Take a look at new shoots growing on a favorite shrub or vine and you’ll see that the bases of these shoots may be beginning to toughen up, their once soft, green outer layer turning brown and woody.
Such shoots, snipped from the mother plants as so-called half-woody cuttings, can be rooted to make new plants.
Cuttings made from shoots still soft and green are called softwood cuttings. Those cut from thoroughly woody, leafless shoots taken in winter are called hardwood cuttings.
Plant hormones, called auxins, play an important role in rooting any of these kinds of cuttings. Under natural conditions, auxins are produced in the buds and growing tips of plants, and are then carried down stems in decreasing concentrations. If you apply auxins directly to the bases of your cuttings, rooting can be hurried along, and you get an increase in the percentage of cuttings that root as well as the number and quality of roots that form.
Natural auxins, once extracted from a plant, decompose too quickly to be of practical use. But we can still use auxins by applying commercially available, synthetic ones, such as IBA and NAA, which are slower to degrade.
These synthetic rooting hormones are available either in liquid or powder form. Concentrations and combinations of auxins will vary with the manufacturer, with higher auxin concentrations generally used for more difficult-to-root species. Fungicides may be added to prevent cuttings from rotting.
Apply a powdered rooting hormone by dipping the base of a cutting into the powder, tapping it to shake off excess, and then sliding the cutting into a hole without brushing off the powder. Because varying amounts of powder might adhere to a cutting, rooting response to powdered hormone preparations is less consistent than it is to liquid ones.
Liquid rooting hormones also are more rapidly absorbed than powdered formulations. Merely soak the bases of cuttings in the liquid formulations. Soaking time required depends on the hormone concentration.
Whether you use powders or liquids to promote rooting, pour only as much material as you need into a clean container, then discard that portion after use. Eventually, the materials do deteriorate. An easy way to test whether a material is still good to use is to snip a leaf from a tomato plant, treat its base, then poke the base into moist sand or potting soil. If the preparation is still good, abundant roots will have formed after a couple of weeks.
You don’t have to go out and buy rooting hormone to root cuttings. Before commercial, synthetic auxins became available, some savvy gardeners would help rooting along by soaking cuttings in water in which stems of willow, an easy-to-root plant, had previously been soaked.
Rooting hormones, whether from a package or from willow stems, are not essential in propagation, nor do they perform magic. No need to use them on species that are easy to root.
The ultimate size and vigor of a plant will be unaffected by whether or not hormones were applied during propagation.
And always follow directions closely for a particular rooting hormone preparation because hormones can be toxic to plants at certain doses. Another synthetic auxin – 2,4-dichlorophenoxyacetic acid – is better known as 2,4-D, a widely used weed killer.