During Winter 2004, I decided to test out the effects of a technique I had heard about but never quite found the time to use before. The technique involves steeping willow cuttings in water for a number of weeks to produce a liquid that could promote rooting and reduce the stress on weakened trees. This is known to have been used as a method for propagation before commercially available rooting hormones were developed.

Willow (Salix species) have long been known to root easily; cuttings from branches up to 5" or 6" thick can be taken at any time of year, placed in regular water and they will root within a matter of weeks. There is no need to apply rooting hormone to the cut end yet almost every cutting will still take root.

This simple technique involves using the water that is left over from this cutting process (Willow water) to promote stronger rooting by using it to water cuttings, air-layers, yamadori (collected trees) and weak trees with poor root systems.

The Theory Behind Willow Water

Aspirin mixed with water has long been used by many to prolong the life of cut flowers and to increase the strike rate of cuttings. Aspirin is a natural anti-coagulant; meaning it stops/slows the processes by which fluids are converted to solid states/congeal. Some may also know Aspirin for it blood-thinning properties. Aspirin would seem to have a similar effect on plants, hindering the ability of the plant to clot up the damaged/cut end of a cutting or root and promoting the ability of water to enter the cutting and so sustain the plant itself whilst it self-heals.

Interestingly, Aspirin is derived from salicylic acid, a product naturally found in Willow bark! There may also be other chemicals/hormones involved that promote rooting but the salicylic acid seems to be the primary cause. 

There is evidence to show that Salicylic Acid has an influence on blocking the wound response and effect of Abscisic Acid. Abscisic Acid is a stress hormone released by all plants in response to wounding or disease; it induces rapid closure of plant stomata and the 'shutting down' of injured areas of a plant. 

So we come full-circle, the Willow cutting releases chemicals into the water, this promotes its ability to absorb water and to take root. This water is then free for us to use to promote rooting in other plants!

Does it Work?

Still dubious, I placed 50 thin trimmings from a Salix purpurea (Purple Willow) into a bucket of water and left it for 4 weeks. As the cuttings started to root (every single one of them) I noticed that the water had taken on a slightly gel-like consistency; the water also seemed to leave a slippery residue on my fingers.

The areas of Willow that were under water also seem to be coated in a coating of a transparent gel; particularly around the area of the cut surface at the base.

I used the solution to water Hawthorn and Oak that I collected this Winter; out of 20+ yamadori I have currently lost only 2, my best success rate so far. The surviving trees have also seemed to be stronger and more vigorous than in previous years.

This of course could be put down to variety of other factors, maybe my collecting expertise has improved, improved success could be due to better care or soil mix or maybe the weather this Winter was more conducive to successful collecting.

However, my opinion has changed now that I have used Willow Water on air-layers. Every Summer for the past 6 years I have religiously taken air-layers from an Acer palmatum that grows in my garden. Rooting is always 100% and I can always accurately judge when roots will first appear from the layering (7 weeks for ½" branches, 5 weeks for 1"+branches). This year I used Willow Water to soak the sphagnum moss before making the layers and afterwards to keep them moist. Rooting has moved forward by a least a week in all cases and rooting has been far stronger so that the layers have also been removed from the parent tree earlier.

Rooting also seems to have been more prolific and vigorous on a Prunus incisa and a Hawthorn that have also been layered this year.

My only reserve is that rooting has taken place over a wider area of the airlayer instead of being confined to the cut area/wound itself. This will mean a more uneven nebari in the future without early removal of these higher roots.

The Willow Water I used for these air-layers was made from the cuttings of Salix babylonica (Weeping Willow) and Salix purpurea (Purple Willow) . The exact species of Salix used would appear to make little difference. I assume the 'strength' of mixture is entirely dependent on the number and thickness of Willow relative to the volume of water that is used.

Willow Water Acidity Issues

A (very concentrated) solution of 'pure' Salicylic acid has a pH of 2.4 if fully saturated into aqueous solution. This level of acidity would be harmful to all lifeforms. However the amount of Salicylic Acid found in a typical solution of Willow Water is not capable of changing the pH value of the water originally used.

 

Conclusion

I cannot offer any scientific or hard fact that Willow Water promotes rooting in plants, only anecdotal evidence. It would appear to me that the use of Willow Water increases the ability of plants (trees) to root and has increased my success rates.

However I doubt that this is some magical rooting preparation; Willow Water will only increase the rooting ability of a plant if all other correct care guidelines and techniques are first applied. If a cutting or air-layer is not made properly or out of season, the use of Willow Water will not make enough impact to ensure that the plant will root nonetheless.