It can seem very promising when most seeds in a seed tray sprout, but once all those seedlings emerge, a crowded mass develops. Plant stems rub against each other, and leaves touch each other, and multiple root systems start to occupy the same space. Thinning is the act of eliminating some seedlings so that the rest of the plants have the light, air, moisture and root space they require. However, you must thin the seedlings without disturbing the ones you want to preserve.
Work in small sections at a time. Identify the seedlings that are growing well, have adequate spacing and appear normal in development, and determine which seedlings you want to keep before removing any. If you find several seedlings emerging close to each other, try using small scissors to snip off the unwanted ones at the soil line rather than yanking them out. This might be easier on the seedlings you wish to save. Once a group of seedlings starts developing a root system, they become intertwined, and pulling one could affect others.
If you are thinning seedlings that are just barely emerging, it will be less problematic than if you waited until the seedlings were too crowded. However, do not disturb seedlings too soon after they have emerged. Wait until they become clearly identifiable, and then thin them before they become too overcrowded.
Dry potting mix may fall away from the roots when you handle it, and overwatered mix may become mushy. It is easier to manage slightly moist mix. If you must extract a seedling rather than trim it off, use your fingers to stabilize the soil around it and then gently lift the plant rather than pulling it up quickly. Inspect the remaining seedlings to ensure they are standing straight and that their roots are still anchored well in the soil.
When seedlings grow too closely together, they are competing for light, and the plants will tend to grow taller. Overlapping leaves will make it harder to observe what is happening to each seedling. They will also be harder to water evenly because one clump of seedlings may be wetter than other sections of the tray. Once you finish thinning, view your tray from above. You should see greater differentiation among the individual seedlings’ stems and leaves, and there should be ample room to examine them. Having space visible like this is an indication that the seedlings will have a higher probability of thriving as they continue to grow without as much competition.
