Here’s how to thin vegetable seedlings:
Thin seedlings in the cool of the day or an hour or so after they have been watered; the seedlings will be plump with moisture.
Thin using a garden scissors to snip off the top of the weakest or most closely crowded. An alternative is to nip off seedlings using your fingernails. Snip seedlings at soil level.
Snipping away seedling tops rather than pulling them up root and all will leave remaining seedlings to root fully and grow strong. The objective in thinning is to leave the remaining seedlings undisturbed.
Remove intermediate seedlings first. When thinning, keep in mind the space each plant will need to spread and grow to maturity. You may need to thin the seedlings of seed broadcast across a planting bed more than once.
Begin thinning as soon as seedlings are large enough to handle. Snip or nip away seedling tops at ground level. The roots you leave behind will quickly decompose, adding nutrients to the soil.
Gently firm the soil around remaining seedlings.
Remove thinnings from the garden; bury them in the compost pile or place them in the trash. The smell of thinnings can attract the plant’s pests to the remaining seedlings.
To avoid thinning, sow seed evenly, seed-by-seed. This can be difficult to do with very small seeds like carrots, lettuce, radishes, and beets. For very tiny seeds, you can spend more to get pelleted seed (seed coated with an inert material) or seed tape (seeds embedded into soluble tape) which are prepared specifically for easy sowing. An alternative is to mix tiny seed with fine, washed sand and sow using a salt shaker or seeder.
If you don’t want to sacrifice seedlings, you can reduce seedling crowding by lifting and repotting seedlings once they have developed a second set of true leaves—usually two to three weeks after germination. Gently lift each seedling using a toothpick or pencil teasing the roots free. Lift seedlings by a leaf to avoid crushing the stems.
Support the seedling and its root ball until you set them into the new container. Firm soil around the transplant and water it in place to grow on. Be sure to firm the soil around the seedlings left in the seedbed.