Chapter 22

Plant Propagation from Seed

Seed Propagation Environment

An understanding of all factors influencing seed propagation in a greenhouse environment is needed for the successful growth and production of high-quality plants. The basic factors affecting plant growth in general play a role in seed propagation; temperature, water, light, and nutrition. The first two factors, temperature and water, play a primary role during germination. Light and nutrition can be considered secondary, to a degree, until roots have initiated. Ideal germination conditions vary by species. Growers not only manage the environment during seed propagation, but also manipulate the post propagation environment—hardening-off (weaning seedlings from the mist system and changing fertility regimes) to assure growth and survival of tender-rooted plants after propagation. The principal objective of seedling production is to develop healthy, stocky, vigorous plants capable of transplanting with little check in growth. The plant propagation period is generally a very narrow segment of a plant’s life, ranging from several weeks for most fast-growing herbaceous plants. Following seed propagation, seedlings are transplanted into containers.

Germination Chambers

Many growers use germination chambers, 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 22.7) to rooms several hundred square feet in size designed to accommodate multiple tall carts of plug trays (See Figure 22.8).

Media

While plants can be grown in all types of root media, seed germination and transplant production have perhaps the greatest requirement for a high quality, uniform medium. It is sometimes noted that coarse or large particle size media are desirable due to the need for high aeration during germination. However, most germination media are actual fine with small particle size in order to provide uniformity for filling small plug cells. Aeration is maintained instead by not over watering or saturating the root medium.

Temperature Management

The initial stages of germination are very dependent on media temperature (as opposed to air temperature). Failing to provide a reasonable substrate temperature during germination will result in lower stands, erratic growth, and slower germination. Media temperature is influenced by the amount of light and the rate of moisture evaporation from the root media, which is influenced by relative humidity. 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.

Light Management

Optimal seed germination may depend on the absence or presence of light, although most species will germinate in light or dark. For growers using germination chambers without lights, keep the trays in the chamber for a minimum time and then move them to the bench to satisfy the light requirement for the latter part of germination. Only low intensity light is necessary for the first stage of germination.

Supplemental Lighting

To overcome the problem of low natural winter light and reduced plant growth, supplemental light can be used over the plants. The best light source for greenhouse lighting is high-pressure sodium (HPS) lamps. Most of the radiation from these lamps is in the red and yellow wavelengths and is very deficient in blue. However, when used in conjunction with the natural daylight radiation, these lamps are quite satisfactory.

Water Management

After the flats or plug trays are filled with media and seeded, irrigate to provide moisture for seed germination. Providing the right amount of moisture during germination is the hardest part of the process. Maintaining proper moisture levels in seedling flats is easier in many ways than in plug trays, which have individual cells that dry out faster. Keep media moist but not soggy during germination. Seed need adequate levels of oxygen and water during germination; hence, proper water management is crucial. Keeping media too dry during germination can prevent or delay germination and cause erratic stands.

Mist Systems

Mist systems are an important part of any greenhouse operation, as they play a vital role in the production of both seed and vegetative plant material (See Figure 22.9). Their fundamental purpose is to deliver an intermittent source of water in low volumes to plant material being vegetatively propagated or to seeds being germinated. Mist systems, when compared to irrigation other systems, typically deliver a smaller particle of water for shorter intervals of time. Depending on how a mist system is used and the type of crop it’s being used on, the crop’s production period can either be accelerated or slowed.

Mist System Layout. There are two types of intermittent mist lines–in-bench or overhead. With in-bench mist systems, the supply lines can be placed along the bottom of the bench under the medium or directly on the surface of the medium. The nozzles are supported above the bench on risers ranging from 6 to 12 inches (15 to 30 cm). Nozzles spray the fine mist out in a circular (360°) pattern.

Types of Nozzles. Two basic types of nozzles are used for misting— impingement or deflection nozzles and whirling nozzles. Impingement or deflection nozzles spread the water from a small stream impacting against a plate or pin. The pattern is a round horizontal circle with a pattern that varies in intensity over its radius. They produce a rather coarse spray from water striking a flat surface. The larger aperture used in this type greatly reduces clogging but uses larger volumes of water. Whirling nozzles use small slots within the body to break up the water into droplets, which are then thrown out in a circular pattern.

Nozzle Spacing. Nozzle spacing is dependent on the size of the area you intend to cover, considering width as well as length. The objective is to achieve an ideal overlapping spray pattern of the mist nozzles to reach optimum uniformity and distribution of the water being delivered. To achieve more uniformity, nozzle patterns are usually overlapped by 50 percent or more.

Controls for Mist Systems. Mist systems can be controlled by time clocks or with variable environmental systems such as the weight leaf system or a controller that measures vapor pressure deficit (VPD). Regardless of how the mist is controlled, it is applied only during daylight hours.

Fog Systems

For crops requiring an ultra-fine mist, fog nozzles may be used. Fog nozzles provide the smallest water particle size available. Fog nozzles usually have a smaller fogging diameter than mist nozzles and require much higher water pressure to obtain their very small particle size. Its advantages include more uniform wetting across the plant area, deeper penetration into the foliage, and higher.

Sub-Irrigation Systems

With the sub-irrigation system, also referred to as the float system, trays are irrigated and placed on the reservoir where they “float,” either continuously or intermittently (sometimes called ebb and flow), until they are transplanted. Capillary mat systems deliver water from a reservoir to the mat where the growing medium “pulls” water into the flat or plug cell by capillary action. Overhead irrigation may be required (for the first irrigation) to settle soil and adequately wet the mix.

Water Quality

Good water quality is essential for propagating quality plants. The salt tolerance of unrooted cuttings, germinating seeds, and tissue culture explants 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/m (less than 525 ppm total soluble salts in ppm), and the sodium absorption ratio (SAR) is 5.

Humidity Management

Good water management is important to limiting plant stress. Generally, the relative humidity in a propagation house is maintained at a minimum of 85 percent and reduced once the plants become established. Care must be exercised to avoid over-misting and over-irrigation, because too much water can be just as stressful as too little water.

Carbon Dioxide Enrichment

Ambient carbon dioxide (CO2) in the atmosphere is around 380 ppm. Sometimes the concentration in winter in closed greenhouses may drop to 200 ppm, or lower, during the sunlight hours, owing to its use by plants. Under adequate light and temperature, but when low carbon dioxide concentration limits photosynthesis, a supplementary increase in carbon dioxide concentration 1,000 to 2,400 ppm can result in a 200 percent increase in photosynthesis.

Fertilization

Fertility is not a critical factor for most species during Stage 1 because most seeds have enough stored nutrients to carry out germination. Also, many germination media have a starter charge of fertilizer which is effective at supplying nutrients as the root emerges and elongates. The most important factors for germination are temperature of the growth medium and a proper balance between moisture level and aeration.

Seedling Disorders Related to Fertility

During Stages 2 and 3 several nutrient disorders may occur: (1) shoot tip distortion or death, (2) misshapen cotyledons or true leaves, or (3) seedling chlorosis. Shoot tip and young leaf abnormalities have been linked to excess soluble salts and deficiencies of either calcium (Ca) or boron (B).

Fertilizer Program Affects Seedling Size

In Stages 3 and 4 the fertility program can have significant effects on seedling height and overall size. Levels of overall nutrition, ammonium (NH4) vs. nitrate nitrogen (NO3) nutrition, phosphorus (P) and calcium (Ca) nutrition have the greatest effects on seedling size. Traditionally, many growers have slowed the growth of bedding plants by not fertilizing or using very low levels. This approach is very effective at reducing height and may improve root growth. However, the disadvantages of low nutrition are that the seedlings become chlorotic and may fall behind schedule.

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. For most species, this is at the second true leaf stage.

Disease Management

Control of disease during seed germination is one of the most important tasks of the propagator. Specific pathogens may infect only certain plant species or cultivars, or specific organs or tissue, which varies with the stage of development of the plant. The pathogenic fungi most likely to cause disease development during propagation are species of Pythium, Phytophthora, Fusarium, Cylindrocladium, Thielaviopsis, Sclerotinia, Rhizoctonia, and Botrytis.

Fungicidal Media Drenches

Fungicidal media drenches can be applied to the container media in which young plants are growing or are to be grown to suppress growth of many soil-borne fungi. These materials may be applied either to media or to the plants. Preferably, a wetting agent should be added to the chemicals before application.

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