Effects of Light on Seed Germination and Plant Growth
Germination occurs when a seed comes out of dormancy and grows its primary root and shoot. Light isn't strictly necessary for germination in most plant species. However, some seeds germinate best in absolute darkness, and others perform well with continuous sunlight. Light does become vital for every species after germination, because the initial sprout will not survive if it cannot reach a light source.
Basic Requirements for Seed Germination
All plants need sunlight to grow and survive, but seeds do not necessarily need light to come out of dormancy. This makes sense when you think about how many seeds you've buried under an inch of topsoil that have successfully germinated. According to the Iowa Agriculture Literacy Foundation, the three necessities for seed germination in all plant species are water, oxygen and a warm temperature.
Each of these must be present in the correct measurements, or germination may not occur. For example, many seeds will not germinate in flood waters because there isn't enough oxygen present. Moist, well-oxygenated soil that's either too cold or too hot also won't lead to germination.
However, there's plenty of variation between plant species. Each has its own critical soil moisture level and optimal soil temperature for germination. What one species considers "too cold" or "not moist enough" could trigger germination in another species. It's important to understand the unique requirements of the seeds you want to grow in order to ensure successful germination.
What Happens During Germination
Germination starts when water softens the protective seed coat in a process called imbibition. Once water has penetrated the seed coat, it reaches the endosperm, a starch reserve that makes up the bulk of the seed. This starch converts to sugar as it interacts with water, and sugar is the primary energy source needed to launch the development of the embryo. A root will begin to grow down and anchor the seed into the soil, while a shoot will begin to grow up.
The shoot will continue to grow upward until it is exposed to light. At that point, the first set of leaves will unfurl, and photosynthesis – sugar production triggered by sunlight – can begin. Until it is exposed to light, the shoot relies on the energy reserves in the endosperm. If the shoot uses all of this energy before it reaches an adequate light source, it dies.
Effect of Light on Seed Germination
According to the Pennsylvania State University Extension, most seeds germinate best in darkness. However, begonia (Begonia spp.), primrose (Primula vulgaris) and coleus (Plectranthus scutellarioides) definitely need light to germinate. Thompson & Morgan also names petunias (Petunia spp.), blanketflower (Gaillardia aristata), gerbera daisy (Gerbera jamesonii), ficus tree (Ficus benjamina) and others as requiring or preferring light for germination. In contrast, seed germination experiments in light and dark conditions show that members of the Allium family, blue tansy (Phacelia tanacetifolia), pot marigold (Calendula officinalis) and delphinium (Delphinium elatum) are among those species that require total darkness.
Thompson & Morgan also point out that many seeds will germinate in both dark and light conditions. The number of fussy seeds are few and far between. In fact, most commercial seeds have been selected over time for easy germination, among other factors.
The size of a seed and its dispersal method both offer some clues as to its light preferences. Large seeds have a much larger endosperm, which means a young shoot has plenty of energy reserves to push through inches of soil and mulch before it can start photosynthesis. In contrast, small seeds have a small endosperm and need to immediately reach a light source for survival. In addition, seeds typically buried by animals may have adapted to prefer darkness when germinating.
Effect of Low Temperature on Seed Germination
If you're struggling to germinate seeds from a plant species that doesn't seem to have a light requirement, low soil temperature could be the culprit instead. Soil temperatures outside a species' optimal range can severely slow germination or prevent it altogether. For example, Pennsylvania State University Extension notes that peppers (Capsicum annuum) germinate in just eight days when soil temperatures remain at 86 degrees Fahrenheit, but germinate in 13 days when soil temperature drops to 58 degrees.
Some species can tolerate a relatively wide range of temperatures for seed germination, but you'll get faster results by planting seeds during optimal temperatures. For example, onion (Allium cepa) has been recorded as having germinated in soil temperatures as low as 35 degrees Fahrenheit and as high as 95 degrees, but germination occurs the fastest when onions are placed in soil that's 75 degrees. In contrast, cucumber (Cucumis sativus) seeds prefer being planted in soil around 95 degrees Fahrenheit and can even germinate in conditions up to 105 degrees.
Therefore, it's important to know whether your seeds are best sown outdoors in the spring or summer. Keep in mind that air temperature and soil temperature are different. In fact, according to Corn and Soybean Digest, soil tends to be cooler than the air in the spring and warmer in summer due to more sun exposure and heat absorption. When starting seeds indoors, consider using a heating mat to better control soil temperature.
Other Seed Germination Tips
Some seeds require unusual conditions or extra-special attention in order to germinate. For example, some species like the lodgepole pine (Pinus contorta) have seeds that must be exposed to fire to germinate. This incredible adaptation gives them a competitive edge, as they are among the first species to repopulate land cleared by wildfires. According to Encyclopedia Britannica, fire melts a resin on the seeds so that water can penetrate the seed coat, or chemicals present in the smoke trigger germination.
According to Diane's Flower Seeds, some species have seed coats that are very tough and require a lot of moisture to soften up. When you're in a hurry to germinate seeds, you can either soak the seeds overnight, scuff the seed coats on sandpaper, or snip the sides with clean nail clippers to help water reach the endosperm.
Other seeds won't germinate until they've experienced a particular sequence of temperatures. For example, columbine (Aquilegia spp.) germinates best after it experiences the cold, moist conditions of winter. The 'Purple Sensation' onion needs to alternate between cool and warm temperatures before it will germinate. Because it can be tricky to simulate all the nuances of nature, sometimes it's best to sow these seeds outdoors and see what happens.
Effect of Light on Continued Plant Growth
Once a seed has germinated, all plants require light to grow. The exact amount of light depends on the plant. Some species have adapted to live in shady environments, whereas others need maximum sun exposure to thrive. Knowing what your plants need can help you keep them vigorous and beautiful.
Furthermore, plants need adequate exposure to the correct type of light. Sunlight emits all the UV wavelengths that plants need throughout their life cycle, including red and blue light. Where gardeners may run into trouble is with grow lights that don't give off sufficient quantities of red or blue light.
According to Greenhouse Product News, red light stimulates plant growth. In fact, when plants are grown only under red light, they tend to become more "leggy" than plants grown with both red and blue light. Michigan State University Extension points out that blue light actively suppresses plant overgrowth and also regulates the opening and closing of stomata, which is crucial in transporting and conserving water within the plant. Blue light may also regulate flowering.
- Michigan State University Extension: Effects of Blue Light on Plants
- Greenhouse Product News: Red Light and Plant Growth
- Diane's Flower Seeds: Germinating Difficult Seeds
- Encyclopedia Britannica: Playing with Wildfire: 5 Amazing Adaptations of Pyrophytic Plants
- Corn and Soybean Digest: 7 facts you didn’t know about soil temperature
- Thompson & Morgan: The Effect of Light on Germination and Seedlings
- Pennsylvania State University Extension: Seed and Seedling Biology
- Iowa Agriculture Literacy Foundation: Science 101: Germination
Cathy Habas has been a full-time freelance writer since 2014. She graduated from Indiana University Southeast with a Bachelor's degree in English. She enjoys writing content that helps homeowners feel motivated and confident in handling projects around the home. Her work has been published around the web, including on home improvement sites like Ron and Lisa.