For Propagating Roses
Rooting hormones are plant growth substances that are applied to influence root formation in cuttings. Although such products, including RooTone, are familiar to many rosarians, less seems to be known as to what they contain and how to use them effectively in rooting rose cuttings. The purpose of this article is to fill in a few of these gaps.
What's in a Rooting Hormone?
There are a number of plant growth regulators that have been shown to influence the rooting of cuttings. The most important of these regulators is auxin, named after a Greek word meaning "to increase". Auxin is involved in many growth processes and its principal function is to stimulate increases in cell length. It also appears to reverse the cellular process of differentiation which is the process whereby cells become specialized in their function. In simple terms differentiation is the where cells become roots, stems or leaves. By reversing the process, auxin in effect, returns the cells of stems and leaves to "square one" so that they can become roots instead. It has been confirmed by many scientific studies that auxin is required for the initiation of roots on stem cuttings.
The chemical name for the auxin naturally produced by a plant is indole-3-acetic acid ("IAA") which is synthesized by the plant from the amino acid L-tryptophan. It was first isolated and identified in 1934, by the scientist F.W. Went at the University of Utrecht in the Netherlands.
After the discovery of IAA, two synthetic auxins, indole-3-butyric acid ("IBA") and naphthaleneacetic acid ("NAA"), were discovered in 1935 to have the same functions as IAA and to be more effective in rooting stem cuttings. Later it was shown that IBA is also a naturally occurring substance in pants. Also both have been shown to be more stable than IAA. As a result IBA and NAA are the active ingredients in most commercially available rooting hormone products.
Types of Rooting Hormones
Rooting compounds are available in various concentrations either as a power or in a solution. In each case the cutting is dipped into the compound. Opinions vary as to which are most effective. Some believe that powder is more effective because it causes the hormone to better stick to the stem. Others believe that the hormone is absorbed better when it is in a liquid form. I have tried both and have observed no differences except that liquid is more convenient to use.
The most familiar product to home gardeners is the powder RooTone, now distributed by Cooke Laboratory Products, Portland, OR. The original RooTone contained IDA, however the product currently on the market contains 0.2% NAA and 4.04% Thiram. The latter is a fungicide used to protect against damping off, seedling blights and to inhibit mold. Another widely available product is Dip 'N Grow, a liquid concentrate distributed by Astoria-Pacific Inc. It contains a combination of 1.0 percent IBA and 0.5 percent NAA and is mixed with water in varying concentrations.
Rhizopon AA powder and water-soluble tablets are available from Hortus USA Corp., New York. A variety of rooting powders are available: No. 1, No. 2 and No. 3, each used for dry-dip application. The soluble tablets are used for liquid dip methods. The powder and tablets contain various concentrations of IBA.
Another product containing IBA that is used by rosarians is SuperThrive by the Vitamin Institute, North Hollywood, CA. Although the peculiar labeling of this product is hardly a model of clarity, it does reveal that it contains 0.09% Vitamin B-1 and 0.048% NAA. The latter is a very small concentration of NAA by the standards of the rooting hormones marketed as such. Other rooting hormone products that are commercially available include Woods Rooting Compound, a solution manufactured by Earth Science Products Corp., Wilsonville, OR which contains 1.03 percent IBA and 0.56 percent NAA. Also there is Hormex powder [Kitty's favorite], manufactured by Brooker Chemical Corp. with six concentrations, ranging in IBA percentages from 0.1 percent to 4.5 percent. Similarly, Hormo-Root powders are available in four concentrations ranging in IBA percentages from 0.1 to 0.4% IBA.
How to Use Rooting Hormones
The concentration recommended by the manufacturer for various types of plants is usually set forth on the label of the rooting hormone. These labels, however, like most chemical labels these days, are often so full of warnings and other information, that it is difficult to find the answer to the question of how much to use. Further the recommendations are often quite general and not specific enough for roses.
In this regard some important new data has been reported that suggests that rooting hormones are more effective at higher concentrations than those recommended by the manufacturer. Dr. Malcolm Manners, Chairman of Florida Southern College has reported on an experiment conducted by his introductory horticulture class in rooting cuttings of 'Fortuniana', a popular rose rootstock in Florida. The rooting hormones Dip 'N Grow and Rhizopon were used in a number of different concentrations. In general, the experiment demonstrated that better rooting was achieved with the more concentrated rooting hormones. The best rooting occurred with Rhizopon #3 powder (0.8% IBA in talc) and Dip 'N Grow at 1/5 dilution. Complete results, for those interested, are reported in detail on Dr. Manners' web site at http://members.aol.com/mmmavocado/MMMspage.html
Prior to reading the study by Dr. Manners it had been my practice to use Dip 'N Grow as my rooting hormone of choice. My reading of the label had indicated that the proper concentration was at a mixture of 1/10 dilution. Although I have had considerable success at this concentration, the study by Dr. Manners has convinced me to increase the concentration to 1/5 dilution. I was also encouraged by his findings that no damage was observed to cuttings at higher concentrations.
It is also of note that the study conducted under the auspices of Dr. Manners showed considerable value to first wounding the stems of the cuttings before dipping them in the rooting hormone. By "wounding" it is meant that thin strips of bark are removed at the base of a cutting with a sharp knife. In general, the students found that the more wounding they did, the better the rooting success was. The best rooting occurred with 2 strips of bark removed. No reason was suggested for this result however it is likely that the wounding exposes the lateral meristems which extend the length of the stem. This is the area where active cell division takes place and a reasonable theory is that by exposing the lateral meristems to the rooting hormone cellular differentiation into roots occurs.
The findings by Dr. Manners confirmed my own practice in this regard. I have in the past tried rose cuttings without wounding and others where I have crushed or mangled the stem end of the rose. Later, I attended a seminar that suggested that wounding by slicing off strips of bark was more beneficial and my own experience has been a significant increase in takes by using this method
Are Rooting Hormones Necessary?
Those reading about roses will inevitably encounter reports of successfully rooted rose cuttings without the use of any rooting hormones. From such reports some rosarians have concluded that rooting hormones are not really necessary. Are they?
The scientific literature is clear that auxin is essential to the initiation of root formation in cuttings. But it should be kept in mind that auxin is a naturally occurring growth regulator in roses. The amount and the location of auxin will vary depending on the variety of rose, the season in which the cutting is taken, as well as other environmental factors. Thus, as Dr. Manners observes, some roses, notably many of the Teas and Chinas, seem to be inherently easy to root from cuttings, almost regardless of the method one uses. Others are more difficult, and some are exceedingly difficult to root. So the answer to the question of whether rooting hormones are necessary is, as it is with many other rose questions: "it depends".
The problem is that it is difficult to know in advance which cuttings will root on their own and which require the boost of a rooting hormone. And, since rooting hormones are cheap and easily applied, the best approach is simply not to guess but to make sure by using a rooting hormone. This is the practice of commercial nurseries and is my recommended practice for those seeking to root rose cuttings at home.
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