Starting seeds indoors or outside is not complicated, but you do need to know what conditions the seeds you're planting need. Many gardeners are unaware that some species, such as begonias, geraniums, and petunias, require light to germinate, and covering them with soil will inhibit their sprouting.
There's a general seed planting rule that says you should plant a seed to a depth that is three times its thickness. That means fat bean seeds should be planted one to three 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, though. Some seeds need the stimulus of light hitting them before they will break dormancy and start to germinate. Very often it is seeds that self-sow that require light. These plants, such as balloon flower (Platycodon grandiflorus) and poppies, drop their seeds on the soil and they germinate where they land. They sprout in response to environmental factors, including having the light hit them.
Seeds That Need Light to Germinate
There are several seeds that germinate best when they are exposed to light. If these seeds are covered in soil, chances are they will remain dormant and not sprout until conditions improve. It seems counterintuitive not to bury seeds, but these seeds should only be pressed onto the surface of the soil and kept moist to germinate. They include:
- Balloon flower
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 underneath them or covering them. Flowers such as alyssum and cosmos will self-seed during their current growing season as well as the next one, whether or not they are exposed to light. Other seeds that will germinate uncovered include:
- Cole Crops
Although the seeds listed above do not require a covering a soil, you will probably get better germination if you follow the recommended planting depth because it will be easier for you to keep them moist and safe from hungry birds.
Keeping Seeds Moist When Exposed to Sunlight
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. Animals, wind, heavy rain, 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 clear 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 directly sown outdoors, another 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. It can usually be found near seed-starting supplies; look for finely ground horticultural vermiculite, as other types are not suitable for gardening.
This may all sound complicated, but most seed packets will tell you exactly what you need to know. And remember, seeds have been sprouting for millennia without a lot of fuss––it's just nice to give them the best chance possible by providing them optimal conditions so they can thrive.
Seed and Seedling Biology. Penn State University Website