There's a general seed planting rule that says you should plant a seed to a depth that is 3 times its thickness. That means fat bean seeds can be planted 1 to 3 inches deep and tiny carrot seeds should barely be covered. Most seed packets will take the guesswork out of the process and tell you how deep to plant the seeds. It's a good idea to follow these recommendations because a seed that is planted too deeply might not have enough stored energy to push itself above the soil line.
There's an exception to every rule, including the best practices for planting seeds. Some seeds need the stimulus of light hitting them before they will break dormancy and start to germinate. Very often it is the seeds that self-sow, that require light. Seeds, such as balloon flower (Platycodon grandiflorus) and poppies, that drop their seeds on the soil and germinate where they land are responding to environmental factors that include having light hit them.
Seeds that Germinate Best if Exposed to Light
There are several seeds that actually germinate best if they are exposed to light. If these seeds are covered in soil, chances are good they will remain dormant and never sprout until conditions improve. It seems counter-intuitive not to bury seeds, but these seeds should only be pressed onto the surface of the soil and kept moist, to germinate. They include:
Seeds that will Germinate With or Without Exposure to Light
While most plants that self-sow in your garden are able to germinate without being covered with soil, that doesn't necessarily mean they absolutely need light. Some plant seeds are indifferent to light exposure and simply need to make contact with soil, whether it is only underneath them or surrounding them. Flowers such as alyssum and cosmos will self-seed during their current growing season as well as the next one, even if they are left exposed to light. Other seeds that will germinate uncovered include:
How to Keep Seeds Moist, When They are Exposed to Sunlight
Although the seeds listed above that are indifferent to light do not require a covering a soil, you will probably get better germination if you follow the recommended planting depth, simply because it will be easier for you to keep them moist and safe from hungry birds.
Being able to sow seeds on the surface of soil makes planting easier, but keeping them moist until germination can be difficult since they are exposed to more than just light. Hungry birds, winds, passing animals and digging gardeners can all disturb or remove seeds from your garden. If you are growing your seeds in flats or containers, you can cover them lightly with plastic wrap, plastic domes or tuck them inside of plastic bags. They will still be exposed to sunlight, but they will not dry out as quickly as if they were left open to the elements.
For seeds sown outdoors or if you prefer not to use plastic, another viable option is to cover the seeds with a thin layer of fine vermiculite, a naturally occurring mineral with water holding properties. Vermiculite is porous enough to let the light shine through while retaining enough water to stay in place and keep the seeds and soil under it moist.
Vermiculite can often be found near seed-starting supplies. It's a mica-type material that is heated up and expanded to increase its water holding. Vermiculite is the small silvery metallic-looking flecks often included in seed starting mixes. The particles soak up water and nutrients and hold them in the mix until the plants are ready to access them.
Look for finely ground horticulture vermiculite, in the plant nursery section of a store. The vermiculite sold in home improvement stores is used in insulation and plaster and does not have the same water absorbing quality. It may even have things added to it that you would not want in your garden.
This may all sound complicated, but as mentioned, check your seed packet to know whether you need to worry about providing light to your newly planted seeds. Seeds have been sprouting for centuries without a lot of fuss, but it is nice to be able to give them a little extra care when we are looking forward to seeing them in our gardens.