Maybe I’m a big ol’ softy. I have trouble thinning my plants. They tried so hard to sprout and grow, and now I’m just going to kill off the extras? But it is something that needs to be done, even if I cringe and make little apologies to every young sprout I snip.

Thinning seedlings is simply removing the extras. We might have extra seedlings because we planted too many on purpose to compensate for a few that fail to germinate, as many seed packets recommend. And of course, we might have been thick-fingered, dropping a few extra seeds by accident. 

Why we thin seedlings

Too many seedlings for the growing space can lead to problems. Imagine tomatoes, planted two inches apart. As those plants grow, they will be fighting for space, sunlight, water (rooting space), and nutrients. A little competition and slightly denser planting can help block out weeds and create a lush, appealing garden. Too much competition, though, and the result is usually spindly, weak plants that produce poorly, whether flowers or vegetables. 

The reason we thin seedlings really comes down to space, but that spacing is important for several reasons. 

Airflow and disease

As plants mature and grow, adding foliage and stems, they need good air circulation to help dry out the inner foliage and prevent the damp conditions that favor fungal infections. Plants that are compacted together in a dense mass of leaves and stems suffer from lower airflow, taking longer to dry out after a rain, the sprinkler, or even just the morning dew. Thinning restores proper spacing, improving airflow and reducing disease.

Sunlight

Plants grown in overly tight conditions will keep reaching vertically, trying to outpace their neighbors to capture a bit of light. Over a growing season, it can result in spindly plants with long but weak stems and little branching. Fewer branches and a thinner canopy mean reduced overall photosynthetic capacity, leading to lower yields. Spindly stems are more likely to break in a storm, or be damaged by the weight of fruit.

Water and nutrients

In containers, rootbound plants can’t access enough water and nutrients to support the growth above. If you’ve accidentally left a potted plant in a small pot too long, you may have witnessed this problem. The result is stunted growth and lower yields. 

The same kind of competition can happen in a raised bed or traditional garden bed if spacing is too tight, even without pot walls. Too many plants competing for water and nutrients in the same spot leads to stunted growth, lower yields, and reduced plant vigor, which can lead to increased plant disease issues.

To sum it up, properly spaced plants are healthier, produce higher yields, and are more disease resistant than overcrowded plants.

When and How to Thin Seedlings

We know we should thin our seedlings as necessary, but when is the right time to do it? 

Young plants may need thinning several times, depending on the type of plant, their spacing needs, and where you started the seeds. 

Thinning seedlings started indoors

Herb, vegetable, and flower seeds started indoors are typically sown in cell packs, which commonly hold 50, 72, or even 128 individual cells in a larger tray. For most applications, only one young seedling will fit in these cells with room to grow, but we often drop two seeds in a cell, either on purpose or by accident, resulting in seedlings growing nearly on top of each other. 

The time to thin these little seedlings is shortly after germination, when they are just sending out their first true leaves. Waiting until the first set of true leaves emerges helps you identify the stronger of the two seedlings.

Thinning seedlings in the garden bed

If you planted your herb, veggie, or flower seeds directly into the garden bed outside, you can likely wait a bit longer to thin them. Since spacing is often an inch or two apart, let them grow until the first set of true leaves is well established and the second set has started to appear. 

Once they start to rub elbows is the right time to think about thinning. Any seedling mortality will likely be passed by then, and you’ll be better able to judge where it’s a bit crowded or where you can leave them be. 

Secondary Thinning

We can often perform an initial thinning, then wait until the young plants are just large enough for an early harvest before doing a secondary thinning. This method works well for vegetables we like to eat in their baby stage as well as their mature size. 

Carrots, beets, lettuces, spinach, kale, and similar crops can be thinned initially, and left a bit too tight spacing-wise, then thinned again in a few weeks. For Simpson Elite lettuce, that might look like thinning to one plant every 2-3 inches when the plants are large enough to start touching, and then again to harvest young leaves (taking the entire plant instead of just a leaf) to achieve final spacing of one plant per 8-12 inches. Baby carrots can be pulled, harvesting every other one, to give the remaining carrots room to grow to mature size.

Thinning technique

How to thin is a personal preference and depends partly on the type of plant, but my recommendation is to use scissors. When thinning seedlings, we do so because they are too closely spaced. Their roots and stems may be entangled. Pulling a small seedling that is nearly on top of another can yank both, or severely disturb the roots of the remaining plant. Instead, use a small pair of scissors to reach in and snip the extra plant off just above the soil line. It’s quick and clean, and it avoids damaging the plants we leave to grow.

For plants in a bit wider spacing, like a few too many pumpkins in a hill, the less desirable seedlings can be carefully pulled and tossed in the compost. 

Later thinning can also be done with scissors, or, of course, in the case of root crops, by simply gently pulling them up and taking them to the kitchen.

FAQs about thinning seedlings

Do seedlings started indoors need to be thinned?

If you planted more than one seed per cell in a cell pack, the answer is likely yes. Otherwise, most seedlings can grow well enough to be ready for transplanting or potting up without thinning. For example, you might plant tomato and pepper seeds in a 72-cell tray, and when they are about 4 inches tall, pot them up into individual 3-4 inch pots to continue growing until transplant day. No thinning required there. 

However, for most veggies and flowers, if more than one sprout in a single cell, keep the strongest one and snip the others.

If I plant at a wider spacing, do I still need to thin?

Maybe not. Plants with reliable germination, such as peas or beans, seed packets often recommend planting at their final spacing, so no thinning is required. For example, peas on a trellis could be planted at a four-inch spacing and never thinned. The risk of planting at wider spacing is only that you might end up with holes or fewer plants than you wished for due to spotty germination, seedling death, or critters. 

Can I transplant the extra seedlings instead of wasting them?

It may be possible. I am a tender heart and often try to separate the extras and replant them. It works with varying success depending on the type of plant. I scatter sow thyme seeds and then thin them by pulling entire clumps of seedlings free and repotting them. While I lose a few, the majority seem to be fine. Transplanting one-inch-tall carrot seedlings will likely be too damaging. I have been known to move an extra pumpkin when they are still in the cotyledon stage. If you’d like to try, it costs nothing but a bit of time. 

Will thinning spread disease?

Spreading disease with our gardening tools, in this case, scissors, is a valid concern. However, two things are working in our favor here. First, many plant diseases are not yet established or even present when we are thinning our seedlings (think late blight of tomato), and second, we are killing the plant anyway. 

If you’d like to be extra cautious, disinfect your scissors with a quick spray or dip in 70% rubbing (isopropyl) alcohol between plants or between varieties. It works quicker than a bleach solution, dries quickly, won’t stain your clothes, and won’t rust your scissors.