Chapter 19

Plant Propagation from Seed

Optimizing Seedling Production

The principal objective of seedling production is to develop healthy, stocky, vigorous plants capable of further transplanting with little check in growth. Ongoing management of environmental conditions (germination facilities, substrates, air circulation, moisture delivery, and fertilization) is required for seedlings development.

Germination Facilities

Indoor seedling production occurs in several types of structures including greenhouses, cold frames, and hotbeds, as described in Chapter 1, Greenhouse Structures and Design. Some bedding plant operations use growth chambers or commonly referred to as growth rooms where seed flats are placed on carts or shelves in an enclosed area and subjected to controlled environments for germination prior to being moved to the greenhouse. These chambers can range in size from a growth cabinet with room to germinate a dozen or more trays of tray plugs (See Figure 19.11) to rooms several hundred square feet in size designed to accommodate multiple tall carts of plug trays (See Figure 19.12).

Germination Media

The germination media requirements for plug trays are quite different from those for flats. Flats hold a relatively large volume of germination medium that supplies water and nutrients for a relatively long time. By contrast, cells in plug trays hold very small volumes of germination medium (especially the 512 and smaller plug trays). Nearly all germination mixes for plugs are soilless. Consequently, because of the small soil volume of each cell, the medium components must have a small particle size. Small seeds should have a finer and more compact medium than is used for larger seeds. Sphagnum peat moss, vermiculite, and perlite are the most commonly used components, but finer grades are employed.

Temperature Control

There is no environmental factor more critical than optimal temperature control for seed propagation. For instance, seed dormancy is broken in some plant species by cool moist stratification conditions that allow the germination process to proceed. The temperature of the air and the propagation medium must be carefully controlled. Although air temperature has a significant effect on root zone temperature, the root zone can be significantly (10°F, 5.5°C or more) cooler than the ambient air temperature in the greenhouse. For this reason, growers often use a heating system to specifically increase the root zone temperature. The bottom heat provided should be 5 to 10 degrees F (3 to 7°C) warmer than the air temperature.

Light Control

For almost all plug crops, light intensity should be as high as possible while maintaining the proper temperature, humidity and moisture level. Because many seedlings are produced in the winter and early spring, once seeds germinate, light quantity is limiting in many parts of the country. Supplemental lighting is usually cost effective for seedling production, since it improves plant quality and greatly improves rooting, which thereby reduces time from germination to transplant.

Irrigation

Water management is the dominant factor in controlling plant height. Knowing when to water generally is determined by experience. Plant seedlings should be watered only when needed, and the plug allowed to dry sufficiently before additional watering. The media should be kept moist but not continually wet. Excessive watering leads to succulent plants with restricted root growth. One of the most common problems in greenhouses is overwatering. Not only does this contribute to poor plant growth and health, it also encourages the spread of pathogens that thrive in wet conditions. Several systems for delivering water to seed flats and plugs are available including automated irrigation systems.

Mist Systems

Continuous misting systems are not recommended because they lower temperature of the rooting medium, resulting in slow rooting, and cause excessive leaching of nutrients from the foliage (See Figure 19.13). In addition, too much water can reduce oxygen in the rooting medium and create a favorable environment for decay-causing organisms. Intermittent mist (i.e., “on” for a specified and “off” for a specified period) reduces leaching of nutrients and helps prevent excessive water in the medium.

Fog Systems

A typical fog system uses a high-pressure pump, distribution piping and nozzles that break the water stream into very fine droplets (See Figure 19.14). Piston pumps are needed to develop the 800 to 1200 psi pressure to get the 10 to 20-micron size droplets. Injected into the air, tiny water droplets of fog remain suspended until they are evaporated. The smallest particles vaporize almost instantaneously. Copper, stainless steel and re-enforced flexible hose are used for piping. Diameter is frequently 1/4 or 3/8 inches as water supply required is only one to two gallons/hour/nozzle. For propagation, lines of pipe are evenly spaced above the crop area. Plastic, ceramic and stainless steel are used for nozzles.

Humidity Control

Maintaining proper atmospheric humidity in controlled propagation environment is important because low humidity can increase transpiration and subject seedlings to water stress whereas extreme humidity can promote diseases. 

Carbon Dioxide Supplementation

Sometimes the concentration of carbon dioxide in winter in closed greenhouses may drop to 200 ppm, or lower, during the sunlight hours, owing to its use by plants. Under these conditions, low carbon dioxide concentration limits photosynthesis, requiring a supplementary increase in carbon dioxide. When supplementary carbon dioxide is used during periods of sunny weather, the temperature in the greenhouse should be kept relatively high.

Plant Growth Regulators

Plant growth regulators are often applied to plug-grown seedlings to decrease internode elongation and to strengthen the stems. Though many label directions recommend application 2 to 3 weeks after sowing, it’s best to apply growth retardants at the correct stage of development.

Fertilizing

The goal in transplant production is to produce a sturdy, compact plant that, when transplanted, will grow rapidly and yield well. In general, too much fertilizer, especially if applied too early in the growth process, will result in a leggy plant that may not establish rapidly in the field. Too little fertilizer will result in stunted plants which are slow to grow when field planted. Therefore proper fertilization directly impacts field performance. No generalized fertilizer program is available for all transplant crops or all environmental conditions. Growers develop their own blends and application schedules based on experience.

Disease Control

Control of disease during seed germination is one of the most important tasks of the propagator. The most universally destructive pathogens are those resulting in “damping-off,” which may cause serious loss of seeds, seedlings, and young plants. Damping-off is a term long used to describe the death of small seedlings resulting from attacks by certain fungi, primarily Pythium ultimum and Rhizoctonia solani, although other fungi—for example, Botrytis cinerea and Phytophthora spp.—may also be involved (See Figure 19.16).

Water Quality

The salt tolerance of germinating seeds is much lower than that of established plants, which can be grown under minor irrigation salinity by modifying cultural conditions. Water quality for propagation is considered good when the electrical conductivity (EC) reading is 0.75 dS (deciSiemens) and sodium absorption ratio (SAR) is 5.

Managing Seedling Maturation and Hardening Off

Mature seedlings will typically have a balance of root and shoot growth—at least two sets of true leaves and an ample root system that holds together the root ball when removed from the growing container. “Hardening off” is the final step in preparing seedlings for transplant and uninterrupted growth. In the final 3 to10 days in the greenhouse zone, seedlings should be exposed to conditions that most closely resemble their future growing environment. This acclimatization process reduces transplant shock, which can occur when seedlings experience an abrupt transition from a controlled environment of the greenhouse to the less predictable environment.  The following developments occur during the hardening off process:

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