While many seeds need to be planted to a depth relative to their size, about 2 times as deep as they are large, some seeds actually need to be sown right on the surface of the soil with barely anything to cover them from the elements. These seeds are usually small, although that is not always the case.
Many annual and perennial flowers have tiny seeds that can be sown on the surface for surface planting. These seeds usually need little more than a light sprinkle of soil on top of them, no more than ¼ inch thick. Flower seeds for surface planting include marigolds, sunflowers, baby's breath, snap dragons, petunias, hollyhocks and begonias.
Many vegetables have small seeds too that are best for surface sowing instead of actually planting These vegetables need a little bit of light in order to germinate properly. Vegetable seeds for surface planting include lettuce, corn, carrots and onions. Other vegetables with large seeds that can handle surface planting are beans, squash and pumpkins.
While tree seeds are usually not small, some of them cannot handle being planted too deeply or they will never be able to reach the surface. This happens with some nut trees, so they need to be planted on the surface with only a little bit of soil on top of them, no more than an inch. Trees seeds for surface planting include: oak, butternuts, black walnuts, hickory-nuts, and chestnuts.