Plant Propagation from Seed
Greenhouse Seed Production Techniques
Commercial greenhouse production relies heavily on the production of transplants, involving highly mechanized operations beginning with seed germination and ending with transplanting the plants into containers. Now, greenhouse producers have adopted plug production as the preferred method for transplant production. A plug is a containerized transplant with a self-enclosed root system. There are several pros and cons to consider when deciding whether to grow plugs from seed or to purchase plugs and grow them to transplant size. The advantages of producing one’s own plugs include rapid production, efficient use of greenhouse space, choice of species and cultivars, and self-reliance. The disadvantages can include extra labor to handle an exacting crop and increased heating costs in winter (since plugs are quite sensitive to temperature fluctuations). For most small to medium-sized growers, especially beginners, it is often more economical to purchase plugs from specialized growers and concentrate on producing finished containers. This avoids the cost of equipment, skilled plug growers, germination problems, and other problems associated with the seedling stage.
Seedling Growth Stages
Seedling production is generally described in four observable stages. The first two stages account for actual seed germination and emergence. Stage one includes the starting process from seed sowing and medium wetting to the emergence of the radical or initial root. Stage two continues from root emergence until the seed/cotyledon leaves are expanded.
Flat Production
Flats that are used for seed germination are shallow, usually 11 inches wide, 20 inches long, and 2.75 inches deep (Figure 24.2). A small volume of rooting medium in a shallow flat is all that seedlings need at the start since most species are transplanted within 2 weeks after emergence. Shallow flats with small volumes of soil are light weight and easier to handle than deep flats.
Advantages and Disadvantages
The use of flats for seed germination has a number of advantages in that it requires very little investment in equipment, can be sown by hand, and good use made of available germination area—up to four thousand seeds of plants like tomato and marigold can be sown in a single flat. However, there are several disadvantages of sowing seed in flats. Diseases in the soil are readily spread from seedling to seedling through adjacent root systems.
Methods in Sowing the Seed
The 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. However, sowing in rows results in less root intermingling between seedlings and less transplant damage. The goal of broadcasting seed is to distribute seed uniformly and quickly over the entire area of the flat. One method to accomplish this is to divide the seed into two equal quantities.
Plug Production
A plug is a seedling produced in a small volume of medium contained in a small cell, of which between 72 to 800 are contained on a single sheet (Figure 24.4). There is a vast variety of plug production trays, composed of materials ranging from molded polystyrene to vacuum-formed plastic or other suitable material that is inserted into a tray. Expanded polystyrene (EPS), or, as many know by The Dow Chemical Company's trademarked name, Styrofoam, does not last as long as molded plastic trays. Over time, a Styrofoam tray will allow roots and disease into the Styrofoam, and thus you get tray-to-tray variation and disease carry-over.
Advantages and Disadvantages
The use of plug trays for germinating seed offers many advantages. Seeding machines (seeders) can sow up to 600 plug trays per hour; therefore, labor costs are reduced. Each seedling is grown in its own container (cell), so it does not have to compete with other plants for water and nutrients. Seedlings produced in plug trays are spaced further apart and thus receive more light. If a soil-borne disease attacks a seedling or seedlings, the disease will be confined to that particular cell(s).
Tray Insert Selection
The number of plants in a tray depends on the cell size needed for each plant. Thirty-two (32) cell trays are great for plants that are more prone to transplant shock, such as cucumbers, melons, squash, and pumpkins. These cell trays will encourage faster growth as there is more access to nutrients and plenty of space. They are recommended for early-season tomatoes, flowers, cucumbers, melons, squash, pumpkin, and other vine crops. Seventy-two (72) cell trays are a good balance between space for growing and seedlings per tray. These are also a good choice for the vine plants; however, you will want to transplant the vine plants earlier than you would with the 32-cell trays.
Shape of Plug Cells
The cells of plug trays are either square or (more rarely) round. Square cells are usually deeper and so hold more germination medium. They allow for better drainage than do round cells. The geometry of the square cell permits greater root medium volume per cell than does a round cell, since square cells “fit together” like pieces of a jigsaw puzzle.
Tray Filling
AAutomated tray fillers help growers improve product quality and save on labor costs and uniformity of the fill, creating a better growing environment for young plants. Cells must be completely filled for their entire depth to wick properly and prevent dry cells, but avoid over packing the cells. Proper media moisture content is critical for adequate tray filling, and the use of a pre-moistened medium is highly recommended.
Dibbling
In order to make a micro-environment that is more conducive to germination and growth, it is common to dibble the growing media prior to placement of the seed. Dibbling refers to the making of a small indentation in the surface of the media where the seed will be placed. Small producers are use dibble boards consisting of rounded or pyramid shaped objects fastened to a base with handles, that allows entire trays to be dibbled at one time.
Types of Mechanical Seeding
Efficient plug production requires the use of a mechanical seeder. The choice of seeder depends on several factors including cost, seeding speed, number of flats to be seeded, and the need for flexibility to sow a variety of seed shapes and sizes. Four types of seeders are commonly available to plug growers—template, needle, drum seeders, and optical seeders.
Plate Seeder. The plate seeder, the least expensive type of seeder, is a good choice for small, entry level operators or medium-sized operations (Figure 24.8). Plate seeders require a quantity of seed to be dumped onto the plate, which typically is a shallow pan (resembling a cookie sheet) with a series of holes in it corresponding to the layout of the plug tray to be sown.
Needle Seeder. Needle seeders use a vacuum and thin needle to pick up seeds and seed them one row at a time to a seedling tray (Figure 24.9). The number of needles or nozzles on the manifold corresponds to the number of cells in a row in the plug tray used. 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 (Figure 24.10). The tubes align with the cells of the tray, so the seeds drop into the cells.
Drum Seeder. The drum seeder has holes drilled into it in a pattern that matches the spacing of the cells in the plug trays (Figures 24.11 & 24.12). Air is sucked into the drum through these holes. As the drum rotates, the holes pass by seeds held next to the drum in a small tray. One seed will stick to each hole, held in place by suction. The drum rotation and conveyor speed are coordinated so that a row of holes on the drum is aligned over a row of cells in the tray.
Covering Seed
Seed of some crops needs to be covered to get the best germination. Generally, the larger the seed, the more covering it needs. A covering will provide more humidity or moisture around the seed for germination, may provide dark conditions to improve germination, and help the initial root to go into the soil. Improper covering will result in reduced or erratic plant stands. You can cover the seed in a number of ways.
Watering Station
Once the seeds are planted, they are usually settled in with water. This gives the seeds good contact with the growing media and results in better germination. For small numbers of seeds this can be done by hose and water breaker but for larger operations it is more cost efficient to use a watering tunnel or water bar.
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