Do You Really Need to Thin Seedlings in Your Garden? Here's What Pro Growers Say

It's always exciting when your seeds start sprouting up in garden beds or in starter trays. So the idea that you should get rid of some of the tender little seedlings to thin out the plants seems almost heartbreaking. Would it really be so bad to just let nature take its course? Unless you're very precise when sowing, here's why experts say thinning seedlings is often key to your garden’s success. Plus get tips for how and when to properly use the technique, and find out when you can skip it.

What Is Seedling Thinning?

Overcrowded seedlings won't grow as well as those that have adequate space. "Thinning is a practice of making space for plants so they aren't crowded. This could be in containers, pots, trays, or even after direct seeding outside in the ground," says Corbin Schulz, farm director for Seed Savers Exchange in Decorah, Iowa.

Thinning seedlings simply means that you reduce the number of sprouts growing in a particular space. For example, if you sowed veggie seeds in a row and the mature plants need 6 inches of space between each plant, you'd remove any seedlings growing closer together than that in your row. If you planted 3-4 seeds in a cell in a seed starting tray, you'd thin the seedlings to just one per cell.

Why Seedlings Need Thinning

Your seedlings are all cute and tiny now, but as they grow bigger, so do their needs for resources both above and below ground. "The roots will eventually start to compete for moisture and nutrients in the soil," says Schulz. "The nutrients will also deplete faster when there are more root systems absorbing them." And for plants that need full sun, any competition for light can slow down photosynthesis and growth, she adds.

Overcrowded seedlings also tend to have more disease problems like damping off. That's because there's less airflow around each plant, which helps dry up excess moisture that can allow pathogens to take hold. Stressed plants that aren't getting enough resources are also more susceptible to disease and pest issues.

When to Thin Seedlings

The best time to thin seedlings is after they've grown a few leaves but haven't started to really crowd each other. "You can do so anytime after the first set of true leaves have emerged," says Diane Blazek, executive director of the National Garden Bureau and All-America Selections. "The first set of leaves are called cotyledons so wait for the next set of leaves, which will look different, more like the mature plant's leaves."

How to Thin Seedlings Correctly

Seedlings are somewhat delicate little plants. Avoid damage to the plants that remain, and space them out properly with these thinning tips:

Check spacing needs. How much to thin out your seedlings in your garden depends on how far apart the mature plants should be for optimal growth. And of course, this varies depending on the type of plant you're growing. Spacing information is usually listed on the seed packet or online.

Keep the biggest seedlings. Some seedlings will naturally look more robust and develop more true leaves faster than others. When thinning, aim to leave the strongest seedlings and remove less vigorous neighbors. "This will allow the biggest and healthiest plant to thrive in the pot or soil on its own," says Schultz.

Snip, don’t rip out seedlings. Besides being delicate, seedlings are tiny, so it can be tricky to only pull the seedlings you want to thin out without inadvertently grabbing neighboring seedlings you want to keep. Plus, Blazek says "there is a good chance the roots are intermingled, and you will also pull up the tiny plant you wish to keep." Instead, thin seedlings by cutting their stems close to the soil surface with small scissors (Blazek recommends manicure scissors). For larger seedlings like beans, some gardeners just use their fingernails to pinch off the stems.

Thin more than once. If you're not sure which of your seedlings seem the strongest, you can do an initial pass and lightly thin a few out that are obviously not as robust. Then wait another week or so before thinning out to your final seedlings. You especially may need to take this approach with very small or densely sown seedlings.

Ways to Avoid Thinning Seedlings

Depending on your planting techniques and types of plants you're growing, you might not have to thin seedlings at all in the following situations:

Planting fewer seeds. Lessen or eliminate the need for thinning by "just sowing fewer seeds in the beginning" says Blazek. That means sowing your seeds very precisely at recommended spacing or using seed tape.

Using fresh, high-quality seeds. Some gardeners plant more seeds than they need and thin later because not every seed is guaranteed to sprout. Germination rates can vary depending on the type of seed and their age. However, Blazek points out that "if you are buying fresh, quality seed, you will have a high germination percentage, which lessens the need for thinning."

Transplanting instead of thinning. Some gardeners prefer to transplant crowded seedlings into their own pots or a different part of the garden. This tactic is best for certain veggies and flowers like tomatoes and marigolds that can handle some root disturbance; other plants have more sensitive roots that won't tolerate transplanting well, such as root vegetables. It's also more time-consuming, but if you want to keep as many seedlings as possible, this option could be worthwhile.

Harvesting early. If you're planning to harvest microgreens or baby greens, you'll usually pick them before overcrowding starts to cause problems so you can skip thinning them. And if you harvest selectively instead of all at once, you can give the remaining plants a little more room to grow until it's their turn to be picked.