Germination is usually the growth of a plant contained within a seed; it results in the formation of the seedling. It is also the process of reactivation of metabolic machinery of the seed resulting in the emergence of radicle and plumule. The seed of a vascular plant is a small package produced in a fruit or cone after the union of male and female reproductive cells. All fully developed seeds contain an embryo and, in most plant species some store of food reserves, wrapped in a seed coat. Some plants produce varying numbers of seeds that lack embryos; these are empty seeds which never germinate. Dormant seeds are viable seeds that do not germinate because they require specific internal or environmental stimuli to resume growth. Under proper conditions, the seed begins to germinate and the embryo resumes growth, developing into a seedling.
Disturbance of soil can result in vigorous plant growth by exposing seeds already in the soil to changes in environmental factors where germination may have previously been inhibited by depth of the seeds or soil that was too compact. This is often observed when you turn soil over or till it to prepare it for planting. This sometimes referred to as a soil ‘seed bank’.
Seed germination depends on both internal and external conditions. The most important external factors include right temperature, water, oxygen or air and sometimes light or darkness. Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to the ecological conditions of a plant’s natural habitat. For some seeds, their future germination response is affected by environmental conditions during seed formation; most often these responses are types of seed dormancy.
Water is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is called imbibition, which leads to the swelling and the breaking of the seed coat.
Oxygen is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling’s energy until it grows leaves.
Heat or a specific temperature range
Temperature affects cellular metabolic and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures.
Some plants surround or infuse the seed coating with chemical compounds that stop or slow down the ability of a seed to germinate. This is designed usually to make sure the seeds survive being moved away from the plant or only germinate when suitable conditions are reached.
For example a plant growing in a dry desert environment may want the seed to only germinate when it has been through a harsh summer dry period and then reach the time when rain or other moisture is more available and the seeds have a better chance of surviving.
Another example is when a seed inside a fruit will be eaten and go through the digestive tract of an animal or bird.
Or maybe the fruit will fall to the ground and then be exposed to the air when the fruit rots, breaks open to the air and can be washed by rain water and successfully germinate into a suitable soil surface that has available nutrients, air and water.
There are many seeds that can have the inhibitors removed by a fermenting and drying process. This is when the seeds sit in moisture for a short while as the coating inhibitor substances ferment and then are either deactivated or washed away.
Some seeds like chili peppers require this fermenting or washing process along with a drying to allow them to germinate. These seeds require both processes to occur. Pepper seeds that are not dried will germinate at a much lower rate than if they are properly cleaned and then dried to 12% moisture content, stored for a time and then rehydrated to grow on.
Sometimes seeds need a time of rest after harvest from a plant to allow for better germination. Eruca sativa – Rocket – needs around 3 months of rest after harvest to fully mature. Although they appear fully ripe picked from the plant as dry seed pods the seed germination will be low until they have had time to rest and fully mature. This is a natural built in germination inhibition to allow for the weather to change to a more suitable pattern of rain or sun light levels and a more desirable temperature.
It is important to consider where plants are originally from and how those local climate and weather patterns may affect the growth. Plants have developed over millions of years in response to these natural cycles and conditions.