Richters Free Lecture Series
SUCCESS WITH SEEDING
Why Do Seeds Fail?
Poor or No Germination
- Planted too deep or too shallow
Improper planting depth is probably the most common reason why seeds fail. Our general rule of thumb is to cover seeds twice the thickness of the seeds when planting in pots, flats, cells etc., and three to four times the thickness of the seeds when planting outside in the garden. Very fine seeds only need to be scratched into the soil surface and firmed in.
- Poor drainage and poor root-zone aeration
All seedlings, even before they emerge from the soil, require oxygen for growth. They also need to get rid of carbon dioxide as they respire. If the sowing medium is too wet, gases cannot move through the soil. Roots cannot grow and may rot. Make sure the soil has adequate drainage to ensure proper growth and development of the root system.
- Light requirement
Some seeds require the complete absence of light; others require light; while most are insensitive to light exposure. This light/no light requirement does not depend on light intensity-even weak light exposure can stimulate or prevent growth.
- Hard seed coats
Some herbs delay germination by means of a hard seed coat that prevents water and gas diffusion. This virtually imprisons the embryo preventing it from breaking through the seed coat. Scarification or scratching is necessary to weaken or puncture the seed coat. This will allow water and gases to diffuse so that the embryo can break through the coating.
- Short viability
Many species of plants have seeds that are short-lived, especially those from tropical areas. For example, Neem seeds (from India) remain viable for only 30 days after harvest. If seeds such as these are planted when they are too old then little or no germination will occur.
- Intolerance to drying out
Some tree, wildflower and many tropical seeds cannot dry out after harvest. They must be kept in moist, cool conditions for germination to occur successfully. Examples are bay laurel, coffee, and bloodroot.
- Improper storage
Most seeds require cool, dry storage conditions. Excess humidity or high temperatures can destroy seeds in a few days or weeks. It is crucial to know in what condition the seeds need to be kept in order to achieve optimum germination.
Good Germination but Seedlings Fail to Grow
- Drying out
This is a very common reason for failure. In fact, it is likely the most common explanation for seedling failure. Once seeds have germinated and the seedlings are growing, they cannot tolerate drying out. It takes only fifteen minutes on a hot summer day for a seed flat to dry out. Watering is crucial! Seedlings must be monitored on a regular basis to ensure they are receiving adequate amounts of water in order to survive.
- Post-emergent damping-off
This is a very serious problem when the correct germinating conditions are not provided. Damping-off is a fungus that attacks the stems, constricting water and nutrient transport to the leaves, causing the eventual toppling over of the seedlings. It occurs in patches and can destroy seedlings overnight. Some causes of this fungus are poor ventilation, poor soil aeration and overly crowded seedlings.
- Poor light
Before seedlings emerge, light requirements are generally less. Moderate exposure to sunlight or room light is sufficient for light-requiring seeds. After germination begins, seedlings require increasing light intensity. Spindly seedlings are an indication that the light intensity is not strong enough. These seedlings should be moved to a brighter window.