Vegetative Plant Propagation
Propagation Environment for Cuttings
An ideal rooting environment for cuttings can be treated similar to seedlings. The propagation environment in the greenhouse is one that maintains hydrated cuttings, minimizes stress, prevents disease, and promotes rapid root formation to support a growing and transpiring cutting. If appropriate cultural and environmental factors are maintained, most annual and perennial unrooted cuttings can become fully rooted liners within two to four weeks. The critical environmental factors to manage during rooting are discussed in the following sections.
Relative Humidity and Airflow Management
It is recommended maintaining the relative humidity in a propagation house at a minimum of 85 percent. This can be done with mist or fog systems. If environmental conditions are ideal (i.e., warm medium temperature, humid still air, and adequate DLI) requirements for misting or fogging should be minimal and frequency can be low. As a general rule, mist should be applied often enough to prevent the unrooted cuttings from wilting and long enough so water evenly coats the leaf surface but does not drip off.
Light Management
Vegetative cuttings require a minimum quantity of light to provide the energy for root initiation and development. Light intensities below this minimum result in arrested root development, leading to a delayed crop or rooting failure. Conversely, too much light can reduce root formation due to excessive stress on the cuttings, and lead to bleached or scorched leaves.
Light Intensity
Unrooted cuttings require indirect, diffused sunlight but never full sunlight. To initiate and develop roots, provide cuttings with sufficient light to support photosynthesis but not so much as to stress the cutting. If light intensities are too low little or no root development will occur, leading to a delayed crop or rooting failure. Conversely, too much light can bleach leaves and reduce root formation due to excessive stress on the cuttings. Generally, light is minimized initially and then gradually increased as the cuttings develop roots.
Daylength (Photoperiod)
Photoperiod is often not controlled during the propagation of vegetative annual or perennial unrooted cuttings. In certain annual plants, flowering is promoted by certain photoperiods (i.e., long days for petunia and argyranthemum) and should be managed to prevent premature flower induction during propagation or finishing.
Light Quality
Light quality (which is commercially manipulated through greenhouse spectral filters, greenhouse coverings, and varying supplementary light sources) plays an important role in seed germination and vegetative propagation.
Temperature Management
Air temperature and especially medium temperature are important for callus and root development. A desirable medium temperature for most species during Stage 1 (callus formation) is 73 to 77 degrees F (23 to 25°C), which usually requires bench heating. Air temperature should be maintained between 68 to 73 degrees F (20 to 23°C) when bottom heat is utilized. However, if bottom heat is not available, air temperature should be increased to 77 to 80 degrees F (25 to 27°C) so that medium temperature is adequate.
Moisture Maintenance
There are many methods for providing moisture to unrooted cuttings during propagation. The idea is to provide cuttings with enough moisture to minimize wilt and keep humidity as high as possible, thereby reducing transpirational water loss. However, a slight degree of stress is useful in encouraging root initiation and development. The most common watering system used in propagating unrooted cuttings is intermittent mist. Mist systems minimize lowers the leaf-to-air vapor pressure gradient and slows down leaf transpiration. Mist also lowers ambient air temperature, and the cooler air consequently lowers leaf temperature by advection, in addition to cooling occurring through evaporation of the applied film of water. Since intermittent mist lowers medium temperature, suboptimal temperatures can occur, which reduce rooting.
Carbon Dioxide Enrichment
With many species, carbon dioxide enrichment can increase the number of cuttings that can be harvested from a given stock plant, but there is considerable variation of rooting response among species. Principal reasons for increased cutting yields are increased photosynthesis, higher relative growth rate, and greater lateral branching of stock plants.
Rooting Media
Rooting media for asexual propagation should be clean and sterile. Cuttings are not susceptible to damping-off, but they are attacked by other fungi and bacteria which may come along in the medium. Most commercially prepared media are clean when purchased. The media should be low in fertilizer. Excessive fertility will damage or inhibit new roots. High-quality artificial mixes sometimes contain slow-release fertilizers. Coarse perlite alone can be used to start some cuttings. This doesn’t hold much water for long, but it is fine for rooting cuttings which would ordinarily rot in higher moisture media.
Fertitilization
Commercial operations often apply moderate levels of controlled release macro and micro elements to the propagation media—preincorporated into the media—prior to sticking cuttings in the media. During propagation, supplemental fertilizer is added by top dressing (broadcasting) with controlled-release fertilizer, by injecting gradually increasing concentrations of liquid fertilizer (fertigation), or by a nutrient mist.
Disease Control
Fungi and bacteria are plentiful in the greenhouse. Pythium, Penicillium, Rhizoctonia, Pestalotiopsis, Glomerella, Anthracnose, and Botrytis are just a few organisms that cause havoc during propagation. Of these, Botrytis is usually the most common and destructive. Preventive fungicide applications and regular rotation of chemicals can keep most diseases in check. There are several other non-chemical disease control techniques
Hardening Cuttings
A few weeks after striking cuttings into the rooting environment, they should be inspected for root development. When cuttings have developed adequate root systems, they need to be hardened for transplanting outside the rooting environment. The goal is to condition stem and leaf tissues and promote secondary root development before transplanting. This can be accomplished by gradually reduce the misting/fogging frequency over a period of 3 to 4 weeks.
Click on the following topics for more information on vegetative plant propagation.
