Nowadays, floriculture growers start their crops using either plugs from seeds or rooted cuttings (also called "liners"). Growers that use plugs either grow the plugs themselves or buy them from other growers who specialize in plug production. Regardless of who produces the plugs, the objective is to have a healthy seedling in each cell of the plug tray. In order to achieve that, plug producers have to buy seeds of the highest quality possible and then follow appropriate germination procedures (right temperature, light, water and fertility at each plug production phase). Doing otherwise represents a loss of money. The extent of such loss depends on the number of nonusable plugs per tray and the wholesale price of each plug (Table 1).
|Table 1. Potential losses for a 512-plug tray (after Paul Karlovich, Raker’s Acres). Although prices change over time, this table clearly indicates that even the loss of a few plugs per tray represents a loss in revenue.|
|Nonusable plugs (%)||Wholesale price per plug ($)|
Take-Home Message Number 1: Use high-quality seeds and follow appropriate germination procedures.
Every floriculture grower starting crops from seed should be aware of important seed properties such as seed viability, seed germination and seed vigor.
Seed viability indicates to which degree the seeds are alive. Sometimes, seeds are alive but do not germinate even when placed under ideal germination conditions. In such cases, it is said that seeds are "dormant." Germination will occur only when what it is inhibiting germination disappears. Hence, seed viability does not equal seed germination.
Seed germination is the emergence and development of the essential structures (root, hypocotyl and cotyledons) that will produce a normal plant under favorable conditions. Reporting the germination potential (expressed as percent germination) of a seed lot is a legal requirement (by law, seed producers must provide this information on packets), and it is measured under ideal germination conditions (in the laboratory) that should be easily replicated. In other words, the germination percentage of a seed lot measured by two different laboratories should be the same.
Having a seed lot with a low germination percentage is a sure indication of germination problems. But having seeds of high germination percentage is no guarantee of a good crop of seedlings. How is this possible? Because growers are more interested in seed vigor than germination percentage.
Seed vigor can be defined as the seed properties that determine the capacity to produce a rapid and uniform emergence of normal seedlings under a wide range of greenhouse (as opposed to laboratory) conditions. It is not a legal requirement, and each company can develop its own system to measure seed vigor.
Take-Home Message Number 2: Viability does not equal germination, and germination does not equal vigor.
If a grower objective is to produce plug trays with 100 percent of the cells having a seedling, seed vigor is paramount. Seed vigor depends on the genetic characteristics of the seeds, storage conditions and age of the seeds.
Seed laboratories can use different tests to determine seed vigor: Cold Test, Accelerated Aging, Electrical Conductivity, Seedling Vigor Classification and Seedling Growth Rate. Some seed companies have developed their own methods, such as the Ball Vigor Index.
Why should growers pay attention to seed vigor? As shown in Figure 1, during storage, seed vigor decreases faster than seed viability (or germination). In other words, the ability of a seed to produce a quality seedling (plug) decreases faster than the ability to germinate.
Seed deterioration is inevitable, irreversible and variable. Depending on how seeds are managed after harvest and stored, the best one can do is slow down seed deterioration.
Growers have to take into consideration two factors in relation to seed storage: temperature and seed moisture content (in turn, a function of air relative humidity).
In general, temperature for storage should be between 4 degrees Fahrenheit and 70 degrees Fahrenheit; the most common is around 42 degrees Fahrenheit. At lower temperatures, seeds might not be respiring as much as they do at warmer temperatures.
It is important to remember that air temperature influences air relative humidity. A reduction in air temperature produces an increase in air relative humidity. The air relative humidity of working areas such as seeding rooms is usually between 70 percent and 80 percent. A seed exposed at this relative humidity for two hours can increase its moisture content up to 2 percent.
Seed Moisture Content
In general, if the air relative humidity is between 20 percent and 40 percent, seed moisture content is around 5 percent to 8 percent. If seed moisture content is above 5 percent, seed vigor might decline; above 8 percent, seed deterioration increases; above 12 percent, the seed might be attacked by fungus and insects; above 25 percent, the germination process starts.
Figure 1. Relationship among seed vigor, viability (germinability) and deterioration, and the area of application of vigor tests. Note how vigor decreases faster than viability (from Delouche and Caldwell, 1960).
Take-Home Message Number 3: Keep seeds cool and dry!
Here are some rules of thumb regarding seed storage for seeds between 5 percent and 14 percent moisture content.
- Seed stored at 75 percent relative humidity and 45 degrees Fahrenheit = 120 (75 + 45 = 120). These conditions are unacceptable and will favor seed deterioration.
- Seed stored at 40 percent relative humidity and 45 degrees Fahrenheit = 85 (40 + 45 = 85). These conditions are acceptable for seed storage.
Seeds should be stored in a waterproof container having a desiccant (Figures 2 and 3). This container should then be stored in a refrigerator.
|Figure 2. Packets containing silica gel desiccant (top left), clear silica gel (top right) and cobalt chloride-doped silica gel (bottom right). Cobalt chloride is pink when wet (bottom left) and blue when dry (bottom right). (Photo by XtremXpert)||Figure 3. A glass jar containing a desiccant made of particles granules of calcium sulfate (97 percent) and cobalt chloride (3 percent).|
Growers' Best Practices Regarding Seed Handling and Conservation
Take-Home Message Number 4: Follow seed-handling best practices!
Finally, keep in mind that the majority of seeds of floricultural crops have short to medium storage lives. Only a few can be stored for long periods (Table 2).
|Table 2. Relative storage life of flower seeds. Note that only a few crops have a long storage life.|
|Coneflower||Dusty Miller||Shasta Daisy|