From time to time, in various parts of the world, I have stuck a willow branch into the ground and watched with admiration as it quickly manufactured some new roots and set about becoming a tree.
Apparently these willows had been trying to tell me something about rooting, but I hadn't been paying much attention. Fortunately, Dr. Makota Kawase, professor of horticulture at the Ohio Agricultural Research and Development Center in Wooster (Ohio State University) was more attentive - though even he describes his discovery as something of an accident.
The discovery: that the willow tree contains a substance that can induce rooting in cuttings of all other plants, including hard-to-root species. Cherry, oak, and beech have all rooted in trials at the research center, while easy-to-root species have done so more abundantly. The particularly nice thing about the discovery is that the home gardener can extract the root-promoting substance simply by soaking willow twigs in water.
Dr. Kawase has yet to identify the rooting substance, but its effects are now well established. In one experiment, mung bean cuttings produced 12 times as many roots in willow-water extract using only 1/3-ounce of willow twigs as the control using plain water. A more concentrated extract produced about 100 roots on 2 inches of mung bean stem while controls produced 4 or 5 roots.
As so often happens in the sciences, the rooting discovery came almost by chance. Some willow twigs were left soaking in a bowl of water when a member of Dr. Kawase's team needed water to moisten some soft wood cuttings. He took the needed water from the bowl containing the willow twigs.
Later it was noticed that the cuttings sent out an extraordinarily high number of roots. In seeking the reason the scientists came back to the willow water. The trial was repeated several times and the team came up trumps every time.
Having established the existence of a root-promoting substance in the willow, the need now is to identify it from among the many compounds that are dissolved in the water. Apparently it is not a rooting hormone, or at least not one that is known to agronomists.
Current experiments are aimed at chemically separating each of the several compounds that are dissolved from the willow. When the exact root-promoting substance is isolated, commercial production will probably then take place.
Meanwhile the home gardener can do what Dr. Kawase and his team originally did - take some willow fronds, remove the leaves, and cut them into small pieces. Soak these twigs in water for 24 hours. Then use them to root cuttings of your favorite specimens.