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 be barely 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.
Seeds that Germinate Best if Exposed to Light
Some seeds actually germinate best if they are exposed to light. If these seeds are covered in soil, chances are good they will never sprout. 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 Light
Most plants that self-sow in your garden are able to germinate without being covered with soil. However, that doesn't necessarily mean they need light. Several plant seeds are indifferent to light exposure. Flowers like alyssum and cosmos will self-seed during their current growing season as well as the next one. Other seeds that will germinate uncovered include:
How to Keep Exposed Seeds Moist
Although they don't 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're exposed. If you are growing them in flats or containers, you can cover them lightly with plastic wrap, plastic domes or tuck them inside of plastic bags.
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. Vermiculite is porous enough to let 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. The vermiculite sold in home improvement stores is used in insulation and plaster and does not have the same water absorbing quality.