I really don't understand why people just don't plant the plants spaced far enough to begin with....why would you plant them once, then "thin" (move them apart, i assume?) them? seems like redundancy
bad germination. Plant many more seeds than necessary to ensure you get enough and when you end up with too much, thin them.
Sounds odd, but I have this packet of Mexican sunflowers that I sowed in the ground last year and got nothing from. This year I sowed them in pots 2 per hole and have about 25% of what I need. Bad seed.
"Thinning" is not moving them apart, it is choosing the strongest plant.
Have you ever bought a 6-pack of vegetable seedlings, and noticed that one or two grow huge and threaten to take over the garden, one limps along and never amounts to much, and the rest behave somewhere in-between? Seeds produce plants that are genetically different (in small ways), so each one responds differently to its microclimate in your garden.
When you (over)plant seeds, you are making sure that some of them survive and thrive, and playing a statistical game that assumes one out of every 4 or 5 in a given spot will be that extra-strong plant. You remove the competition when the plants are young to give the chosen plant a better chance.
The best way to thin is to cut (with a fine-tipped set of shears) the weakest plants in the bunch. That ensures that you do not disturb the roots of the plants you want to keep. It's meant to be a time-saver, so that you need plant a given space only once and do not end up with empty spaces where seeds did not germinate or were weak.
Of course, if you are growing a plant that you know tolerates root disturbance, you can try to separate and plant out each tiny seedling (I've done it successfully with basil and some native plants), but it is rather tedious and worth doing only if your eventual planting space is not available when you want to plant the seeds. For instance, you can plant a handful of seeds in a pot, and then in a few weeks when they are big enough to transplant, transfer them to the garden.
And by the way, test it for yourself by doing some test plots with some carrot or lettuce seeds: plant some at the final spacing, and plant some at the recommended (to be thinnned later) spacing. In the recommended spacing area, leave some plants to fight it out on their own, and thin the others as recommended. Pull some out by the roots, and cut some at soil level. See if there is a significant difference (in yields, vigor, or health of the plants). See if you prefer one way or another.
clumping plants usually need thinning after a while because you plant one and as they multiply after a few years they can get congested and often dont flower as well.
Many times on the seed packet you will see something like 'sow seeds 1/4 inch apart and thin to 2 inches apart after the first true leaves appear'. That means pinch out any in between, remove them which is preferable to attempting to transplant them because of the damage you could do to the other plants roots. Part of that has to do with germination, part with staying with old information.
thanks all, makes sense to me now8
Also, some plants simply can not be planted at optimal spacing do to the nature of the seeds. Beet seeds, for instance, have a pod that contains at least two actual seeds. There are others plants which have clustered seeds, though I can't think of them now.