Gardeners rightly sow seed heavier than the suggested seeding rate on the seed packet to ensure they get a good row of snap beans, for example. This heavier seeding easily can be justified from past experiences. Perhaps the soil crusted over and emergence was poor, or the squirrel followed you down the row and ate a bunch, or the rains came and the seed rotted.
The smaller the seed, the more easily it will be impacted. Lettuce has a tough time getting through a crusty soil, while a big seed like beans will just break through (though to be honest, we can lose some cotyledon leaves in the process and you get a few runts). A nice trick to be sure crusting does not happen is to cover those smaller seeds with sand. It will not cake or crust over and you get to see where the row is!
The seed packet will take into account all the environmental influences that cause a decline in emergence and plant stand in the row. So you do not really need to sow heavier than that, it is just a habit gardeners get into because “Heck, we have always done it that way.” Gardeners truly get excited to see the first signs of seedling emergence; it’s a great feeling of success and satisfaction that is well-deserved.
However, once those rows are up, it is time to thin them. Radishes and carrots, for instance, need the room to expand. Too crowded, and the carrots are misshapen and the radishes push each other apart leaving flat sides and shoulders pushed out of the garden soil. We thin snap beans to allow each plant to fully develop a canopy full of flowers and later beans. You will get more beans from a properly spaced row than if we do not thin and they are crowded together. Properly spaced rows also will produce longer before finally deciding they are naturally done.
Vine crops, as the name suggests, can easily get out of control and take over a great amount of space, so thinning can help that by limiting the number of vines. Smaller gardens will benefit from those “bush type” cucumbers for example.
There is one crop that traditional thinning can be altered a bit. It is leaf lettuce, especially if you sow your lettuce in a wide row. Harvest those larger plants for the dinner salad, leaving those smaller plants to continue to grow for harvest later. If you have over-sown and need to thin, consider them “micro greens” and add as garnish or work them into other dishes.
The seed packet will provide a lot of other valuable information to get you off to a good start. There may be a picture of the seedling vegetable so you do not pull it out while removing weed seedlings. There also should be a suggested thinning distance for best quality and production.