Seed packets and gardening guides often mention the need to “thin seedlings” at a certain point in time. It is important to know exactly what this means, along with when to do it and how to do it.
So, what does thin seedlings mean? Thin seedlings means to cut or pull up some of your seedlings from the soil. This leaves more space between the remaining seedlings so that they have room to grow without competing for water or nutrients. Thinning also prevents the spread of disease and keeps seedling roots from getting tangled together as they grow. This makes it easier to transplant them outside later.
Thinning seedlings is necessary when you end up with more seedlings than you can reasonably plant in your garden. This can happen if you get a higher than expected germination rate from the seeds you planted.
However, this is better than the alternative, which is to plant too few seeds, get a low germination rate, and end up wanting more seedlings for your garden.
Let’s take a closer look at thinning seedlings, why to do it, when to do it, and how to do it properly.
What Does Thin Seedlings Mean? (Why Should You Thin Seedlings?)
When you thin seedlings, you remove any extra seedlings to leave more space between plants. You should aim to remove the weaker seedlings and leave the strongest, healthiest ones to continue growing.
However, that still leaves us with the question of why to thin seedlings at all. There are several good reasons to thin your seedlings, although it can be difficult to kill perfectly healthy seedlings.
Thin Seedlings To Prevent Competition
First of all, thinning your seedlings prevents competition for water and nutrients in the soil. Every seedling needs enough space in the soil to grow to its potential.
Leaving enough space allows a seedling to build a proper root system in the soil. This root system will be large enough to support a fully grown plant with healthy leaves, flowers, and fruit.
These healthier plants will be more resistant to disease, pests, and harsh environmental conditions outdoors. They are also more likely to survive when you transplant them outdoors from inside your home or from a greenhouse.
I know it seems cruel to pull up young seedlings that seem healthy, and it can be difficult for some gardeners to do it. However, you need to do it for the sake of the remaining plants!
Think about it this way: you would not think twice about pulling up a weed that grew too close to one of your plants. You know that the weed will compete with your plant for water and nutrients, making your plant weaker or even killing it.
In a sense, extra seedlings growing too close together are a type of weed, since they will compete for nutrients and hold back the growth of the seedlings around them.
For more information, check out this article on thinning seedlings from the University of Nebraska Extension.
Thin Seedlings To Prevent Weak and Tangled Roots
Thinning your seedlings also prevents the problem of weak and tangled roots. When plants grow too close together, their roots can become tangled.
This means competition for nutrients and water (as mentioned earlier). It also means that it is difficult to separate the roots (and thus the plants) when you want to transplant them outdoors later in the spring.
Remember that plants are more likely to suffer from transplant shock if the roots are disturbed or damaged when they are moved. This is much more likely to occur if you need to pull and tear some of the roots to untangle two or more plants that were grown too close together.
Thin Seedlings To Prevent the Spread of Diseases and Pests
There is one more good reason to thin seedlings to leave more space between plants. Common garden diseases and pests are less likely to spread between plants that have enough space between them.
In some cases, leaving more space between plants will only slow the spread of diseases or pests, rather than preventing these problems entirely.
However, the extra time can give you the chance to pull up and remove infected plants. This will keep the disease or pest from spreading to nearby plants, allowing you to save most of your harvest from these threats.
When To Thin Seedlings
Hopefully, you are convinced by now that thinning seedlings is a good thing (or at least, a necessary evil). Now we need to figure out exactly when to thin your seedlings.
The proper time for thinning seedlings will vary by plant species. However, a general rule is to thin seedlings after one set of true leaves emerges on the plants.
The first leaves that appear at germination time are called cotyledons, and they are embryonic leaves, not true leaves. They can last a few days, or perhaps a year or more, depending on the plant.
For more information, check out this article on cotyledons from Wikipedia.
The next set of leaves to appear on a seedling is the first set of true leaves, or leaves that are not cotyledons. Usually, these first true leaves will appear a few days after germination.
To see the difference between cotyledons and true leaves, take a look at the picture at the bottom of this PDF from the Utah State University Extension.
Once the first set of true leaves appears, you can thin the seedlings (more on how to do this later). Do not wait too long for thinning, or else the roots of the seedlings will become tangled, and you will have trouble transplanting them later.
For more information, check out this article on sowing seeds from the Penn State University Extension.
Also, check out the table below to get an idea of when seedlings will germinate, when you might want to thin them, and how much space to leave between the remaining plants.
|Time to |
|Time to |
|Lettuce||2 to 15||4 to 19||8||20|
|Pumpkin||3 to 10||5 to 14||30||76|
|Radish||3 to 6||5 to 10||4||10|
|Spinach||6 to 10||8 to 14||3||8|
|Squash||7 to 14||9 to 18||30||76|
|Sunflower||4 to 11||6 to 15||6||15|
|Tomato||6 to 11||8 to 15||12||30|
How To Thin Seedlings
Now that you know why and when to thin your seedlings, it is time to find out how to do it!
To thin your seedlings, first get a set of thin scissors or gardening snips (they should be fine enough to work with small plants).
Next, make sure to clean your scissors or snips with alcohol before you start clipping. This will prevent the spread of diseases between plants. This is a good practice for any garden tools that come in contact with soil or that are used to prune and cut plants.
Then, decide which seedlings to leave growing, based on which ones look the strongest. You should also take into account the recommended spacing between plant, which you can find on the seed packet or online. There is also information on spacing for some common plants listed in the table above.
Now, use your scissors to cut away the other seedlings at the soil line. Avoid pulling the seedlings up, since you can disturb the roots of nearby seedlings if you do this.
You can compost the remains or give them to chickens if you have them. You can eat them yourself, either as microgreens in a salad or in a stir-fry.
This works well if you have lettuce, spinach, sunflower, or other seedlings that are commonly used for microgreens. For more information, check out this article on microgreens from Wikipedia.
Finally, make sure to clean your scissors with alcohol after you finish your thinning.
Note: if you are growing in a seed tray with separate cells, such as a standard 1020 tray, then thin to one plant (the strongest plant) per cell.
For more information on germinating seeds in trays, check out my article on 1020 seed trays.
Remember that cutting extra seedlings at the soil line to thin them is probably your best bet. Pulling up an entire seedling by the roots can damage nearby plants, especially if you wait too long and the roots become intertwined.
If you decide to pull up an entire seedling by the roots, then you do have the option of trying to replant the seedling somewhere else to see if it will grow into a healthy plant.
By now, you know what it means to thin seedlings, and why you should do it. You also know when and how to thin seedlings for some common plant varieties.
I hope you found this article helpful – if so, please share it with someone who can use the information. If you have any questions about thinning seedlings, please leave a comment below.