The removal of crowded seedlings, branches, or flower buds to reduce competition, improve air circulation, and increase remaining plants' vigor and production.
In plain terms
When seedlings or buds are crowded, thin by removing extras, leaving remaining plants with more space, light, and nutrients. Thinning root vegetables (carrots, beets) early prevents small roots. Thinning tree fruit buds early ensures larger, quality fruit. It seems wasteful (throwing away plants) but necessary for proper development.
Why this matters
Thinning early (when plants are small) prevents root crowding and stunting. Waiting until plants are large makes it harder and results are poorer.
In practice
Examples
Carrot seedlings thinned to 2 inches apart; grow into full-sized roots; unthinned ones are tiny and misshapen.
Peach tree fruit thinned to one fruit per 6 inches; remaining fruits are large and high-quality; unthinned fruits are small.
Overcrowded seedlings thinned early; remaining plants are strong; unthinned ones are weak and spindly.