Vegetative Plant Propagation
Propagation Environment for Plant Cuttings
An ideal rooting environment for cuttings can be treated like 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 2 to 4 weeks. The critical environmental factors to manage during rooting are discussed in the following sections.
Humidity and Management
It is recommended to maintain 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 daily light integral), requirements for misting or fogging should be minimal, and frequency can be low. Achieving optimum humidity and medium moisture can be one of the most challenging aspects of successful propagation with cuttings.
Vapor pressure Deficit
A more useful and absolute measurement of humidity is vapor pressure, which is the measure of water vapor (gaseous form) in the air, and vapor pressure deficit (VPD), which is the difference between the actual vapor pressure (moisture in the air) and saturation vapor pressure (maximum water vapor pressure or how much moisture the air can hold when saturated).
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. Several methods are used to reduce the amount of light coming into the propagation space, such as shade cloth, retractable shade curtains, and whitewashing the exterior of the greenhouse.
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.
Moisture Maintenance
here 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 the leaf-to-air vapor pressure gradient and slow 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.
Temperature Management
Air temperature and especially medium temperature are important for callus and root development. In grafting, heating devices are sometimes placed in the graft union area to speed up graft union formation while the rest of the rootstock is kept dormant under cooler conditions. It is often more satisfactory and cost-effective to manipulate temperature by bottom heating at the propagation bench level rather than heating the entire propagation house. A desirable medium temperature for most species during Stage One (callus formation) is 73 to 77 degrees F (23–25°C), which usually requires bench heating.
Rooting Media
Rooting media for asexual propagation should be clean and sterile. 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. Coarse vermiculite alone has excellent water-holding capacity and aeration but may dry out rapidly via evaporation if not covered in some way. A mix of 50 percent peat moss and 50 percent perlite favors good aeration. An equal mix of peat moss, vermiculite, and perlite is also good and favors moisture retention. Different rooting media components are used depending on the species being propagated. Commercial mixes are available that often include fine Canadian Sphagnum peat moss, perlite, or vermiculite to further improve air porosity and resist reduce compaction.
Fertitilization
Commercial operations often apply moderate levels of controlled release macro and micro elements to the propagation media—pre-incorporated into the media—prior to sticking cuttings in the media. The media should be low in fertilizer. Excessive fertility will damage or inhibit new roots. Typically, growers will start out with a 50 to 100 PPM nitrogen solution once after sticking. When cuttings begin to callus, apply another 50 to 100 PPM nitrogen drench and also again at 100 to 200 PPM nitrogen once roots emerge from the callus tissue. When roots grow to the bottom of the plug, constantly feed at 100 to 200 PPM nitrogen.
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.
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.
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