Light is not necessary for a seed to germinate, no. The majority of seeds grow most effectively in the dark. Light, which is essential for seedling growth, may actually hinder the germination process. The three primary and necessary conditions for a seed to germinate are water, oxygen, and temperature.
Not all seeds have the same light requirements. Most seeds germinate best under dark conditions and might even be inhibited by light (e.g., Phacelia and Allium spp.). However, some species (e.g., Begonia, Primula, Coleus) need light to germinate (Miles and Brown 2007).
Days 1 – 3: Place your seedlings outdoors in a shady spot that will only get 3-5 hours of direct sunlight throughout the day. Days 4 – 5: Place your seedlings in a slightly sunnier spot that will get about 5-6 hours of direct sunlight. Days 6 – 7: Place your seedlings in a full sun spot (6+ hours of direct sunlight).
The only seeds that will germinate faster with light are the ones mentioned above in this article (or other seeds that require sunlight to germinate). Most seeds do not need grow lights until after they germinate. Most seeds germinate faster if they are warm.
Some plants can be sown outside direct into a shady spot. An area shaded by deciduous trees or hedging, which won't be in full shade until May or June, is ideal for crops that don't need much warmth to germinate, such as chard and beetroot.
Once those seeds sprout, proper seedling care is essential, and that includes supplying the growing plants with the right amount and quality of light. It is true that seedlings need lots of bright light to grow healthy and strong—but they also need a period of darkness in order to thrive.
For many years, fluorescent shop lights were the go-to seed starting bulbs for gardeners, but LEDs have quickly replaced them as the standard choice. LEDs are usually more expensive than fluorescent bulbs, but they are very energy-efficient and long-lasting.
Pre-Emergent Herbicide: Prevents seeds from germinating or kills germinating seeds before seedlings emerge from soil; must be applied before weed seeds germinate. A common example of a pre-emergent herbicide is a Crabgrass preventer, which prevents Crabgrass seeds from establishing new plants.
Most seeds will not germinate without sunlight and will perform best with 12 to 16 hours each day. Indoors, place seed containers in a sunny, south-facing window and give the container a quarter turn each day to prevent the seedlings from overreaching toward the light and developing weak, elongated stems.
The light slows stem elongation through hormones that are sent down the stem from the tip of the stem. In the darkness, the hormones do not slow stem elongation. The seeds in the dark-grown condition rely upon the stored chemical energy within their cells (lipids, proteins, carbohydrates) to power their growth.
For the same reason you don't store your bag of potatoes in a well lit place, you should not store your seeds in one. Light is another important cue to the seed that it is time to sprout. When storing your seeds in see-through glass and plastic containers, it is important to put these containers in a dark, dry place.
While the speed of germination varies slightly across varieties, seeds should absorb moisture within two days and produce a root and the first leaf within four days. At this point, the seed is considered to have germinated.
Some seeds like Allium 'Purple Sensation' and Eryngium giganteum need a long period of cold-moist stratification with alternating temperatures. These are the most difficult seeds to germinate.
Until seeds have sprouted, keep the seed bed moist, never allowing it to dry out. Water with a fine-spray hose nozzle or watering can which will provide a fine misty spray and not wash away the soil. Water often enough (usually about once a day) so that the soil surface never dries out, but remains constantly moist.
Soaking can accelerate germination and increase the proportion of seeds that successfully germinate. Seeds have evolved a protective coating around the embryo so the embryo can survive being dispersed by some means. The coating protects the seeds until they find themselves in a suitable place to grow into a new plant.
Some seeds don't need light to break through their seed casings and sprout. Most seeds germinate best with controlled amounts of UV generation, but there are seeds that germinate in the absence of light.
For the average home gardener starting seedlings indoors, a fluorescent or LED lamp will usually be the best choice to ensure that your plants get the quality, intensity, and duration of light they need to stay in peak condition.
Fluorescent or LED Lights
For most home gardeners, a plug-in shop fluorescent or LED light fixture is the perfect type of light to use for seed starting.
Will grass seed germinate on top of the soil? Yes; in fact, germination will suffer if too much soil is placed on top of the seeds. The experts at Jonathan Green recommend placing a thin layer of mulch or topsoil over them to help keep them moist and warm and promote growth.
Small seeded plants that form soil seed banks for instance, are expected to germinate only in the presence of light, as a mechanism to avoid germinating too deep in the soil where they would deplete seed nutrients before reaching light for photosynthesis (Pons 2000).