A Primer on Seeds and Seed Germination
- Aug 31, 2017
- 1 Comment
A typical question many gardeners ask is: “what happened to my seed germination?” There are a lot of reasons why your seeds did not germinate. Some seeds are by nature difficult to germinate, especially when they are grown outside of their natural habitat, like those imported seeds for instance. First thing we must remember is that not all seeds are alike. In general however, seed germination is affected by various ecological conditions, as well as the physiological qualities of the seeds themselves. Understanding these basic principles would help us increase our chances of success in germinating seeds.
The temperature of the soil (not the atmosphere) affects the seed germination rate. Generally, most seeds germinate between 70 – 85 degrees Fahrenheit (around 21 – 29 degrees Celsius). For each type of seeds however, there is an optimal temperature level at which germination success rate peaks. Gardeners should also be aware of the temperature range (minimum and maximum) outside of which a particular type of seeds will not germinate at all (see “Grow Great Vegetables” for a more detailed table). Some seed companies provide for these information in their seed packet labels. In the Philippines however, these information are not usually provided by the local seed companies, so I often need to do a little bit of online research.
If the optimal soil temperature can not be achieved outdoors, starting the seeds indoors might be a better alternative. Some gardening sites suggest using a heating pad, especially if the seeds are the type that grow best in warmer conditions. I try using the top of my refrigerator to serve as the heat pad for the seeds during the cold or wet months in the Philippines.
For more information about seed starting, check out my article on how to start seeds in a seed tray.
Moisture triggers seed germination
Moisture triggers the seeds to germinate. The seed absorbs water which causes the embryo to swell and crack through the coat. Before planting, soaking the seeds overnight speeds up the process. For very tiny seeds however, pre- soaking is not recommended because seeds tend to clump together after soaking.
The soil must be kept consistently moist throughout the germination stage. Seeds are easily destroyed if they are left to dry after germination has started. Moisten the soil by spraying or misting the seed trays with water once or twice a day. One technique I learned is to cover the seed tray with transparent plastic, creating a green house effect that helps sustain the moisture level in the soil. Another technique is to use a damp paper towel.
Seeds need to breathe
Remember that seeds also need to breathe or take in oxygen from the air. This oxygen comes from the pore spaces in the soil. Therefore, if the soil is too waterlogged, or if the seed is buried too deeply, the seed may not be able to absorb enough oxygen properly. Make sure that the soil is evenly moist, but not very soggy. If available, use a seed starter soil or potting mix that is light and airy, so that air and water can flow through easily. Some sites suggest using a porous material like a small sponge or coffee filter for starting seeds.
What is ‘Damping Off’?
Moisture may invite disease or fungal infection, causing the seeds to rot before or after they start to sprout. This is called “damping off’ and it usually happens in wet and cool conditions (see https://en.wikipedia.org/). Damping off can also happen to seedlings and more mature plants. To prevent this, use a solution of hydrogen peroxide and water. Simply mix 1 tablespoon of hydrogen peroxide per 1/2 liter of water and use this to water the soil or for seed soaking.
Some seeds need light
Most seeds germinate with or without exposure to light. Examples are tomatoes, eggplants and pepper. There are some (like for instance, lettuce and some flowering plants) that will germinate only with the help of light. They are called “photoblastic seeds”. For this type of seeds, expose the seed tray to light by placing it beside a well-lit window or below an LED grow light.
Note that there are also seeds whose germination is inhibited by light (“negative photoblastic’), like for example some varieties of onion seeds. Negative photoblastic seeds should be germinated in an area with minimal exposure to light. Once they begin to sprout, they can already be gradually exposed to sunlight.
It is important to understand the specific characteristics of each type of seeds. Don’t be afraid to experiment to determine the best germination conditions for a particular seed.
Inherent Qualities of the Seeds
Another factor that affects seed germination is the quality of the seeds. Seeds that have been improperly handled, or that are too old, may not germinate successfully. In general, seeds have the highest germination rate if planted within the first two years from packing. Likewise, growing the seeds in their natural habitat increases the chances of success. From experience, I had greater success with locally produced seeds than with imported ones.
Improper handling and storage affects seed quality. To avoid getting stale, seeds should be stored in a dry area at room temperature. Extreme temperatures (whether hot or cold) may destroy the seeds. Once the packet is opened, store the seeds (with the packets) inside resealable plastic bags or vacuum-tight containers. Keep track of the date when the seeds were packed. Ideally, seeds must be used within 2 years for best results. Before discarding old seeds, check whether the seeds are still usable by sprouting a few seeds using the damp paper towel technique.
As a final note, don’t blame your thumb when your seeds don’t germinate. Seed germination can be a little tricky and even expert gardeners fail at this occasionally. Oftentimes, I use trial and error until I get it right. For me, failure is just part of the entire learning process.
Happy gardening everyone!