Most people with a beginner's knowledge of plant growing accept that seeds need light to germinate. By starting seeds indoors, you know growing lights are required when sunlight is not available. But did you know that there are some seeds and plants that don't need light to germinate?
The basic needs for growth are light, water and soil. These three contribute to photosynthesis. Levels of these essential elements are required, but sometimes sunlight or light in general is not required for seeds to germinate. Sometimes the sun inhibits seeds that germinate in the dark.
Some seeds do not need light to break through their seed coats and sprout. Most seeds germinate best with controlled amounts of UV generation, but there are seeds that germinate in the absence of light.
In addition, there are plants that get enough light in heavily shaded areas of a garden or even in the dark.
What do seeds need to sprout?
Do seeds need light to germinate? Let's find out! Source: Myrialejean
Germination occurs when a dormant seed comes to life by interacting with moisture, oxygen, light, and soil content. Let's cover the basics of germinating seeds through an examination of the components required.
Seeds themselves contain small amounts of moisture, but they need to be planted in moist soil in order for them to be brought back to life from dormancy. Extra moisture triggers a process called soaking. At this stage, the seeds fill with water and enzymes are activated that help pressurize the seed's casing and aid in the emergence of seedlings from the surface of the soil.
It is important to properly balance the amount of moisture in your soil for seed starting. Each seed has different requirements. For example, quinoa requires very little moisture and a flat ground cover to germinate. Therefore, the soil should be kept dry for some time before seedlings develop. Too much water will rot quinoa seeds.
Seeds like nasturtiums or sunflowers have a thick seed coat that requires plenty of water to break through, and too little moisture in this case either prevents germination altogether or causes the seedling to die if it shoots up. Often these are pre-soaked to ensure that they germinate.
Seeds need oxygen and adequate airflow to produce enough energy to bounce back from resting. This is due to glucose, which aids the aerobic respiratory process. At this stage, seeds release energy from grocery stores and need oxygen to do this.
Without enough oxygen, seeds cannot balance enough water and carbon dioxide to produce the energy needed to grow.
So how do we make sure seeds have enough oxygen? Understanding a seed's planting depth is one way we can help it germinate. In most cases, you will plant seeds under a thin layer of moist soil.
If you plant seeds like amaranth or mugwort too deeply, you will keep the seedlings away from the oxygen they need to soak up and, in turn, prevent seeds from getting into the food supplies necessary for germination. This is because the seed coat of these seeds is thin.
Some seeds with a thin layer do not even need to be covered with soil and can be sprinkled on the surface of the soil in a thin layer. However, there are seeds with a thicker shell that will germinate best when scarified or when the seed coat is scratched or streaked. The process of banding allows seeds with a thick layer to access the gases, such as oxygen, needed to use food supplies for energy.
There is a lot of discussion among gardeners about how deep to plant seeds. Often times, packets of seeds will say to plant 0.25 inches deep, but this may be too deep for some seeds. A general rule of thumb for planting depth is never to plant a seed more than twice its diameter.
As mentioned above, smaller seeds with thinner seed coats may not even need to be covered. Another way to ensure proper oxygen flow is to avoid compressing the soil. Instead, let the seedlings emerge from lightly spotted soil.
This picture shows each stage of the germination process in rice. Source: IRRI images
While many say seeds need light to germinate, they really believe that seeds need the right temperature to germinate. The germination rates are a big factor here. When a seed has a high germination rate, temperature control may not be as necessary.
For seeds with a lower germination rate, gardeners need to plant more seeds for a decent yield. In this case, it is best to set the correct temperature conditions in advance so that seeds are not wasted.
Sometimes planting seeds at the right time and season is the best way to ensure that temperature conditions are right for germination. Some seeds love the heat of spring or summer, and some varieties prefer cool fall conditions. Some even require planting in the fall in order to face cold stratification in the winter that promotes germination in the spring.
In other cases it will be necessary to start indoors and use a tray with a heating mat underneath to start seeds that do not require direct sowing. Most seeds take 75 degrees Fahrenheit to germinate. Seed starter mats can increase the heat of a seed coat by 10 degrees. In this case, a greenhouse or interior would work with a base heat of 65 degrees Fahrenheit.
Seeds that require set temperatures to germinate before planting in your garden can only access their needs if they are planted at the correct depth and in adequately moist conditions.
Sometimes lights can provide enough heat if the seeds are exposed to the right amount. But seeds planted shallowly could be killed in the process.
It's best to keep up with seed germination requirements to see how fussy you have to be with germination.
And now to the light!
Interestingly, most commercial seeds germinate with or without light as they were chosen for their viability outside of the required conditions. However, there are some that need to be exposed to light. More information is ahead of us on specific seeds that may or may not require light to germinate.
Scientists have been studying light requirements for germination for almost a hundred years. By experimenting with different heat and light controls, Kinzel (1926) learned that there are seeds that need light and seeds that need darkness in order to sprout.
Those in need of light sometimes do not need to be covered with earth. This allows the seeds to access the light that will adequately aid the germination process. Those that require darkness to germinate may need to be planted deeper into the ground to block out UV rays that could stop germination.
Is light even necessary?
After germination, you can see the seedling's anatomy: roots, stems and cotyledons. Source: kaibara87
Here's the truth: all seedlings need light to take root. Without them, they become long-legged as they strain to reach for a source of light. The amount of light each seedling needs varies from plant to plant.
Those who need less light in adulthood do not need as much as those who enjoy full sun throughout their life cycle. Seedlings that are exposed to too much light or heat will wither and die.
But when it comes to seeds, some need light to germinate and some don't. Knowing this is important when deciding whether or not to plant seeds indoors or outdoors in your garden. Let's take a moment to look at photosynthesis and examine why some seeds germinate in light and others don't.
The key to understanding which seeds need light to germinate and which ones need darkness lies in plant phytochromes. Phytochromes are light receptors in plants that support photosynthesis through interactions with red ultraviolet light.
There are two types of phytochromes in the plant world: Type I, which is activated by far-red UV light, and Type II, which is activated by red UV light.
For example, trees in the canopy of a rainforest can withstand direct sunlight or red light because of their type II phytochromes. When this light is absorbed, leaves in canopy trees reflect far red light not only back to the light source that received them, but also to forest floor plants below. These plants at the foot of the forest contain type I phytochromes and have a tendency to burn themselves when given red rather than far red light.
So the answer to this question for plants with leaves is always yes. Light is necessary, but the type of light is very important here. Depending on the nature of a plant, such as herbs, the wrong type of light can cause damage. The opposite is also true. Plants given the right kind of light will thrive when all other needs are met.
How much light is enough
If you're dealing with seeds that need light to germinate, you can start them in a tray of grow lights or in a sunny south or north window (depending on which hemisphere you live in).
Alternatively, if it is warm and sunny enough, the seeds can be sown directly on the ground in a thin layer. A greenhouse in direct sunlight in early spring can also provide enough light and warmth for germination.
Seeds that do not require light to germinate should be sown in trays. To make sure they aren't exposed to light, try covering them with black plastic. While covering, monitor the temperature to make sure it stays in the correct area for germination.
Plastic retains a lot of heat and keeps the soil moist, which makes your tray damp.
Seeds that need light
Roots typically develop first, and then cotyledons appear out of the seed coat. Source: Aronalison
Most vegetables require UV radiation to germinate. So we won't list them all here. But here are a number of plants that are commonly grown from seeds and will not germinate without light. All examples here are plants with tiny seeds that have thin seed coats.
- Lettuce: Here we have an example of a seed that doesn't need to be covered in soil to germinate in your garden. Lettuce seeds need to be exposed to light in order to grow into seedlings. You can sprinkle them on soil or vermiculite and then cover them with a thin layer of soil or vermiculite.
- Carrots: Similar to lettuce to germinate carrot seeds, expose them to light by sprinkling them on the surface of the soil. Use the same method as lettuce: plant seeds in a row on top of soil or vermiculite and cover with soil or vermiculite.
- Rose: Rose seeds germinate best in direct sunlight. Keep the soil moisture at the right level and the seedlings will emerge in about six weeks.
- Certain Salvias: Check the strains here as there are some Salvia species that prefer darkness. After sowing directly in the soil of your garden, keep the soil moist, but don't wash away tiny seeds.
Plants that prefer the dark
As we mentioned earlier, there are several species that prefer to exist in the dark. Many of these are popular vegetable garden varieties with thick seed coats and germinate well when deeply covered in dark soil.
- Nasturtium: The benefits of gardening with nasturtiums are many. Directly sow nasturtium seeds about three times their diameter and cover them with abundant organic soil. Soon after, cheerful round-leaved saplings will appear!
- Marigold: Sun inhibits the growth of the marigold. Cover these worm-like seeds in 1 inch of organic soil out of direct sunlight. Keep the soil moist when gardening with marigolds.
- Sunflower: Mammoth varieties need a planting depth of three times their diameter and are best placed under a plastic cover before they germinate. As soon as the first sign of germination shows, remove the plastic cover and expose the seedlings to direct light.
- Onion: Allium seeds are large with a thick seed coat and do not germinate in direct light. They actually prefer long nights during germination. So you can start them indoors under plastic, remove the plastic and put them under light once they have germinated.
Seeds that don't really care
After germination, your new seedlings will need light to thrive. Source: Acme London
The following plants will sprout either with or without light. Your seed sizes tend to fall between the large and small categories of the above.
- Tomatoes: start these indoors or outdoors with the right temperature conditions and in some places a tomato crop will grow by mid-fall.
- Cucumber: Personally, I can't stop a cucumber seed from sprouting. They germinate in almost any condition that affects the soil. If you're looking for a high germination rate, save seeds from your favorite cucumber variety. One thing to remember about cucumbers is to leave out the vermiculite when starting the seeds.
- Eggplant: While eggplant seeds don't need light to germinate, it doesn't hurt either.
- Zinnia: Zinnias must be sown directly in a sunny place under light ground cover. If you've ever tried transplanting a zinnia seedling, you know it is in transplant shock. So avoid starting it indoors. They are a great accompaniment to your tomato and lettuce harvest.
When you are overwhelmed by all of these different factors, start small. Plant a few different types of seeds and keep track of their needs. Follow your progress too! You will learn what is and what is not best for your conditions.
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