all the vital information:
What about records and assessments?
What else is important?
A personal input into your experiment - plus the ability to look for explanations for the results other than those you were testing for.