Chapter 20

Vegetative Plant Propagation

Managing the Vegetative Propagation Environment

Most herbaceous annual and perennial propagation occurs from December to March in the Northern Hemisphere. An ideal rooting environment for vegetative propagation 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.

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

Some seeds require light to germinate while others require darkness. Examples of plants requiring light for seed germina­tion are ageratum, begonia, browallia, impatiens, lettuce, and petunia. Conversely, calendula, centaurea, pansy, annual phlox, verbena, and vinca germinate best in the dark. Other plants do not respond to any light conditions and will germinate in light or darkness. When sowing seed that requires light, these small seeds must be at or near the soil surface to be exposed to light, which enables germination to occur.

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 greenhouse. Shade cloths are rated by the amount of shade they produce, ranging from 30 to 80 percent.

Daylength (Photoperiod)

Higher plants are classified as long-day, short-day, or day-neutral, based on the effect of photoperiod on initiation of reproductive growth. Long-day plants, which flower chiefly in the summer, will flower when the critical photoperiod of light is equaled or exceeded; short-day plants, such as chrysanthemums, flower when the critical photoperiod is not exceeded. Reproductive growth in day-neutral plants is not triggered by photoperiod. Photoperiod influences virtually every aspect of a plant's life cycle including seed germination, stem elongation, leaf expansion and number, branching, rooting of cuttings, bulb formation, flowering, and resumption of growth in spring to name a few plant responses. With most photoperiod responses there is generally a critical night-length at which a transition occurs from one response to another. Examples include flowering versus vegetative growth, seed germination versus seed dormancy, or active plant growth versus dormancy.

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.

Daily Light Integral

Daily Light Integral (DLI) is the amount of photons (light particles) received by a particular area in one day. DLI greatly affects plant growth and development, and the measurement of DLI is important to many growers. DLI can be measured, and its measurement is particularly important to commercial farmers and gardeners that use greenhouses.

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.

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.

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

The root media in which a plant grows must be of good quality. It should be porous for root aeration and drainage, but also capable of water and nutrient retention. In order for a plant to form a new root system, it must have a ready moisture supply at the cut surface. Oxygen, of course, is required for all living cells. The coarse-textured media choices often meet these requirements. Most commercially prepared mixes are termed artificial, which means they contain no soil. The basic ingredients of such a mix are sphagnum peat moss and vermiculite, both of which are generally free of diseases, weed seeds, and insects. Rooting media for asexual propagation should be clean and sterile.

Pathogens and Monitoring

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.

Click on the following topics for more information on vegetative plant propagation.