Seed Planting Depth

Plant a seed at the proper depth and it thrives. Plant it at the wrong depth and it dies. So, how is a gardener to know how deep to plant each seed?

The general rule of thumb for seed planting depth is to use twice the seed width or diameter. This means a seed that is 1/8” wide should be planted 1/4” deep. Too often, seed packets rely on a standard 1/4”  or 1/2” planting depth, which is not always in a seed’s best interest. ​​

A seed’s life

Once a seed reaches maturity, it is protected by a hard outer shell called its seed coat. This layer allows the seed to roll, float, fly, or pass through a digestive system unscathed and ready to germinate. Only when heat, light, and moisture levels reach the right combination can the seed coat allow moisture in, which triggers the growth of the embryo within. Hopefully, the seed is at the proper depth to grow into a healthy plant.

Planting seeds too deeply

Seeds planted too deeply are less likely to germinate or sprout. Even if they do get that far, those seedlings have to work harder to reach the soil surface. This uses up valuable energy resources that should have been available for producing leaves and stems. It also reduces their growing season.

The very tiniest seeds should simply be sprinkled on the soil surface and only dusted with a covering of soil or vermiculite. And a few seeds, such as lettuce and dill, actually need light to germinate, so they should not be covered at all.

Planting seeds too shallowly

At the other end of the planting depth spectrum, seeds planted too shallowly end up with roots too close to the surface, causing them to wither and die. Even if they don’t die, shallow roots are less able to take up water and nutrients or to anchor plants in the soil. Seeds planted too shallowly are more susceptible to temperature and moisture level changes. These plants are also more likely to bolt, rather than producing a crop.

Seed planting depth also depends on soil structure. Seeds planted in rich, loose loam or sandy soil can tolerate deeper planting than seeds planted in heavy, compaction-prone clay. 

Crops by seed depth

Here is a list of some of the more popular garden plants and their proper planting depths:

•     1/8 to 1/4 inch - celery
•     1/4 inch - arugula, broccoli, cauliflower, peppers, radish, tomatoes
•     1/4 to 1/2 inch - cabbage, carrots, eggplant, onion
•     1/2 inch - beets, cucumbers, melon, spinach, Swiss chard, watermelon
•     1 inch - pumpkin, squash
•     1 to 1-1/2 inch - soybeans
•     1 to 2 inches - beans, peas
•     2 inches - corn
•     3 to 4 inches - potatoes

Tools, such as a dibble, can help ensure that seeds are planted at the proper depth.

Finally, do not press down on the soil once a seed is in place. This compacts the soil. Instead, the act of watering will tuck your seeds in nicely while still giving them a chance to grow and thrive.