Seedling Thinning Philosophy? ( Weak vs Strong )

I'd always heard that if you want healthy tomato plants, you need to select the seedlings that have the thickest stalks and thin the rest. Is that really true? Is there any possibility that certain seeds actually overdevelop too quickly and later lack performance?
I'm also reminded of the folly of choosing a tomato seedling at the nursery that is big and showy with lots of foliage. It looks nice, but I've heard it's better to go with a smaller plant, because the root ball won't be stunted. So choosing the perceived "weaker" seedling can actually pay off.

So there are a lot of variables here. If you start your own plants from seed at home, yes select the most vigorous, robust plants from the lot. If you're buying plants from the nursery or any retail place, here's where looks can be deceiving. For example, the bigger, healthier looking plants may just in fact be older, and have had more time to grow, and the smaller ones will eventually look just like the bigger healthier one next to it, given another week or two to grow. Another example, perhaps some have been given a dose of high nitrogen (urea nitrogen specifically) fertilizer, giving the plants height and a lush, healthy deep green appearance, somewhat disguising the less vigorous plants making them appear great. The interesting thing with crops is much of the plants later fruit production ability and vigor is decided early on in life. If the seedlings and baby plants are allowed to wilt or get stressed with lack of (or excessive) nutrients, it can have a negative affect on the plants fruit production weeks and months down the road. If I were buying plants from a nursery, I would still choose the best looking plants.

When seeding at home, you want to select the best looking plants, cull the weaker ones.   It is unlikely that a weaker plant will catch up and outproduce a stronger one, but... it is not impossible... the plant might just be rooted on a stick or rock in the soil, and once transplanted, could take off to better it's more robust neighbor.  The strongest looking, healthiest looking plants in the lot are usually going to continue to be so.

 If in a nursery, it is sometimes best to go for the nicest looking plants which are actually younger than and smaller than some others, as this gives you the better transplanting potential, and the more chance that you can further influence it's bountiful growth with your own amazing techniques.

I was kind of thinking, what if the thinner ones actually have traits that would be good to have. Like disease tolerance, or healthier fruit. Idk, just some off thing you'd never know about.

The only way to determine disease resistance traits when purchasing plants at a nursery is by the name of the variety. Unless disease tolerance is directly noted on the label, the best way to go about this is a few minutes with the smart phone and check the googles. For tomatoes, for example, often the hybrids for sale have been selected for disease resistance, and sometimes for other qualities like skins that resist cracking. Better Boy is a very common hybrid sold all throughout the country. It has a nice smooth skin and has good resistance to Verticillium wilt, Fusarium wilt and undesirable nematodes. As far as having healthier fruit, that is largely determined by the soil they are grown in. Soils that are well draining, teeming with biologically active life, containing compost and organic matter, and well balanced with organic nutrients will yield the healthiest, most nutrient dense fruit.

what if the thinner ones actually have traits that would be good to have. Like disease tolerance, or healthier fruit. Idk, just some off thing you'd never know about.

 This is always the case in anything in genetics.  You may be onto something, but nature always culls... so you have to consider that.  If you planted say four seeds in a two inch pot, and if they all came up you'd best cull some of them, probably three, so that the one that you wanted would be able to get the most out of the space.  Now you could cull to keep thinner seedlings, or whatever trait you saw that you thought interesting... there is nothing wrong with that, and scientifically there could be merit in it.  You could hedge your bets by culling some of your pots this way (keeping the thinner looking ones), and some the other way (keeping only the strongest looking), clearly labeling them as such and seeing the results.  

Like I said, with genetics you never know for sure, and mutations happen all the time.  Many of the great human inventors were people who were somewhat outside of the norm in society... but without them society would not have made many great leaps forward.

I do think that if you are starting multiple tomato seeds in a pot, and one of the starts is spindly because the seed was further away from the light, you would probably be better off removing it because it'll never recover from that early stunted growth. Or so I've heard.

Seedlings that don't get enough light do indeed stretch. That little seedling spent energy stretching towards light that it could have used for initial root growth and its first set of leaves. Wether or not it will never recover or not is debatable. If there are many sprouts to choose from it's always best to cull the ones that obviously stretched.

I tend to cull the smaller plant. Here's a photo of two okra plants, sown on the same day. I know which one I culled!!!

Isn't it possible to let the plants 'fight it out' by themselves? I mean: leave them all there and the weaker ones will disappear because the stronger ones 'win'? (this is only a theory, a philosophy, ... )

If you sow individual seeds in 1 & 1/2 " tubes 3 inches tall filled with quality weed free compost ,  you'll soon see which plants are best for planting .  Push the selected plants up out of the tube from the bottom so you don't damage or disturb the delicate root balls or the fine cell structure of the tender young stems .  Standing them in 3 inches of water for two hours beforehand helps tremendously .


TIP  
Notice the fine hairs on the stem below the leaves . Don't damage them ,handle them as little as possible. Instead handle the plants by the largest leaves that formed after the first seed leaf stage .

 Either make a deep enough hole to pop in the whole plant in that you've slid out of the tube , so you get a lot of those hairs below the ground as they'll make roots too  
or
Scrape a shallow trench about four inches long , just deep  enough to cover the tube form of then root ball when ht plant is laid on its side & long enough to take the root ball and as many of the fine stem hairs as you can whilst still leaving the leaves out the ground . Again the fine hairs will make lots of roots to fed the plant . Then re cover & gently water through a fine watering rose head so you don't wash the covering away .

The roots will form from nearly all the fine hairs you've put below the soil  and then start to grow downwards .

Both these methods will give your plants the best possible chance of good crop production so long as you correctly :- Water , Nourish , treat for pests & give  enough warm dry moving air around the plants .


Plant spacing is so important , right from the time you sow the seeds through to setting the plants out to grow at two feet (600 mm ) apart .  Sow them too close & you get any number of plant complaints from damping off of the seedlings to tomato blight if they are too close & it's too humid .

Tomatoes love growing in warm dry sunshine with a consistent level of root moisture & nutrients related to the stage they have grown to .

Culling by cutting or squashing a seedling in the pots or beds next to the best plant is widely practiced but not recommended ,  I say this for you have a tender root of the crop dying and attracting plant species pests / diseases t to the immediate area . Hence me using the growing tubes method ..I grow all my plants from seed or cuttings that need it well away from the final planting areas .

Simply pulling the seedlings out the soil to cull them is a folly .  if you get a good magnifying glass & a 20 x microscope  you will be disgusted at the amount of cell structure damage you've caused on each plant
. You also be able to see where the microscopic feeding hairs on the hair roots have been stripped off .   Yes the plants can usually over come this damage over a few days , but with the method  I use it just does not happen. My plants don't get any transplant  damage at all .

Why in tomatoes is everyone talking about culling tomato plants?  If you get it at the 2 to 4 leaf stage you simply pull the plant and poke it in another pot or in the ground.  Nearly every plant I move at this age grows and if they don't no great loss.  I normally plant 2 seeds in every cell.(ideally diagonally opposite corners so I can get my fingers in.)  Then if both come up I pull one and move to another location.  If you let them get above 4 leaf you will see long term damage from transplant shock but in the 2 to 4 leaf stage some die but most live with no visible damage from this early transplant.(so the cotyledon leaf pair and the first pair of true leave or before.)  Occasionally the root breaks off pulling them in which case they are culled but most the root pulls with the seedling and is easily replanted.  They will be set back for a few days but rapidly catch up with no loss I can see from then on.  In fact seeding transplanted at that stage sometimes actually seem to do better.

Why in tomatoes is everyone talking about culling tomato plants?

Probably because this allows one to select for stronger genetics; even if it might be possible to save the weaker plants, they might be better discarded.