What Does Thinning Plants Mean?

The seeds are too tiny.

I was short on time.

The seed package exploded all over my garden.

Whatever the reason, it often happens that some plants end up sprouting and growing too close to their neighbors. I do like to pack in the plants in my raised garden beds, but there comes a point where plants are just too dense to to grow to their full potential.

This is especially true for root crops. They're extra sensitive to having anything too close to them, especially other roots.

When too many plants are trying to grow in the same space, you'll have to thin them. That simply means removing some so that the remaining plants have more room to grow.

What Happens If You Don't Thin Seedlings?

You've gone through all that effort to plant something, you've watched it grow, and now you're just supposed to pull it up and cut off its potential?

I know it's rough, but the answer is yes. Because here's what happens if you don't: None of your plants will have the room they need to grow to their fullest potential.

Overcrowded plants often grow leggy (they basically grow too tall and thin in their search for more resources) and show signs of stress. Plants that spend too long all stressed out will never produce as well as plants that grew up coddled.

Stress aside, there's also something called the “pot-size effect”. This is the idea that plants given smaller pots in which to grow will stay smaller than those given larger pots.

Scientists for the Society for Experimental Biology did a study that used 3D MRI root scans to see how plants extend roots to the edges of their container to get a sense of how big they can grow. Plants are smart, and they know when they haven't been given enough space. If your radishes or lettuces detect other plants nearby, you've essentially given them a too-small container (an imaginary pot), and they'll never grow to their full size.

We gardeners thin some plants so that others will have large enough imaginary pots around them and plenty of access to resources like sunlight and airflow.

It's like the train problem (you know, where the trolley or train is barreling down the tracks and you have to decide whether to let it continue on and kill five people on the tracks or divert it and kill one).

Okay, maybe that comparison is a little dramatic, but still. Thinning plants has always felt like a complex moral dilemma to me!

Plants That Might Need Thinning

The plants you'll most likely need to thin include:

• arugula
• beets
• bush beans
• carrots
• cucumbers
• kale
• lettuces
• radishes
• squash

When to Thin Seedlings

It's best to thin while plants are still young.

If you're starting seeds indoors, you can take the chance to thin multiple seedlings that have germinated in the same cell while you're potting up your seedlings.

You'll typically thin plants growing in your garden once they've formed their first set of true leaves. As a refresher, the first little leaves that emerge after a seedling sprouts are the seed leaves, aka the cotyledon. These often don't look anything at all like smaller versions of the plant's mature leaves. The second set of leaves are the true leaves. These leaves might look a little more recognizable, and it's when you see them that you should come out and thin overcrowded seedlings.

Remember, hesitation in thinning will only cause all of your plants to experience prolonged states of stress over whether they'll receive the nutrients and water they need to grow.

Step One: Think of the Plant Fully Grown

Imagine the fully grown counterpart of your little seedling. What would this carrot look like at the grocery store? How round would that radish be on the produce shelf?

In actuality, you'll typically harvest roots from your garden when they're a little smaller than produce from the store, but it's better to give them a little too much space.

If you'd like your homegrown carrots to be about 1.5 inches wide at the top, then you need to give each carrot sprout at least 1.5 inches of space from the next carrot.

If you'd like to harvest lettuce heads that are 8 inches in diameter, then—you guessed it—you need to give each lettuce seedling 8 inches of space.

Step Two: Select Which Plants to Remove

It's time for the difficult task of selecting which seedlings will get to grow to maturity and which will not. This is gardener selection, not natural selection, and it can feel brutal. (When I said these were easy steps, I didn't mean emotionally!)

Keep the seedlings that look the most robust; remove those that look a little more spindly than their neighbors. This is survival of the strongest.

Step Three: Cut the Seedling at Soil Level

Use a clean pair of scissors to cut seedlings you're going to remove at soil level. You can quickly and easily clear an entire area this way, and cutting at soil level instead of pulling up the entire seedling ensures you don't disturb the roots of the plants nearby that you want to keep.

Here's the good news: you can eat a lot of the plants that you're thinning. Carrot tops, radish leaves, beet leaves, and of course, the stems and leaves of lettuce and arugula plants are edible (and super nutritious). Toss today's sacrifices into your salad bowl.

Instead of cutting, you can also try to pluck the entire seedling from the soil. This has the advantage of removing all plant parts that might take up room around your sensitive little root crops. If you're super careful, you can try your luck and replant the thinned seedling.

How to Thin Seedlings Without Killing Them

I've had some luck replanting thinned radishes. Instead of cutting plants at the soil level, you'll gently pull on the leaves of the seedling that needs to be removed. Dig around the seedling a bit to expose more of the root if it doesn't come up easily.

Treat your thinned plants gently since you've already disturbed their roots by pulling them up. Don't leave them hanging around for too long before replanting.

Use a dibber to dig them a new hole that's wider than their roots so there will be minimal root disturbance while getting the plants back into the ground. Bury the plants a little deeper than normal. Push some soil around the stems to support them. Water them immediately, and baby them for the next three to five days, watering at least daily to help them recover from the stress of moving.

Some might make it, and some will not. This is a gamble. If some make it, then you end up with more radishes! At the very least, giving replanting a try might ease some of the guilt that comes with thinning plants.

How to Thin Tomato Seedlings

I recommend starting tomatoes indoors about 6 to 8 weeks before your last frost date. Tomatoes will go into your garden as transplants (young plants), so you shouldn't need to worry about thinning them once they're planted where they'll grow.

If you're starting your own tomato plants by seed indoors, then you will probably need to thin them once they're seedlings, unless you held strong in only placing one tomato seed per cell or seed starting block.

This year, I sowed way too many tomato seeds into each of my little soil blocks. One block had six seedlings all competing for space. Some would clearly have to be culled. (I mean, do I really need 100 tomato plants? A better question would be, Where would I even put 100 tomato plants?)

Once the seedlings had a couple leaves, I pulled up each block and gently tugged apart the individual plants by their roots. The ones that looked the strongest were then given their own little plastic pot, where they would continue to grow until it was time to harden them off and plant them in the garden.

Make sure to replant them deeply, right up to their first set of leaves.

Those plants that didn't make the cut can go into your compost pile. You won't be eating these little guys.

Take the Time to Space Out Seedlings

Most of the plants that typically need to be thinned are planted by seed directly in the garden, but the problem is these seeds are tiny—radish seeds, carrot seeds, beet seeds, and lettuce seeds especially. Simply separating the pebble-like seeds can be difficult on its own but holding each seed in its place and being sure each one is planted apart from one another takes next level gardening skills (or just lots of patience).

When you're sowing seeds, it helps to imagine how large each plant will become. When a carrot is ready to be pulled, it will take up as much space as one of those little condiment bowls you have in your kitchen. A beet will be like a tea saucer. Having a visual for size can really help.

It’s easy to be lazy with your planting and just toss out a bunch of seeds all at once. (Trust me, I know). But I’ve found that managing closely planted seeds takes way more time than just slowing down in the beginning and planting more methodically. You might still have to thin a bit (especially if you're planting beet seeds, which are actually little clusters of two to four seeds called multigerms), but you'll overall save yourself some trouble later.

Use Spacing Tools

There are awesome tools for those of us who tend to over-plant. (Heads up: Some of these are Amazon affiliate links, which means I earn a small profit if you purchase my recommendations.)

One tool that I really recommend to make your life easier when planting small seeds is a little seed spacer, AKA planting ruler. You can shop my beloved spacer here, or you could make your own. This little tool keeps you from over-planting seeds. For those with arthritis, you might find seed dispensers really helpful in separating out tiny seeds.

Some dibbers (like the one pictured below) come with multiple tips that you can stamp into the soil to mark holes where your radish or carrot seeds will go. These are also called soil diggers. Tools like this are super handy to plant root crops or small leafy greens in rows or blocks and guarantee your seedlings will have enough space.

Don't Over-Sow Seeds

Some seed packages will direct you to plant two or three seeds per hole. This obviously increases your chances of having to thin later if you get good germination. You might prefer to plant only one seed per hole instead. Then, note the days to germination and watch for signs of sprouting every day. You can always come back and sow more seeds in the holes that have not shown any signs of life once your days to germination range has come and gone.

Make sure to use plant labels to mark where you've planted seeds so you don't come in and double-plant by accident.

Thinning Is Just a Part of Gardening and Growing Pains

I encourage you to experiment in your garden and figure out which works best for you: taking time to space seeds out or ruthlessly thinning seedlings later; cutting your radish seedlings at the base or gambling on radishes surviving the move to a new spot.

I'll probably always over-sow my seedlings and then feel terrible about having to thin some of them. If they're radish seedlings, I'll at least try to replant them, even though most gardening professionals would tell you it's not worth it. This is your sign that you don't always have to follow traditionally accepted methods. Making your own way is what makes gardening so much fun (moral dilemmas and all)!