Willow Tea...

Here's the "scoop" on willow water.
In the 1970s, into the early '80s, Dr. Makato Kawase, of the Ohio State University, did research on root-promoting compounds, and he discovered what we now know as willow water. He worked with birch and spruce species which were known to be exceedingly difficult to root from cuttings, with or without the use of synthetic auxin hormones. He made an extract of willow stems (willow is preposterously easy to root from cuttings with no pretreatment), and soaked the birch and spruce cuttings in the extract. Afterward, the spruce and birch cuttings rooted at a high rate of success. That much is good science. For an article by Dr. Kawase, here's a reference:

Southern florist and nurseryman. South Florist Nurseryman Sept 11, 1981. v. 94 (26)
p. 17-18, 23, 25, 29.

Unfortunately, Dr. K. died rather young, and his research was not carried on by other researchers. His theory was that the formation of adventitious roots requires two trigger materials -- auxin (already well-known) and a second, previously unknown substance, which he named "rhizocaline." This name, and its concept, were first introduced by Bouillenne and Went in 1933, but their rhizocaline was later demonstrated to be IAA (the most common naturally occurring auxin). Kawase's concept, though, is that his "rhizocaline" is a different, non-auxin substance, and that a plant needs some of both, to successfully make adventitious roots. He never elucidated the chemical nature of the substance; he only postulated that it must exist, and it must be present in willow.

So plant species may be divided into several groups.

1. This group has plenty of endogenous (internal, naturally occurring) auxin and rhizocaline. This group roots easily from cuttings with no special treatment.

2. Another group of plants has adequate endogenous rhizocaline, but lacks sufficient auxin. This group is difficult to root from cuttings without an added auxin-based hormone, but if treated with the hormone, can be rooted easily. It is this group for which the commercial rooting powders and liquids are most useful.

3. A third group has adequate auxin, but lacks rhizocaline. This group is difficult to root, with or without auxin-based hormone treatment, but can be rooted easily after a treatment with willow extract.

4. A fourth group lacks both auxin and rhizocaline, and the way to successfully root these cuttings would be to treat with willow water, followed by an auxin-based compound.

5. The last group lack the ability to make adventitious roots, regardless of treatment, and so cuttings always fail no matter what you do.

To date, no one has discovered what the mysterious substance in willow water is. What we do know is that it is NOT auxin, and it does not substitute for auxin. We did some work with it here at FSC (see link below), with roses. We found that willow water alone did not promote rooting, but in the particular rose we were using, it enhanced the effect of auxin-based root promoters. Also, one of our chemistry professors (who has a hobby of growing camellias -- a genus for which willow water is popular) did quite a lot of work trying to discover the active ingredient. Like others, he was not successful.

So there's the "good science" of willow water. Because of those successes, it has taken on rather a cult following. People "believe" in it in the religious (as opposed to scientific) sense. Among its fanatical advocates are rose, rhododendron, and camellia growers. Much of what they write may be true, as it relates to better rooting success. But their reasons for its working are often specifically not true. Their writing usually suggests either that
1. the active ingredient is natural auxin (IAA) or
2. the active ingredient is aspirin or some other salycilate.

Both of these conjectures are almost certainly false. We did quite a lot of work with aspirin and other salycilates (I should say Dr. Spencer, our chemist, did), with no positive results at all. And for that reason, the concept that willow water "works just as well" or "is better than" commercial auxin compounds, is not necessarily true, and the comparative result will vary wildly among species you're attempting to root.

For any who want to try willow water for yourself, here is the "standard" method:

1. Collect stems of nearly any species of willow (Salix spp.). Weeping willow (S. babylonica) is probably most popular, but we use S. caroliniana with good success.

You want young first-year twigs, with green or yellow bark; not old enough to develop brown or gray bark.

2. Strip off and discard all the leaves. All you want are the twigs. Cut the twigs into 1" lengths. Now you have what looks like a pile of small matchsticks.

3. Add enough water to barely cover your twigs. At this point, methods vary among workers. You can either heat the mixture almost (but not quite) to the boil, and brew it like tea, letting it soak until thoroughly cool, and for several hours more, OR you can not heat it, and just let it soak, like "sun tea" for several days, in the room-temperature water. In either case, when the liquid develops a greenish-yellowish-brown color, rather like weak tea, You filter off the solids, keeping the liquid. It will keep in the fridge for several weeks, or may be used immediately.

4. When ready to root your cuttings, make a fresh cut at the base of the cutting, and place it in the willow water, like flowers in a vase. Leave it there several hours, so it has time to take up a significant amount of the willow water. At the end of the soak time, you can rewound the base and apply an auxin-based hormone, or not, depending on the type of cutting. Then root the cutting in your normal way (we use intermittent mist).

On the efficacy (or lack thereof) of auxin-based rooting compounds, as I've said, I've never seen much effect from Rootone. I think it's just too weak for most of the woody cuttings that I tend to want to root.

The other powders, two brands of which are Hormodin and Rhizopon (there are others) rate their concentrations by the use of numbers, and lucky for us, everyone seems to use exactly the same numbering system. So their #1 compound is always 0.1% IBA in talcum powder, #2 is 0.3% IBA in talc, and #3 is 0.8%. Dip-'N-Grow, a liquid, is 1.0% IBA plus 0.5% NAA dissolved in alcohol, and you dilute that down to whatever concentration you want.

With roses, citrus, and other woodies, I generally find that the #1 powders, or DnG diluted to 1:10 or 1:20, are about as useless as rootone. However, the #2 and #3 powders, or DnG diluted to 1:4 or 1:5, often give dramatic increases in percentage of cuttings that root at all, and number and size of roots produced by those cuttings which do root.

There has been research to show that with these compounds, as you increase the concentration of the auxin, the rooting percentage increases, up to some critical concentration (which varies from one plant to another), at which the material becomes toxic and the cuttings are killed. So for any given species or cultivar, the trick is to find out how high a concentration you can get away with, short of killing the cuttings, and then back off just enough to avoid the toxicity. At that high concentration, then, you'll maximize your rate of success.