Thinning perfectly healthy seedlings may seem counterintuitive. I’ll admit I had difficulty doing it myself when I started farming because it felt unnatural. But once you get the hang of sowing seeds in a way that gets you maximum germination and how to space out certain crops properly, you’ll see thinning is a necessary task for high yields and peak performance. Let’s get into it.
What Is Thinning?
Thinning out your seedlings can help promote more growth and yield better crops.
Thinning seedlings is the intentional and timely removal of seedlings that inhibit proper spacing for crops to grow to their full potential.
Why Thin Seedlings?
Thinning seedlings creates airflow and space for better root development.
Thinning seedlings is necessary if you want the following:
- Proper airflow
- Decreased disease pressure
- Less competition for nutrients or water
- Appropriate space for root development
- Less stress on plants
- Decreased legginess
- Healthy growth
Whether you start your seeds indoors or direct sow only, thinning is an important part of successful gardening.
How to Thin Seedlings Before Transplant
Thinning your seedlings is a quick and easy way to reduce crowding.
About two weeks after germination, take a small pair of garden shears, very sharp, clean scissors, or just your fingernails and pinch out the weakest-looking seedlings from each cell. Don’t just pull undesired seedlings out, as this can disturb the roots of other seedlings! It’s best to nip them off right at the soil’s surface, leaving the tiny roots in place to decay into the soil.
Be sure to do this before the seedlings get so large that they compete for nutrients. Remember, the soil in seed cell trays only contains so much seed-starting mix and nutrients, so you want the “keeper” to have what it needs as it grows.
Crops To Thin Indoors Before Transplant
Multiple crops will get an earlier start if started indoors.
Some crops typically started indoors will have multiple seedlings per cell. Thin each cell to contain only one before transplanting!
These seeds are multigerm seeds that produce multiple sprouts.
Beets and Swiss chard, interestingly, are the same vegetable botanically, Beta vulgaris, and have the same ancestors. Beta vulgaris is a multigerm seed instead of a monogerm seed, so there are multiple “seeds” within the seed you receive from the seed company.
Their tiny flowers fuse with neighboring flower petals, so their seeds are also fused. The coolest part about this fact is that when Swiss chard seeds start to germinate, not only will there be multiple sprouts, but they might all be a different color.
Choose the least leggy Swiss chard seedling with the thickest stalk as the “keeper” and pinch out the rest. Be sure to do this before the stems become tangled up in one another.
Lettuce seedlings are very tiny, making it difficult to thin them out.
Head lettuce seeds are small and pale in color. This task can become painstaking, inefficient, and time-consuming when seeding many trays at a time. Some growers use tweezers or a hand seed sower, but I think it’s easier to drop 1-2 seeds in each cell and thin them later because time is worth more than the cost of a few extra seeds.
Lettuce seedlings are hardy and can stand some manipulation during transplant without damage. If there is more than one seedling per cell tray, you may separate the two and transplant both gently. Only do this if both seedlings seem healthy. Keep roots intact when separating, and both seedlings should grow into a full-size head. Alternatively, you can snip any extra seedlings from the cells before transplanting, similar to the Swiss chard above.
Pro tip: Many seed companies today offer pelleted seeds. These are coated in a layer of clay that makes handling and sowing them easier. They are more efficient in commercial seeders because they’re round and are believed to improve germination rates. There is an added cost for pelleted seeds, but you’ll only need to sow one per cell.
Green Beans/Sugar Snap Peas
Green beans and snap peas are fast-growing and typically ready to transplant in a few weeks.
While these crops are not traditionally started indoors, if you grow in a cooler region, doing so will give you a head start on spring crops. Just be sure your cells are properly sized to accommodate the large seeds. These crops are quickly germinating and fast-growing, so have your garden bed ready for transplant in just a few weeks.
Transplanting these legumes is a great option if you are experiencing a wet spring or have rodents like voles or mice lurking in your garden. Extended periods of rain can cause rot easily, and they’re an easy snack for a critter to grab.
Cucumbers and summer squash will only need one or two seeds per tray.
Both crops have fairly high germination rates, so I only sow one seed per tray, with one exception. If I have leftover seeds from the previous year, I assume the germination rate will have decreased, so I always sow two seeds per cell in this case. This may result in two cucumber or summer squash seedlings in every cell, so thin to one when they’re large enough to see which seems larger and healthier.
Pro tip: Always look at the germination rate on the package before you begin sowing. If it’s on the lower side, plan on sowing multiple seeds to get at least one healthy seedling per cell and thinning the others.
Beets are typically easier to direct sow rather than transplanting them.
Some growers choose to direct-sow their beets rather than transplant them. Remember that beets are “multigerm” seeds, so more than one seedling will sprout from one seed.
Interestingly enough, beets can be left intact if desired. Since they produce such a wide, bulbous taproot, they will nudge each other apart as they grow. You may end up with beets with strangely shaped sides where they are pressed against other roots, but they will still produce viable and tasty roots nonetheless.
If you want perfectly-round beets, it’s especially important to thin, and when they’re young is best. Give each beet about 3” for round beets, or allow clusters of 2-3 beets to grow about six inches apart if maximizing your harvest is more important than shape.
How to Thin Direct Sown Seedlings
Once your direct-sown seedlings are about two inches tall, they are ready for thinning.
Depending on germination and growth rate, direct sown crops should be thinned about one to three weeks after germination or when they are one to two inches tall. Using just your hands, go through each row of seedlings and gently remove some until the remaining ones are about the spacing you’d like.
To thin, use your thumbnail or a pair of pruning shears to clip off the seedling right at the soil’s surface. Discard them into a pile or gardening bucket and remove the debris when finished – or, if it’s a plant like lettuce, eat your seedlings as if they were microgreens! You may leave a little extra to account for any early loss and thin again in a week or so.
Pro tip: Remove any small weeds as you are thinning to save time later pulling large weeds.
Direct Sown Crops You Should Thin
Some directly sown crops will succeed more when thinned out early.
Crops that have been direct-sown outside should be monitored as they germinate for spacing, crowdedness, and airflow. Keep in mind thinning doesn’t need to happen if you sowed just the right amount of seeds and germination was just right. Sometimes, this will happen as you become more familiar with the crops you’re sowing, your soil, the timing of sowing, etc.
If you’re using a mechanical seeder, once you hone in on those skills, you may not have to thin much at all. If your seedlings are overcrowded, thin them to the recommended spacing. Refer to the seed packet to get this info.
Thin carrots once they’ve hit about 2″ tall, and be careful not to disturb the roots of the ones you’re keeping.
Carrots should always be directly sown; depending on your sowing method, you may need to thin. Ideally, carrots are spaced at ¾”-2” spacing, depending on how large you want your roots to grow. Smaller carrot types can be closer together than larger ones. They can take up to 3 weeks to germinate, so be sure not to let the soil surface crust over and dry out to avoid gaps in your rows.
Wait until the carrot tops are about 2″ tall to thin. Any sooner, and you risk pulling out more than you’d like. Do your best not to disturb the delicate carrot seedlings that remain in the soil – do not pull these out root and all, but clip them off.
Seed packets may recommend sowing 30 seeds per foot and thinning to one every one to three inches. After experimenting with what works best in your soil, you’ll learn how much you need to sow and when to thin. You may think if you don’t thin, you’ll get more carrots, but ultimately, you’ll have larger, healthier carrots if they have the space and access to nutrients they need to grow down and out.
Your leafy green seedlings don’t need much space if they’re grown for baby greens.
Greens include mesclun mixes, spinach, arugula, mustards, mizuna, baby kale, etc. Since you want these to remain small, you don’t have to give them the space they would need as full-size versions of themselves, but rather just an inch or two.
If you’re creating a mix of your own, be sure the crops have similar germination times and days to maturity to avoid one shading others out. Consider alternating greens in rows so they’ll grow together in a garden bed without competing or shading each other out.
Pro tip: If you want to practice the “cut-and-come-again method,” cut about an inch above the crown or your greens when harvesting so they can grow back.
Radish seedlings generally need about 2 inches of space.
Radishes can be sown early in the spring, as soon as soil can be worked, and do well in cool weather. Rows only require about an inch in between, so you can fit a lot in a small space. When you start to notice that little radishes have formed below the surface, thin to two-inch spacing. The great thing about radishes is you can use the small, thinned radishes in a salad, so there’s no waste!
Pro tip: Different radish types may differ in care needs. Be sure to water globe radishes at least an inch per week. They require consistent water to fill out their shape and stay juicy in their short maturity time, so don’t skimp.
Green onions typically need more space and water if they are bulb-forming onions.
Some growers prefer to plant one green onion ½-1 inch or so in a single row, while others plant clumps of 4-6 every few inches. I’ve tried both and gotten similar results, so it’s a personal preference. Growing a non-bulbing onion type as scallions will ensure they won’t try to bulb out, which requires more nutrients, space, and water.
Similar to head lettuce, green onion seedlings are hardy and can be messed with during transplant without damage, so don’t be afraid to separate them and clump them together as you wish. In fact, many people will start onions en-masse in a seedling tray, wait until the onion greens reach about 4″ tall, then separate and individually transplant the seedlings!
How you thin turnips depends on your desired root size.
‘Hakurei’ turnips are Japanese salad turnips that are incredibly fun and easy to grow. Thinning is key, however, because while you can harvest them small, they can grow to the size of a baseball. If you prefer them to be radish-sized, thin to 1-2” apart, but if you’d like them larger, give them 3” per cluster.
Sow these in rows or clusters, giving each turnip room enough to expand out. Don’t wait too long to thin because the greens can become a jungle. Similar to radishes, you can consume the small turnips you’ve thinned, along with the greens.
Thinning Full-Sized Plants
Sometimes, it’s necessary to move large plants so they can reach their full potential.
When growing greens like kale or Swiss chard that will become quite large, try transplanting them with tighter spacing at first and harvest them as “baby” greens. In a 30” bed, space them three across at 6-9 inches. Then, as they mature, thin out the center row and every other plant on the outside rows, ultimately giving you the recommended spacing of 12-18” for full-sized plants. Remember to feed your soil as needed. This method gets you more bang for your buck!
The same concept can be used when growing tightly-spaced baby spinach. In the fall, remove every few plants so the remaining plants can produce large leaves. Harvest these late in the season to freeze for winter soups and casseroles.
It might seem counterintuitive to thin your seedlings at first, but once you see the benefits of giving them each the appropriate space to grow and that you can select the healthiest seedlings to grow into healthy plants, you’ll see why it’s a necessary task.
Thinning your seedlings will lead to an abundant garden of healthy, properly spaced-out, and productive plants. Happy thinning!