Why and How to Repot Seedlings
One of the problems with starting seeds indoors is that they sometimes end up outgrowing the small cell they were started in. In a healthy seedling, just about as much effort is spent toward developing strong roots as stems and leaves. So here’s why and how to repot seedlings.
Why Repot Seedlings
If you’re starting your seedlings in a cell tray with a reservoir, eventually when you lift up the cell tray to refill the reservoir, you’ll find that the roots have made their way out the drainage hole at the bottom of the cell and into the reservoir itself.
Once you find this happening, you’ll need to repot them. Why? Because it leads to two bad things:
- The seedling’s taproot will get bound up and tangled, making removal difficult and damage much more likely.
- The roots that don’t make their way down into the watering hole will wind themselves around and around the cells, resulting in the seedling being “rootbound.”
What’s a taproot?
The taproot is the longest, strongest root of a plant. It helps anchor the the seedling. Until a more extensive root system develops, the taproot is the main source of water and nutrients for the seedling. Plants with a long taproot are drought tolerant and often more easy to care for. (Here’s more about the purpose and benefits of a taproot.)
Not all plants rely significantly on a taproot to establish themselves. These plants can withstand damage to the taproot and come out the other side perfectly healthy. But they must be repotted or transplanted before the taproot begins to mature.
If a seedling’s taproot is damaged once it’s begun to mature, the seedling will have to put effort into repairing it, rather than growing taller and stronger. For some plants, if the taproot is too badly damaged, and not enough other roots have developed, the seedling may die.
What’s it mean to be rootbound?
You know how kids outgrow their shoes SO FAST? Secondary seedling roots—the roots that are additional to the taproot—act the same way. They grow really fast, and if they don’t have room to spread out or down, they’ll start circling around and around the inside of the pot.
Eventually, with nowhere for the roots to go, the seedlings will stop growing. This keeps them from getting as big and strong as they can before transplanting them.
The solution? Repot them!
How to Repot Seedlings
It can feel a little scary to uproot a seedling from its cozy little cell it was started in. So here are my tips for how to repot seedlings in a way that helps make sure they stay healthy.
- Wait until the plant has grown at least one set of true leaves. At this point, it is able to photosynthesize, and should be strong enough to handle the move to a larger pot.
- Fill the larger pot with growing medium—I like to stick with seed starting mix just to be safe.
- Create a hole for the transplant by digging a few inches into the soil.
- Remove the seedling from its cell as gently as possible, disturbing the root ball as little as possible. Turning the cell over into your hand usually works the best.
- Don’t remove, shake or brush the soil from the roots—the less they’re bothered, the better.
- If your seedling is extremely root bound, loosen some of the smaller roots on the outside of the rootball. This will encourage them to spread out in their new home.
- Gently set the rootball of the seedling down into the new pot. These 4″ pots are usually big enough for the seedling to live in until transplant.
- Lightly pack soil around the stem, making sure the roots are completely covered.
- Water the seedlings in their new, larger pot to help them re-establish their root systems.
Waiting for roots to re-establish
After you repot your seedling, you’ll probably notice a significant slow down in growth. Don’t worry—this is totally normal!
Whenever you transplant a seedling, even if you’re reeeeeeeally careful, the microscopic root hairs that grow out from each root are damaged. As young as the plant is, it takes a fair amount of energy to regrow them.
Check out these tiny little root hairs on this newly germinated spinach seed!
So you will likely see little to no growth happening in that first week or so after repotting. Just make sure the soil stays moist (but not completely saturated) to encourage rooting. I put my 4″ pots on old cookie sheets and fill the cookie sheet with water so they can continue to be watered from below.
As long as the seedling doesn’t appear to be dying or wilting, it will eventually start growing again.
More seed starting advice:
Handling seedlings can be scary, but these tips for how to repot seedlings can help make sure they stay healthy! Let me know if you find them helpful.
Seedlings often quickly going to outgrow the little cells they're started in, so you'll want to repot them. Here's why and how to repot seedlings.
5 Signs It’s Time To Repot Your Seedlings
It’s important to be able to recognize when it’s time to transplant seedlings from the seed tray to a larger pot. Transplanting seedlings a few weeks after starting should be part of your seed starting routine.
We start the seeds in smaller containers because we can control moisture and temperature much better that way, and if you’ve heard me talk much about seed starting then you know my motto is that moisture and temperature are the most important factors in getting good germination.
But once the seeds sprout, they quickly outgrow their seed starting container. It would be a bad idea to let them continue to grow in a pot that is too small. They’ll become nutrient deprived and their roots will grow round and round into a big knot.
Transplanting them into larger pots, helps them develop healthier roots and grow faster. By not restraining their growth indoors, you’re training them to grow big and strong when it’s time to put them in the garden.
But how do you know when to repot seedlings?
There are a few simple things you can look for that are dead giveaways your plants need a bigger pot.
1. They have one or two sets of true leaves
The ideal time for transplanting your seedlings is about 3 weeks after they sprout or when you have 1-2 sets of true leaves. It’s better to get them in new containers before they start to show the signs of stress listed below.
2. The cotyledons are turning yellow and falling off
Cotyledons are the first leaves that emerge from a seed. They are different than the “true leaves.” True leaves are the second and subsequent sets of leaves that grow after the cotyledons emerge.
It is normal for cotyledons to yellow and fall off, but if they’re doing it when you only have 1 or 2 sets of leaves, your plants really need to be transplanted.
3. The true leaves are turning yellow
It’s definitely time to transplant if the true leaves are yellow. This is a sure sign that your plants are starved for nutrients.
4. The roots are wound around and around the root ball
You definitely want to see roots in your growing medium when it’s time to plant, but if they’re circling around the edges of the root ball, then they’re getting too crowded.
5. They’re crowded
You don’t want to overcrowd your plants when they’re young. Some plants will grow taller than others and that will affect how much light the others get. You’ll also get the larger plants sequestering all the nutrients and that will stunt the growth of your other plants as well.
Why transplant the seedlings at all?
You might wonder why we would go to the trouble of repotting seedlings at all? Why not just give them some fertilizer, or better yet, start them in a larger container to begin with?
You’ll be much more successful germinating seeds if you start them in small containers. This allows you to have more control over the temperature and moisture in the seed starting container. We’ve found we have much better sprout rates in the smaller cell trays as compared to using other types of seed starting containers.
You can dose your plants with some fertilizer but that will stimulate growth. They’re already telling you they need more space, why stress them by making them grow bigger in the same small space?
Both of those options are viable alternatives, but you’ll have healthier plants if you transplant instead.
How to repot seedlings
Transplanting seedlings is quite easy to do. You simply need a new container and some potting mix. We recommend a container that is twice as big as what they are in now and a high quality potting mix like Fox Farm Ocean Forest.
We like to mix the potting mix and the seed starting mix in a 50:50 ratio. This is especially helpful for young seedlings who still have tender roots.
Before filling your containers, wet down your soil mixture to ensure even watering after you plant. Then nest your seedling in the new container, filling in around the base of the plant and pressing down to seat it in well and remove air pockets.
For tomatoes, bury the stem leaving only 1 or 2 sets of leaves above the soil line. For all others, plant them level or bury the stem about 1/4 inch or less.
Water your transplanted seedlings well and place them back under the light.
What about fertilizing seedlings?
You can fertilize young seedlings after transplanting as needed. Use a liquid organic fertilizer diluted to half strength. If they tolerate the half strength and seem like they need more, you can up it to full strength.
Don’t fertilize your seedlings until after you transplant them. You don’t want to stimulate growth in a space that is too small.
Do you transplant or repot your seedlings?
Tell us about your experience in the comments below!
You've conquered seed starting. Woot! But do you know when to repot seedlings? Transplanting seedlings is a step you can't skip! Learn the signs your plants are telling you they've outgrown their seed starting tray.