Always check for special germination requirements with each type of seed. Germination requirements include light, temperature, nutrients, moisture, and medium. Each germination requirement can be controlled by the grower for each bedding plant.
Exposure to light influences seed germination. Some seeds must have darkness to germinate. Bedding plant seeds requiring darkness include celosia, cleome, gazania, periwinkle (Vinca), portulaca, pansy, verbena, and viola. It may therefore be necessary to see that the seeds are well covered. Other seeds require exposure to light. Seeds that need light to germinate in elude snapdragon, begonia, browallia, impatiens, sweet alyssum, petunia, and salvia. Still, others do not seem to be affected by exposure to light.
Proper temperature is important for successful germination. Most seeds germinate quickly with a medium temperature. It is important to note that temperature refers to the soil temperature and not the air temperature. Check the soil temperatures with a soil thermometer. Soil temperatures that are too cool remain wet for a longer period of time Wet, cool soil conditions reduce oxygen exchange with the seed and slow germination.
Seeds are started in a special medium, a soil or soilless material. The media used for germination purposes are easily purchased as prepackaged mixtures. The mix is fine-textured, uniform, and often consists of a peat moss and vermiculite. It is also free of disease organisms and weed seeds. Medium plays a minor role in germinating seeds.
The most important factor in seed germination is the moisture level in the medium. A rule of thumb is that once the medium and seeds have been moistened they should be kept moist until sprouting. If the medium is allowed to dry out during the germination process, the seeds die. It is best to wet the medium before sowing After the seeds have been sown, the flats can be covered with a clear plastic cover or sheets of poly to reduce evaporation. The coverings are removed once the seeds start to sprout. Some growers mist their flats regularly to maintain proper moisture levels.
Different seeds prefer slightly different moisture levels. Seeds that prefer a wet medium or one that glistens and is wet to the touch are coleus, begonia, and alyssum. Moist medium that is wet, but not saturated, is prescribed for impatiens, petunia, geranium, salvia, vinca, and pansy. A dryer medium is best for verbena, zinnia, and most perennials.
In general, fertilizers are not used in the germination process. Fertilizers are soluble salts in the medium. Soluble salts inhibit the germination of most bedding plant seeds. Two exceptions are petunia and snapdragon. Low fertility levels actually enhance seed germination for those two crops.
Hand Sowing Seeds
The depth at which the seeds are planted varies with the size of the seed and their light requirements. Larger seeds have more stored energy in their cotyledons enabling them to push up through the medium as they germinate. Tiny seeds have limited stored energy, and therefore lack the ability to push up through the soil if planted too deep. A rule of thumb is to plant the seeds at a depth no greater than three times the diameter of the seed. Hand sown seeds are placed evenly in rows rather than being broadcast over the entire flat. This practice protects the loss of all the seedlings by limiting the spread of disease if introduced.
After the seedlings develop their first true leaves, they are transplanted or moved to pots or cell packs. Cell packs are molded plastic containers divided into two, three, four, or six separate growing compartments. It is in the pots or packs that they will be finished or grown to saleable size. During the transplanting process, the seedlings must be carefully divided to avoid damaging the plant. It is recommended that the technician only handle the seedlings by their leaves since the stems are very easily-bruised.
The Plug Revolution
Propagation of bedding plants has changed dramatically with the industry move toward automation and the increased size of greenhouse operations. Small operations may still rely on hand seeding, but many small operations have moved away from seeding all together. Instead of propagating their own plants, they purchase plugs from large companies. The trays of plugs are easily custom grown for the medium and small greenhouse growers. Today, roughly two-thirds of all the seeded crops begin as plugs. It is safe to say that plugs have revolutionized bedding plant production worldwide.
Plugs are small plants grown in a small amount of medium in divided trays. The cells in the trays determine the size and number of plugs to be produced. The sizes of the plugs are of an inch or larger. Plugs are very convenient as they can be popped out of the tray for transplanting to cell packs or pots. Plugs also allow for automated transplanting. The number of plugs grown in a tray varies. The larger producers of bedding plants use automatic seeders to produce plugs. The automated seeders greatly increase the speed at which plants can be seeded and the number of flats that can be produced. Automatic seeders place a seed in each cell of a plastic plug tray that has been filled with a germination mix or medium Some growers use more than one seed per cell, especially when germination rates. Petunias and begonias may be double-seeded to get a fuller tray. Five to six seeds per cell is common for portulaca or alyssum.
The materials used in the medium are extremely important. Vermiculite is another common plug medium ingredient. It retains water, it has good nutrient exchange, and it provides calcium and magnesium Perlite is sometimes used to provide aeration. Calcined clay is used for aeration and for its nutrient exchange capacity. In some cases, soil is used in the medium Different growers use different mixes. One example of a medium used consists of soil, with lime and fertilizer added. The mix has excellent moisture-holding and aeration capabilities. After the seed is sown, the medium is covered lightly with vermiculite and watered. The key to successful and uniform germination is a medium that provides proper moisture levels and is well aerated. Each tray is labeled with the plant variety to avoid confusion. Some growers then place the plug trays in specially designed germination chambers that provide the optimum temperatures for seed germination.
The goal of the grower is to produce plugs that are uniform, have short internodes, and have a high number of dark green leaves. To accomplish this, the plants move through four growing stages. The type of plant determines how long each stage lasts. Stage one involves germination of the seed. In stage two, the plants are kept warm and given low levels of fertilizers. Stage three in volves higher levels of fertilizer to promote growth and development. In stage four, plants are prepared for shipping. To slow growth prior to shipping, the plants’ fertilizer levels are reduced and they may be treated with a growth regulator. The entire process of producing quality plugs takes 6 to 10 weeks.