If you cannot get some seeds to germinate, the reason could be not having the right temperature, or it could be the light.
Most seeds, especially medium and large-sized seeds, do not need light to germinate. Some of the large seeds even require darkness to germinate. Only a small number of seeds, especially the tiny ones, require light to germinate.
In this article, we list out all the common vegetable and herb seeds that require light and darkness to germinate and will explain why they need it to do so.
Tiny seeds smaller than ¼ inch (6 mm) generally require at least 8 hours of light to germinate.
They should be sprinkled on the soil and patted down lightly in soil no deeper than the height of the seeds themselves which is between ⅛ to ¼ inch (3 to 6 mm). This is to retain some moisture while maintaining contact with light.
But there are some exceptions where tiny seeds such as tomato seeds need to be covered with soil to prevent the passage of light despite their small size. The full list of such seeds is in the next section.
Here is a list of vegetable and herb seeds that require light to germinate.
Lettuce seeds are incredibly small and lightweight and require sunlight to germinate. As such, lettuce seeds should be sprinkled on the soil or patted down to a depth of ⅛ inches (3 mm) only.
Lettuce seeds are flat, oval-shaped, with different colors like near-white, grey, and dark brown depending on the variety.
Celery seeds are tiny and range from light brown to dark brown in color.
They should be planted at a depth of ⅛ inch (3 mm) or simply patted onto the soil surface.
Although celery seeds requires light to germinate, they should be covered lightly with soil or a thin transparent film to keep them warm, as celery seeds don’t germinate at low temperatures.
The seeds of the dill plant are small, flat, and round. These seeds require light to germinate, and as such should be planted at a shallow depth of around ⅛ inch (3 mm) or simply spread on the soil and patted down lightly.
Rosemary has small, brown, egg-shaped seeds. The seeds are sown at 1/4 inch (6 mm) in the soil, and with a light covering of soil.
Thyme seeds are tiny, round, brown and grain-like. They require light and warmth to germinate and thus should be planted at a depth of 1/16 inch (1.5 mm) or simply sprinkled over moistened soil and patted lightly.
6. Lemon Balm
The small lemon balm seeds are brown or black with an oblong shape. These seeds need light to sprout and should be planted at no deeper than ⅛ inches (3 mm).
Oregano produces tiny oval-shaped, dark-brown seeds. They should be planted at a depth of ⅛ inch (3mm) or simply patted onto the soil surface.
Large and medium-sized seeds generally do not need light to germinate. And roughly half of these seeds actually require darkness to germinate while the other half do not have a preference and can germinate in both light or darkness.
The seeds that require darkness to germinate are “negatively photoblastic”, meaning light hinders germination. These seeds contain phytochrome A, which reduces the production of germination-stimulating enzymes when light hits.
Seeds that only germinate in darkness are often big seeds up to 1 inch (2.5 cm) long. Since they need to germinate in darkness, they have to be sown at least 1.5 inches (3.8 cm) deep in the soil to ensure that the entire seed is in contact with the soil.
Some seeds are “neutrally photoblastic”, and can germinate in light or darkness. For these seeds, germination is influenced by soil moisture and temperature and not by light. But there is still a preference for darkness because of moisture availability.
There are exceptions, however. Some seeds such as tomato and onion seeds are quite small but they also need darkness to germinate. They need to be covered with some soil to prevent contact with light during germination but not too deep due to their small size, best at ¼ to ½ inch deep (6 to 12 mm).
Asparagus seeds are medium-sized, round, and black. They don’t require light to germinate and are generally buried at a depth of 1 inch (2.5 cm) in moist soil.
Artichokes produce moderately sized oval seeds that could be brown, off-white, or slightly green.
The sowing depth for these seeds ranges between ¼ to ½ inch (6 – 13mm).
Basil seeds are small, black, and round. They can germinate in dark conditions but should be planted no more than ¼ inch (6 mm) deep, and covered lightly with soil for warmth.
All varieties of beans do not require light for germination, and can be buried at depths of 1 inch (25 mm).
Beets or Beetroot seeds are unique, as each seed contains multiple seeds rather than one single seed. The seeds are light to dark brown and have a boxy shape. They germinate in the dark, at depths of ½ inch (13 mm).
6. Broccoli, Brussels Sprouts, Kale, Cauliflower, Kohlrabi
These vegetables are all brassicas, originally derived from a single plant. As such, they have very similar-looking seeds that are round, small, and range from orange to dark brown.
The seeds are sown about ½ inch (13 mm) deep and they will germinate in 8 to 10 days at an optimal temperature of 70 to 77 F (25 C).
Carrot seeds are small brown and oval with ridges. They can be planted at a depth of ¼ to ½ inches, and they’ll sprout after 15 to 21 days.
8. Coriander and Cilantro
Coriander and cilantro are different parts of the same plant. Coriander and cilantro seeds are medium-sized and light brown in color with ridges.
They should be sown at a depth of ½ inch (6 mm). Germination takes 7 to 10 days.
Chives seeds are small and flat.
They need to be planted shallowly at depths around ¼ to 1/2 inches (6 – 13 mm). Seeds will germinate for 10 to 14 days at 77 F (25 C).
The seeds are flat and oval and could be cream, tan, or white colored, measuring about 7mm in length. These seeds are sown at a depth of 1 to 1.5 inches (25 – 38 mm). They germinate after 7 to 10 days.
Eggplant seeds are mid-sized, flat, tan-colored, and vaguely bean-shaped. Recommended sowing depth is ¼ to ½ inches (6 – 12 mm) and germination takes 7 to 14 days.
Mint seeds are small, dark brown, and round. They do best at depths of ¼ inches (6mm), and a light covering of soil.
Onions produce black, flat, round seeds. They are very small and weigh little. They are best planted to a depth of ¼ inch (6 mm), and covered with a bit of soil.
The crescent-shaped seeds are small and lightweight with light to dark brown coloring. Parsley seeds should be buried ¼ inches (6 mm) in the soil.
Pea seeds are medium-sized, round, and brown. Sow the pea seeds at a depth of 1 inch (25 mm).
Peanuts are large seeds. They could be red or brown colored and are best planted 2 inches (51 mm) into the soil.
Pepper seeds are flat, circular, and cream-colored. The seeds germinate in darkness at depths between 1⁄4 to 1⁄2 inches (6 to 12 mm).
Radish seeds are small and look like orange-colored pebbles. The ideal planting depth is ¼ to ½ inches (6 to 12 mm).
19. Watermelon, Squash, and Pumpkin
Watermelon, Squash, and pumpkin are all from the same genus and family (Cucurbitaceae) and have similar-looking seeds that are flat and oval, and about ½ inch (10mm) long.
All three types of seeds should be planted at the same depth of 1 inch (25 mm) in soil.
20. Sweet corn
Corn seeds come in red, yellow, and white. The seeds are thick and have a triangular-like shape. The recommended depth for sowing corn is 1.5 to 2 inches (38 – 50 mm).
The seeds of the tomato fruits are small and flat. The right sowing depth is ¼ inches (6 mm) and the seeds should be covered with loose soil.
Tiny seeds generally require light to break seed dormancy in order to germinate because they have limited food reserves to help them push through layers of soil to reach the light.
Seed germination is an energy-consuming process that uses the food stored in the seed. Burying a seed deep under the soil would require it to have more energy to push through the soil to the surface for the seedling to receive light.
Since the quantity of resources stored in the seed is relative to the size of the seed, tiny seeds in particular need to use their reserves more efficiently than the larger ones.
Because of that, tiny seeds are programmed to germinate only when hit by light. This is an evolutionary trait that ensures that small seeds only germinate at the surface of the soil, rather than deep in the soil where they can’t emerge.
For these seeds, light breaks seed dormancy by triggering their photoreceptors (called phytochrome B) to suppress the production of abscisic acid which maintains seed dormancy, and to stimulate the production of the gibberellins hormone to germinate.
The reason why large seeds prefer to germinate in the dark is believed to be related to the higher level of moisture in the depths of the soil compared to the surface layer where there is less moisture.
According to a study of 54 types of seeds with different sizes, the light requirement for germination is found to be lower with bigger seeds (Milberg et al, 2000). This indicates that bigger seeds do not require light to germinate.
Take a look at this experiment of large seeds. Those that are put in darkness germinated much better than those put in the light to sprout.
The size of the seed determines how deep it should be sown. A general rule of thumb is to plant seeds at a depth twice the length of the seed.
Smaller seeds are planted at or very near the soil surface because their size limits the food stored in the seeds, which will not be enough for new seedlings to push through several layers of soil to get to the surface.
On the other hand, bigger seeds need to be planted deeper into the soil to have access to more moisture in the soil and they have more energy reserves to get the seedling to the surface.
Sawada, Y., Aoki, M., Nakaminami, K., Mitsuhashi, W., Tatematsu, K., Kushiro, T., Koshiba, T., Kamiya, Y., Inoue, Y., Nambara, E., & Toyomasu, T. (2008). Phytochrome- and Gibberellin-Mediated Regulation of Abscisic Acid Metabolism during Germination of Photoblastic Lettuce Seeds. Plant Physiology, 146(3), 1386–1396. https://doi.org/10.1104/pp.107.115162
Ha, M, & Morrow, M. (2022, May 4). 18.4: Germination. Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/Unit_3%3A_Plant_Physiology_and_Regulation/18%3A_Development/18.04%3A_Germination
Flores, J., González-Salvatierra, C., & Jurado, E. (2016). Effect of light on seed germination and seedling shape of succulent species from Mexico. Journal of Plant Ecology, 9(2), 174–179. https://doi.org/10.1093/jpe/rtv046
Arana, M. V., Burgin, M. J., De Miguel, L., & Sánchez, R. A. (2007). The very-low-fluence and high-irradiance responses of the phytochromes have antagonistic effects on germination, mannan-degrading activities, and DfGA3ox transcript levels in Datura ferox seeds. Journal of Experimental Botany, 58(14), 3997–4004. https://doi.org/10.1093/jxb/erm256
Milberg, P., Andersson, L., & Thompson, K. L. (2000). Large-seeded spices are less dependent on light for germination than small-seeded ones. Seed Science Research, 10(1), 99–104. https://doi.org/10.1017/s0960258500000118