If, like most gardeners, you want to have good, full rows of plants,
you should probably thin some of your vegetables, flowers, and herbs.

-What exactly is “thinning”?

Thinning is prepared by planting seeds more densely (that is, closer together) than you want your plants to grow.
Allow the plants to grow for a week or two after they emerge from the soil, then remove some of the plants so that the remaining plants (the biggest, healthiest-looking ones) are approximately the recommended distance apart.

-Why is it so essential?

While you may not want to waste seeds, they are relatively inexpensive. It is far preferable to plant seeds densely and then thin the resulting plants than to plant seeds widely and then discover that too few have germinated to yield the desired crop.

Seeds must also physically push their way out of the soil. When planted close together, they aid each other in this sometimes difficult task.

When planting seeds densely, it is essential to thin the plants out. Otherwise, the plants will be too close together and will compete for light, water, and nutrients. They will remain small and will not produce a large amount of what you require (leaf, fruit, root, etc.).

Planting plants in a small, dense seedbed is a variation of thinning. If you want to grow basil seedlings in warm conditions, plant 50 or 100 seeds very close together (say, in a square 60 cm/24 in on a side), then cover them with a cold frame (see season extension techniques) or a floating row cover. When the plants reach about 10 cm (4 in) in height, dig them all up and transplant them into rows 60 cm (24 in) apart. They can live without the extra heat once they reach that size.

Similar methods are used for broccoli, cauliflower, cabbage, and other related plants. The purpose of the cover for these plants is not extra heat, but rather a protection from flea beetles. The flea beetles can kill very small seedlings but not larger transplants.