Note: There have been many excellent articles written detailing various procedures and methods of taking dahlia cuttings. This article is intended to help growers recognize and solve the most common problems encountered when taking cuttings.
Dahlias can be propagated from seeds, tubers, or cuttings. Seeds will produce varieties different from the parent plants, so propagation from seeds is used primarily to develop new cultivars. Propagation using tubers is the method most commonly used by, and familiar to, dahlia growers. Tubers will produce a clone, or a plant exactly like the plant of the “mother” tuber. Propagation of dahlias by taking cuttings is the best method to increase stock of a favorite plant or to over-winter varieties that have tubers that are difficult to store. In addition, cuttings are clones of the mother tuber so they provide a quick and easy way to increase the stock from a single tuber. Cuttings transplanted to the garden produce healthy tubers that store well over the winter.
Success with cuttings depends largely upon the proper manipulation of three environmental conditions: humidity, temperature and day length
After the cuttings have been dipped in rooting hormone and placed in the rooting medium (either sterile soil or sterile rooting cubes) they should be placed in a humid environment until well rooted. The humidity should be high enough to keep the cuttings from wilting, but not so great as to add moisture to the rooting medium. The rooting medium should be kept damp, but not saturated. Light misting with a sprayer can help increase the humidity if it is necessary. The temperature of the air surrounding the cuttings and of the growing medium itself should be kept between 65 to 75 degrees.
Dahlias are photosensitive, and this fact is often overlooked in the process of taking cuttings. Most varieties of dahlias will develop tubers when the day length is 12 hours or less, and will develop feeder roots when the day length is longer than 12 hours. To assure that feeder roots develop, I recommend rooting cuttings with the day length artificially set to 14 hours or longer. The intensity of light necessary to initiate the chemical reaction, which tells the plant “how long the days are”, is not, the same intensity required for plant growth. It is only necessary to provide 10-20 foot-candles of light during this period. A fluorescent 40-watt bulb, or incandescent 100-watt bulb, 4 to 5 feet above the cutting bed is sufficient. If you are rooting cuttings indoors, and are already supplying a stronger intensity of light necessary for growth period, then just leave the source of light on for at least 14 hours a day. If you are taking cuttings in a greenhouse or outside, then supplement the natural daylight with additional light so that the total length of light is 14 hours or greater.
To summarize: Use a rooting hormone and a sterile rooting medium, keep the humidity high enough to prevent wilting and the temperature between 65-75 and provide daylights of 14 hours or greater. Most dahlia varieties should form roots within 10-14 days when the proper environmental conditions are provided.
Below is a list of some of the problems that might be encountered while taking dahlia cuttings.
1. Symptom: Cuttings turn black and rot at the soil line.
Cause: Damping off, caused by too high temperature or a rooting medium that is not sterile.
Solution: Keep the temperature at 65-70 degrees. Use only sterile soil or sterile root cubes.
2. Symptom: Cuttings root rapidly, but then wilt and die.
Cause: Usually caused by waterlogged soil, which deprives the roots of oxygen.
Solution: Keep the rooting medium damp, but not saturated.
3. Symptom: Cuttings look good, but develop callused looking nodes instead of roots – even after a month in the rooting medium.
Cause: Day length too short.
Solution: Artificially provide at least 14 hours of light each day.
4. Symptom: Rooted cuttings, planted in the field, develop club shaped tubers with few eyes.
Cause: Tubers allowed to develop in small pots during short day lengths.
Solution: Re-pot cuttings as the roots touch the sides of the pot. The cutting should be grown to 8-12 inches in short day lengths with the plant lying down in a 2-3” trench and all leaves covered except the top set uncovered in the trench. At each node well-formed tubers will develop
5. Symptom: Rooted cuttings, planted in the field, develop hair roots but few, if any, tubers.
Cause: Cuttings were grown only in long day lengths.
Solution: Short day lengths are necessary for tubers to develop properly.
This article originally appeared in the March 2004 Bulletin of the American Dahlia Society.
– Bill McClaren