Chapter 19

Plant Propagation from Seed

Greenhouse Seed Production Techniques

Traditionally, bedding plants and vegetables have been produced by germinating seeds in flats and transplanting seedlings to larger containers. At the present time, greenhouse producers have adopted plug production as the preferred method for transplant production. Many small- to medium-size growers essentially choose to purchase plugs from specialized plug producers more economically than producing seedlings themselves. This avoids the cost of equipment, cost of skilled plug growers, germination problems, and other problems associated with the seedling stage. Specializing only in finishing crops results in the ability to tightly schedule the production area for rapid crop turnover. Many larger growers may grow plugs not only for their own production but also for sale to other growers. In-house plug production is done to reduce the cost per seedling unit, control plug quality, and grow cultivars that can be difficult to find from specialist plug growers. Often, larger growers need to sell plug flats to other growers in order to justify the cost of equipment.

Flat Production

Seed is sown in flats by one of two ways—broadcasting or sowing in rows. Either way gives good results. The germination medium must be firmed into place and thoroughly moistened before the seed is sown. Broadcasting is a faster method than sowing in rows. The goal of broadcasting seed is to distribute seed uniformly and quickly over the entire area of the flat. Broadcasting is a quick method of sowing seed, but most growers sow seed in rows when using flats. Sowing in rows results in less root intermingling between seedlings and less transplant damage. First, small depressions or rows must be pressed into the medium. These rows usually extend the length of the flat.

Plug Production

A plug is a seedling produced multi-cell plastic trays, also called flats. A flat is typically 11 x 21.5 inches (28 x 55 cm) and 2 ¾ inches (7 cm) deep (See Figures 19.2). Trays can be fitted with removable sheet inserts containing the cells. Inserts come in a wide variety of combinations of packs per flat and cells per pack. Cell size dictates the length of time a crop of plugs takes to produce and the time required for the bedding plant grower to finish the crop. Trays for transplants vary in size from 32 cells to 512 cells. Plug trays with 512 or more small cells are used to produce small “plugs” (or started plants). Trays with fewer than 512 cells are used for producing large plugs. The number of plants in a tray depends on the cell size needed for each plant. Large cell sizes such as 32, 50 or 72 are often used for vine crops and early harvests.

Advantages and Disadvantages

Plug technology provides numerous advantages over conventional flat seeding such as optimization of greenhouse space, reduces the time and labor required for transplanting, shorter production cycles, high degree of seedling and final uniformity, reduces the spread of disease organisms, and reduces transplant shock.

Seedling Growth Stages

Germination and growth of seedlings in plugs or flats have been divided into four easily recognizable stages. These stages are useful because environmental and cultural requirements for the plant changes in each stage.

Growth Stage One

From sowing to radicle (root) emergence. For uniform germination, the temperature of the germination medium must be kept constant throughout Stage 1. Like seeds sown in flats, seeds sown in plug trays vary in their optimum germination temperature requirements depending on the species. Bottom heating is excellent for maintaining constant germination media temperature. Moisture levels are critical during Stage 1. The seed must receive enough water and oxygen for germination, but it should not be soaked with too much water. Plug trays should never be inundated with large volumes of water which will “drown” or displace the seed.

Growth Stage Two

From radicle emergence to emergence of the first true leaves. The environmental requirements of seedlings in Stage 2 are generally similar to those of Stage 1 except that in Stage 2 the moisture level of the germination medium can be somewhat drier but varies depending on the crop. The primary goal for Stage 2 is root development; therefore, uniform moisture is critical. Allow the medium to dry out slightly between irrigations; however, seedling roots are extremely sensitive to drying out and are killed easily.

Growth Stage Three

From the first true leaf to 4 or 5 leaves. This is the longest period of plug production. During Stage 3, the seedlings make a significant increase in size. Watering should be done daily, if sunny, warm conditions warrant but allow the germination medium surface to dry between irrigations to promote stronger seedlings. Apply fertilizer once or twice a week during Stage 3, depending on weather conditions and rate of seedling growth. Use nitrate nitrogen at 100 to150 ppm to produce strong seedlings. Ammonium nitrogen is handy to use for rapid greening of seedlings that become chlorotic from lack of nitrogen.

Growth Stage Four

Seedlings in Stage 4 are ready for transplanting. They should be somewhat "hardened" to make this transition from the plug cell to the finishing container. Hardening can be achieved by decreasing fertilizer applications and avoiding those that contain ammonium nitrogen. If needed, temperatures can be lowered into the low to mid-60 degrees to hold seedlings until they can be transplanted. Decreasing water can also help harden seedlings for transplanting. Applying a 0 or small negative DIF continues to be helpful in controlling seedling height.

Seed Quality

Producing plug-grown bedding plants has resulted in a demand for high-quality seeds with high germination rates that germinate uniformly. The combination of mechanical seeders and market demand has placed pressure on seed companies to consider seed quality as part of the overall effort to breed new cultivars. Several seed treatments are used in an effort to improve seedling stands in plug flats. These are the following:

Graded Seed

These are cleaned seeds that have been physically separated by size, shape, weight, or density. Grading seed for size uniformity improves seeder performance while also producing more consistent rates of emergence.

Primed Seed

Seed priming is done to increase the speed, uniformity and overall percentage of seed germination. Seed priming involves soaking the seed prior to planting. Soaking starts the germination process, but the seed is planted before germination is completed. Seeds that have been primed may be then dried down and stored for a period of time, or they may be planted soon after priming.

Pelleted Seed

The practice of coating seed, called seed pelleting, was developed to improve the ease of handling and planting of small, irregularly shaped seed. In many cases, pelletized seed is used for ease in mechanical seeding. Pelletizing increases speed of seeding and allows singulation (one seed per cell) within the tray. In the process of seed pelleting, the surface of the seed is covered with an inert material, such as clay, diatomaceous earth, polymers, sand or some other material.

Mechanical Seed Sowing

Mechanical seed sowing is more commonly used for large-scale production of vegetable and flower seeds for greenhouse production because seed size is small and large quantities of seeds must be precisely sown in each small plug in the flat. Automated seeders do the job of sowing seed many times faster than can be done by hand.

Template Seeder

The template seeder uses a template with holes that match the location of cells in the plug flat (See Figure 19.6). Template seeders use a vacuum to attach seeds to the template releasing the seeds either directly into the plug flat or into a drop tube to precisely locate seeds in each cell of the plug flat.

Needle Seeder

The needle seeder is fully mechanical, requiring little input from the operator (See Figure 19.7). Individual needles or pickup tips, under vacuum pressure, lift single seeds from a seed tray and deposit one seed directly in each plug cell or into drop tubes for more accurate seeding.

Cylinder or Drum Seeders

The cylinder or drum seeders have a rotating cylinder or drum that picks up seeds using vacuum from a seed tray and drops one seed per plug cell (See Figure 19.8). This is the fastest, most precise, and most costly of the commercial seeders.

Top-Dressing Seed

Once seeded, the cell generally is top-dressed with vermiculite (See Figure 19.9). Vermiculite holds water in, thereby reducing losses to evaporation, and it resists large shifts in temperature, providing for a more uniform germination environment.

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