Many years ago when I first started to garden for myself, I acted like every seed and every seedling was sacred. I just couldn't bring myself to purposely kill a seedling. I don't know why I was like that, but it caused me grief in the long run. I often ended up with rows that were far too crowded to properly produce. When I sowed pots in my greenhouse, although I tried to only drop one seed per pot, with the tiny seeds especially, I'd end up with a clump. Then I'd go carefully separate each baby seedling and repot them. And of course, when i was successful in planting only one seed per pot I'd end up with lots of empty pots where I was wasting the potting soil, the fertilizer, and the space it took up. If the seeds had only 80% germination, then 20% of the pots were empties! Duh.
Although I still tend to be quite frugal, I no longer treat every seed as a Demi-god to be saved and nourished regardless of sensibilities. Ha, now it's more like "off with their heads"! Well, except for the real expensive seeds. Those I still coddle.
I've learned that some seedlings are very easy to transplant in the very early stages, before they even get true leaves. Thus when I find I've sown them too closely, I can easily tease the excess seedlings out of the soil and plant them somewhere else. All the brassicas are easy....cabbage, kale, cauliflower, broccoli, etc. So are tomatoes, eggplants, peppers. I'm fairly successful with lettuce too. I can thin these veggies without sacrificing or discarding any of the seedlings.
(Onion seedlings planted one week ago. In a couple of weeks they will be large enough to tease apart and transplant. Right after taking this photo a lightly mulched the ground to help promote faster growth.)
Some veggie seedlings are far more fragile, making handling them more difficult for me. I'm definitely less successful. Things like beets, carrots, many herbs. I have a tendency not to bother trying to tease these crowded seedlings apart. I'll just sacrifice the extras. I'll use a scissors to snip the culls off at the soil. Sometimes the leaves are big enough to use for salads, soups, etc. so I'll save the snipped off culls for the kitchen. Why use scissors? Pulling the culled seedlings out can disrupt the soil, causing the one plant that I wish to keep to get accidently pulled out too. Boy, I've done that way too many times until I figured out to use a scissors instead of my clumsy fingers.
My neighbor sowed some onion seeds for the gardens here. He planted way too many seeds per pot. We decided to transplant them to the garden "as is", then let them grow up a bit to a better size for handling. After a bit of growth we gently separated the seedlings and transplanted them. It worked out fine. We had none die this way. But if we had tried to separate them earlier, I'm guessing we would have lost half, if not more.
The same neighbor also planted broccoli seed too thickly. As an experiment, we transplanted the seedlings into the garden still in their little clumps. As the plants grew, we snipped off the slower growing individuals....eating them in stir fries. As the plants grew bigger, we again snipped the smaller ones out, using their leaves and tips for the dinner table. We eventually ended up with the most robust plants being left to produce the broccoli heads.
Why bother to thin at ll? Because when there are too many plants too close, they compete for the resources -- the space, the sun, the water, the nutrients. The most robust seedlings win, but at a price. They get stunted to some degree because of the struggle competing with others. With some vegetables, they won't produce well at all if too crowded. The first time I sowed beets, I got only a couple nice big beets. The rest of the bed was nothing but tops. Too crowded.
Many plants do better when planted within reasonable distance from companions. I'm guessing it has to do with wind protection and higher humidity around the plants. Just a guess. But I've seen that ypung plants spaced 2 foot apart grow slower than those same plants planted 6 inches apart. So I take advantage of that quirk by planting with closer spacing than my final goal. Then over the next few weeks of growing I'll thin out the extras. Which to discard? The weaker, smaller, odder looking plants. This method works good for me with most veggies.
So what to do about those expensive seeds that I do to want to waste? I will sow the seeds individually if I can. Any seedling clumps will be teased apart and transplanted. Out in the garden I will use a quick crop, like radishes, as their companions to promote faster growth...then thin out the radishes.