In general, consider thinning once the seedlings are showing a pair or two of true leaves rather then the seed leaves they first germinate with. If you're thinning seedlings growing in containers, the aim is usually to leave a single, healthy seedling behind in each pot, cell, or pellet.

When should I thin seedlings?

How big should seedlings be before thinning? It's usually best to wait until seedlings have their first set of true leaves before thinning them. That way, you'll know which ones have successfully unfurled. But don't wait too long, or overcrowding could begin to stunt their growth.

How do you pick out seedlings to thin?

Use garden snips or a scissors to thin out crowded seedlings. Nobody likes to thin seedlings. It's fussy work, and always hard to decide which ones to save and which to toss. Here's and easier way: When the first true leaves appear, snip off the extra seedlings at the soil line.