Information on how to plant your seeds in often printed on the seed packets. This information includes how far apart to space your seeds and the depth to which the seeds should be planted. The seed packet may also list how many days until germination of the seed. The optimum planting depth and the days to germination, though, are influenced by several factors.
The size of a pumpkin seed is approximately one-quarter inch in diameter. Garden folklore recommends planting seeds at three times their diameter. This indicates a pumpkin seed should be planted at three-fourths-inch depth. A 1-inch depth, then, provides you with a bit of leeway when planting your pumpkin seeds.
The smaller the seed, the less depth required for planting. Marigold seeds, for example, are so small that a planting depth of less than a quarter of an inch is sufficient.
An ideal soil, loose and rich in nutrients, allows you to plant seeds deeper into the soil. The emerging seedling is able to break through and establish a strong root system. The depth gives the seed the time it needs to convert the stored food reserves within the seed to energy, in turn producing a seedling. The denser the soil, though, the shallower you should plant your seeds.
Seeds need water to aid in the conversion of their food stores. The larger the seed, the more water is required. The soil surrounding the seed retains moisture, giving the seed a water supply. If the planting depth is too shallow, the seed isn't able to draw in enough moisture to germinate as the surface of the soil dries out. Too deep, though, and the seed may "drown" from lack of oxygen.
The temperature of the soil influences germination. Warmer soils promote germination, while cool, damp soils may inhibit the seed and may cause the seed to decay. In spring, your garden's soil may still be cool, and the deeper you plant your seeds, the colder the soil surrounding them. Should a seed be chilled for an extended time, even as little as two weeks, it may decay. A decayed seed does not germinate.
Though some seeds require colder environments to germinate, most plants require soil and air temperatures of between 60 and 75 degrees to germinate and establish root systems. Decrease the planting depth for early spring plantings, increasing the depth as the soil reaches warmer temperatures.