This practice is especially important when handling hard-to-root species. The most effective and widely used root-promoting hormone is indolebutyric acid (IBA). IBA may be used in combination with naphthalene acetic acid (NAA), but the latter is less effective when used alone. Rooting hormones may be formulated as a powder (dust) or liquid (solution) and available as ready-to-use, or can be prepared to the desired strength from powders or salts. To avoid contamination in case any of the cuttings are diseased, they should not be dipped into the same container of powder or solution. For powder application, a puffer duster (squeezable bottle) may be used; a spray bottle may be used for liquid applications to spray the ends of the cuttings. Cuttings should never be dipped into the original container of the hormone. The amount needed should be poured out or removed and any leftover amount discarded. Rooting hormones may be mixed with fungicides to protect the cut surface from rotting under humid conditions. Fungicides are especially helpful in species that require a long period of time to root. A suggested ratio of hormone to fungicide (e.g., captan) is 9:1 by weight. Reagent grades of these hormones are not water soluble, requiring the use of alcohol as solvents. Salts of IBA and NAA, though more expensive, are water soluble, but require more of the substance to be effective. Prepared solutions store well for a long time at 40–45°F in darkness (wrap bottle with aluminum foil and place in a fridge.) Juvenility The physiological age of the cutting significantly affects its rooting success. Cuttings from mature woody and fruiting stock plants are less successful than those from newer shoot growths. The juvenile stage is thus more conducive to rooting. Cuttings are commonly made from one-year-old plant parts. Some propagators prune stock plants to obtain fresh growth as needed. Position of Plant Part The position of plant parts used as cuttings plays a role in their capacity to root. Lateral shoot cuttings root better than terminal shoot cuttings. 10.6.2 ENVIRONMENTAL FACTORS Time of Year Cutting Is Made Whereas cuttings may be taken any time of the year for herbaceous and perennial plants, hardwood cuttings root better when the materials are obtained in late winter when plants are dormant. Similarly, softwood cuttings from deciduous woody plants are best taken in spring; semihardwood cuttings root best when obtained in midsummer. Darkness Darkness promotes etiolation in plants. Although this condition indicates improper development, materials obtained from etiolated plants root better than plants exposed to full light. A practical application of this condition is the keeping of basal plant parts of shoots to be used for cuttings in darkness for a period of time. This technique may be applied in species that are very difficult to root. Light Even though etiolated plant materials root better than normal ones, rooting cuttings in full light under a mist makes them root more quickly. Optimal conditions for photosynthesis are needed for producing healthy liners. However, when light intensity is excessive, plants may be in jeopardy of moisture stress. Producing short and stubby products. During such periods, shading is required. Similarly, in winter, supplemental lighting is required. 10.6 Factors Affecting Rooting of Cuttings 319
Temperature 320 Chapter 10 Asexual Propagation Bottom heat in cutting beds warms the soil to induce quicker rooting. It is important to have adequate rooting before the shoot starts to grow vigorously. The bottom heat is usually about 6°C (10°F) higher than the air temperature, which should be maintained between 18 and 27°C (65 and 75°F). Supplement heating is critical during cold periods to avoid injury. Moisture and Humidity Intermittent mist and fog system, controlled by a timing device, are commonly used for propagation. The misting frequency and duration depend on light, radiation, and air temperature. Cuttings do not have roots and hence are unable to absorb moisture. However, the exposed plant parts are subject to evaporation. To reduce moisture loss, cuttings are generally maintained under conditions of high moisture by misting them. Initially, misting may be required almost continuously. As time goes by, the misting schedule is modified, being less frequent and less intense. In some greenhouses, a mist is replaced by a fog. Nutrition No fertilization is needed for unrooted cuttings since they can take up very little of it. Light fertilization may be applied through the mist. Once rooted, a complete fertilizer application of 20:20:20, for example, at a rate of 1 pound (0.45 kilogram) per 100 gallons (378.5 liters) of water may be applied. Rooting Medium Certain species, including coleus, African violet, and Philodendron scandens, can root in water alone. Most cuttings are rooted in solid media that must be sterilized, freely draining, and of good moisture-holding capacity. Sand is well drained but poor in moisture-holding capacity. If sand is used, the rooting should be conducted under shady conditions and should be hand watered. Vermiculite and perlite make good propagation media, especially when combined with peat most. The medium for propagation must be very loose to enable rooted cuttings to be readily removed from the rooting medium and planted with little loss of roots. Propagation may also be conducted in outdoor conditions, provided the medium is well drained and aerated to avoid saturated water conditions. In place of the conventional rooting media, a variety of preformed, lightweight materials are widely used. These media include rockwool media, compressed peat pellets, and other artificial materials marketed under various brand names such as Oasis root cube, Oasis wedge, and Horticubes. Sometimes cuttings are rooted directly in the finish pots. Species such as geranium and poinsettia are propagated in this way. Sanitation Cuttings have exposed surfaces and hence are prone to disease attack. The propagating medium must be sterilized before use. Steam sterilization is effective in controlling most soilborne diseases. 10.6.3 TAKING AND PREPARING CUTTINGS Select the material according to age and size. Cuttings from plant parts that receive optimal sunlight are most desirable. If cuttings are obtained from stocks in the field, they should be wrapped in moist burlap or other suitable material to reduce water stress, and be kept out of sunlight as well. If cuttings will not be prepared right away, the material should be stored in a fridge or a cool place, or misted.