A seed is the reproductive body of a plant that can grow into another plant. It consists of an embryo, stored food and a protective coat.
In plants, seeds are important for reproduction and dispersal. They are also used to produce oil, fiber, and food products. Some seeds have medicinal properties.
Germination is the process by which a seed starts to grow into a plant. All seeds need the right temperature, water, oxygen and sometimes light or darkness to germinate.
Most seeds have a protective layer that protects them from the elements and insects, but it can also inhibit germination. The seed’s coating can clog up the nutrient flow to the embryo.
To break this barrier, the seed needs to swell and split open. This happens when the seed is soaked in water, which softens the coating and allows the nutrients to get through.
This process can take as little as one day, but can be longer depending on the species and its unique germination conditions. Once the root has broken through the seed coat, it goes down into the soil looking for water and nutrients. Next, the shoot grows up in search of sunlight. The whole germination process takes one to two weeks to complete.
Seeds need water and oxygen, but they also require optimal soil conditions. These can include proper temperature, light and adequate moisture to germinate properly.
During germination, a seed’s protective coating breaks down and the embryo’s cells start to enlarge. It then takes in water and oxygen through the seed coat.
Soil conditions that promote germination (or emergence) can be determined by soil texture, moisture availability and ground cover. Soils that are sandy or drain well tend to warm up faster than heavy, water-logged soils.
Wet-thermal models use germination trials conducted in the laboratory to determine the rate of seed germination as a function of soil moisture availability and temperature. They have been shown to be highly predictive of germination responses measured by hydrothermal models in field tests (Hardegree et al., 2018).
While seed germination is dependent on many factors, including seed vitality, soil moisture and air quality, the soil temperature plays an important role in the speed of germination. Whether you are directly sowing seeds in the garden or starting them indoors, achieving optimal soil temperatures will increase the germination rate and result in more vigorous plants.
Using a soil thermometer, insert it 3 to 4 inches deep into the soil surface and note the temperature. Planting at the optimum temperature for a particular crop will ensure that hearty seedlings will be ready to harvest.
All seeds have an optimum temperature at which they will germinate. Depending on the species, this range is between 55 and 80 degrees Fahrenheit or 21.1 and 27.7 degrees Celsius.
The light that a seed receives can either promote or inhibit its ability to germinate. This is largely down to the wavelengths of light, which are in the red and blue spectrums.
The effect of light on germination is complicated by the fact that different seeds have varying degrees of sensitivity to light. It is sometimes only part of the seed that is light sensitive, like the micropyle on phacelia, or even just one point on the surface of lettuce.
In other cases the whole seed will need to be exposed to light for it to germinate, such as in the case of a tiny flower bulb. In such a case, the seed will have to be covered with a thin sprinkling of vermiculite after sowing and left in diffused light until germination takes place.
Most gardeners use fluorescent shop lights or light-emitting diode (LED) bulbs for seed starting. LEDs are energy-efficient and available in a wide variety of shapes and sizes.