biology and procedure of seed germination
Special hints specific to the sowing of a particular species will be found on our productpages of each species.
This article is meant as a general approach to successful sowing and create a deeper understanding about the biology and procedure of seed germination.
Understand germination to make your garden flourish
When you look at a giant redwood or simple tomato plant it is difficult to imagine that they both started at the same state, the seed. They were no bigger than a chip of a pebble but somehow they grow to hundreds and even thousands of times their initial size.
Germination is the process through which plants grow from a seed to form a seed to become a new plant. The seed’s metabolic systems reactivate when the conditions are just right and begin to develop the root and shoot of a new plant.
Inside every seed is an embryo. Just as embryos develop into children in animals, so does the embryo in plants develop into the new plant. The seed also has reserves of food that the embryo will use in its initial development. Most seeds have a coat that protects them from the environment. Before the conditions are appropriate, seeds stay inactive. This is an evolutionary response to give the plant inside its best chance of survival.
The process of germination itself is simple enough. First, the seed takes in water. This process is referred to as imbibition. When the seed takes in enough water, the seed coat breaks open. After that, the radicle and plumule begin to emerge. These are the parts of the embryo that develops into the stem and roots respectively. As they develop, the parts of the seed storing food and covering them, the cotyledons, unfold. These are easy enough to identify as they make up the largest part of the seed. This is about the two halves of a nut or bean. As the embryo uses up the food stored in the cotyledons, the cotyledons slowly develop into the plant's first leaves. That way, they continue providing food for the plant well into its early life.
The right conditions
Like we had said before, seeds do not just germinate. Much like a romantic evening or first date, nothing can develop unless the conditions are just right. These conditions are both within and outside the seed. The most significant external conditions include water, temperature, oxygen and in some cases light or the absence of light. Not all plants are born alike. These conditions vary from plant species to plant species. Different plants have evolved over millions of years to germinate differently in response to their natural environments.
Regardless, water must be present for any seed to germinate. When seeds are fully formed, they are usually dry. This helps the seed survive long spans of time without decomposing. Before the seeds metabolism can reboot, imbibition must occur. Seeds will soak up a lot of water causing them to swell significantly. As the seed continues to swell, its coat ruptures. The cotyledons hold reserves of food in form of protein, oils and starch. Imbibition starts the process of breaking down these reserves. The water absorbed activates hydrolytic enzymes in the seeds that break down the reserves into simple chemical forms that the plant can use in its development. Once the stem and root emerge and the food stores are used up, the seedling will start to use photosynthesis to make its food.
Seeds use aerobic respiration to create most of the energy they need to grow into young plants. This means that they use oxygen to break down food and create energy. Oxygen is the gas in the air that sustains all life including our own. When you breathe, you are carrying out aerobic respiration. Oxygen is in the air and in pores in the soil. When the seed is buried too deeply, the soil packed too tightly or soaked with so much water that air cannot get in, then the seed will choke. Sometimes, the seed coat is so thick, it cannot let oxygen in. The seed, therefore, stays dormant until the seed coat is worn out enough or broken so the seed can take in oxygen and water.
The temperature also has to be just right. The temperature will affect how fast the seed will grow. The ideal temperature varies from species to species. However, each species has a temperature range within which it will germinate. Beyond this range, germination will not occur. Most germinate at room temperature. Some need the temperature to continuously fluctuate between warm and cool. Other seeds germinate at temperatures just above freezing. Some seeds need to feel the cold before they can break their dormancy. They will not germinate even in optimal conditions. They have evolved to stay dormant through winter and start growing in spring. For most seed, light is not necessary for them to germinate. However, there are some which depend on direct light. Other seeds will only germinate after extremely high temperatures. These temperatures could occur in the instance of a forest fire. The heat cracks the seed coats of and allows the seeds to germinate and develop very fast because now there is no other plants left to compete for light, nutrients, water etc...
As diverse as all those mechanisms seem to be, they all have one in common: To advantage survival of the seedling and ensure the plant has its best shot once it germinates.
Preparation of seeds
Germination is not entirely in natures hands. You can help the process along by using a number of time-tested tricks and techniques to improve the odds of your seeds germinating.
Scarification is the process of weakening the seed coat to make it permeable for water which is absolutely mandatory for germination. Depending on what prevents water passing into the inner seed, different procedures can be used to scarify the seed:
If the seed coat simply is too thick or hard the seeds may simply be sandpapered or filed with a metal file to allow entry of water. You should take care to never file too deep and injure the inner seed, just stop right before that point, then let them soak in water for a day then plant it into your substrate. Another possibility to scarify hard seeds is using diluted acids and bases and give your seeds a bath for some minutes. Sulphuric acid is particularly popular.
Glossy seeds are often covered with a wax to make them impermeable to humidity. For such kinds of wax-protected seeds a hot water bath will usually do it’s job to remove the protective layer. Rubbing the seeds with some sand while washing them with water is also very effective.
With the so called “cold-germinators” sprouting is controlled by two hormones which are influenced by humidity and temperature. It’s a bio-chemical reaction that allows sprouting not until the shell of the seed is moistened and softened after the rise temperatures after a longer period of cold weather. This is the matter with a lot of trees and other plants growing in temperate zones or the northern hemisphere. So such seeds need a special treatment for sprouting ( so-called stratification ):
One way is to let nature run its business by sowing the seeds in the open or in a plant pot on the balcony in late autumn and wait for germination in spring and make sure there is also some water present. If the season doesn’t fit in you can use your refrigerator. Just put the seeds into the fridge in a pot with some damp sand. Usually four weeks will do. To prevent drying-up you should cover or close the pot. After taking the seeds out of the fridge germination will start with temperatures rising, what is actually simulating spring.
The use of Phyto hormons like Gibberellic acid is another way to make such seeds grow even without a cold treatment.
Seeds depending on light:
There are seeds that need light for budding. These shouldn’t be covered by earth but should only be strewn on the surface and lightly be pressed against the substrat for contact and to avoid dehydration. These are seeds that mostly are so small and frail that covered by earth they don’t have the necessary strength to break through to the surface. In general, all those very tiny, dust-like seeds should be surface sown only. However there are also a few species available with bigger seeds which also require to be surface sown, e.g. all bamboo, cacti and Ephedra seeds.
One common problem with light germinators is, that drying up once the seedcoat has opened would kill the seeds in no time. This can happen very very fast since they are not surrounded by moist soil. To avoid fast evaporation the plant pots should always be covered with some transparent foil or plastic bag. This will produce a greenhouse effect which does not only keep the moisture but also lead to a more constant temperature. To avoid mold a daily ventilation is mandatory. Simply uncover your plant pots for some minutes.
Tropical and semi-tropical plants:
Tropical and semi-tropical kinds cope well with temperatures between 22-25°C. Temperature often has a considerable impact on the period of germination. Some plants such as Heliconias take higher temperatures. This means the equipment acquisition of a small heated greenhouse in most cases.
Covering the plant pot by a transparent plastic film may help to keep humidity and temperature. However the danger of mildew may then rise drastically and daily ventilation will be absolutely necessary.
Depth for sowing:
There is a rule of thumb: The depth of sowing should be twice the diameter of the grain. Bear in mind that tiny grains should be dealt with like seeds that depend on light.
Substrate for sowing:
Useing the adequate substrata for successful germination is very important. The frail roots of germ buds are quite damageable and wouldn’t prosper in normal manured potting compost, they would „burn“. There is a special kind of compost for the purpose of sowing available in garden stores. A good sowing mix should have a low nutrient level. Never use fertilized soil. For fast and good root development the substrata should be light and permeable so that water as well as oxygen can reach the roots. Therefore the sowing compost should be mixed with some perlite or vermiculite.
If the substrate is too wet it might moulder. The substrata is alright if formed to a bowl in your hand and then pressed there shouldn’t be any water coming through.
My personal number one favorite substrate for sowing is “coco peat”. This substrate already combines all those mentioned requirements in one single products. Coco peat is leightweight, it perfectly stores water, does not contain nutrients and is very very resistant to mold. Once your seeds have germinated and developed some roots, it is getting time to transplant them into a more nutritive soil
The pot should be just the right size. If it is too large, your seedlings will be vulnerable to root rot since the soil will dry too slowly. Something around two inches in diameter will work to start with. Also, consider the material. Terracotta and clay are porous and are great for plants that need dry well-aerated soil. Otherwise plastic is cheaper and lighter. Any pot you choose should definitely have holes to allow water to drain away.
Like anything worthwhile, you will have to tend to your seedlings diligently to for them to flourish. As long as you are paying attention to the main factors influencing germination, you should be fine.