HUGE Growth by Potting up Seedlings with Worm Castings

This blog post is a subsection of our Worm Castings 101 - Everything You Need To Know guide. In the 101 guide we cover multiple topics about worm castings, so if you'd like a more thorough walk through of what worm castings are and what they have to offer then click through to read more!


In the blink of an eye, the tiny seeds we recently started indoors are already growing big and strong. As a matter of fact, within the first few weeks of germinating most will have outgrown their home. So, today we're going to go through the process of potting up your seedlings into a larger container with even more worm casting seedling mix.

And as you'll see, they'll thank you by putting on HUGE growth over the coming weeks!

What does it mean to be potting up?

The process of potting up is really quite simple. It simply means to be moving our seedlings into larger pots. For instance, it's a very common practice to start seeds in a 1" seed cell and about 14 days after planting, the now growing seedling is beginning to outgrow its 1" home - making it the perfect time to move it up into a larger 3" pot.

After a few more weeks and plenty of new growth, that 3" seed cell starts to be a bit small - making it once again time to pot up.

This simple process continues until it is warm enough outside for the plants to be transplanted into the garden beds.

Personally, I prefer to start my seeds in 3" seed cells. Why's that? Simply to skip that first step. By starting multiple seeds in 3" seed cells, I no longer need to pot up from 1" seed cells to 3" seed cells. 

Why pot up seedlings?

The purpose of potting up seedlings is because we want our plants to have sufficient space and soil to develop strong, robust root systems. In its earliest days this root system will be just a few small hairs. However, after a few weeks in our worm casting seedling mix this root system will rapidly develop.

The challenge is that it will quickly run out of space in its existing seed cell.

This occurs when the roots begin to circle the outside of the seed cell. When this happens, the plant begins to feel stressed and that it is actually at the end of its growing season. When this happens, it stops putting out foliage growth and puts all of its energy towards flowering in hopes of passing along its seed for future generations. 

So without potting up, you run the risk of our plants bolting and going to flower before we even get them into the garden - just like this zucchini below! 
A stressed zucchini seedling ready to be potted up
This Zucchini is displaying stress in the form of very early floral growth. 

When do I start potting up?

You will know that it is time to pot up your seedlings when the roots begin to circle the inside walls of the seed cell - this is called root bound.

You can check if this is happening with your plants by:

  • Hold onto the seed cell with one hand and with your other hand weave the stem(s) of the plants through your fingers.
  • Flip it upside down, so that the plant is facing the ground (but being held by your hand that has the stems weaved through it).
  • With the hand holding the seed cell, gently tap it to loosen it from the seedling mix and potentially roots inside and pull it off
  • Observe the root system

If you notice lots of roots and that they are beginning to circle, you want to begin the process of potting up the seedling into a larger seed cell with more worm casting seedling mix. 

image of a root bound seedling ready to be potted up
This root bound seedling is needing to get potted up as soon as possible.

How do I pot up my seedlings?

Now that you know your seedling is ready to be potted up, simply:

  1. Grab a new pot that is at least 2x the size of the current pot (i.e. if it is in a 3" seed cell, use a 6" pot/container to pot it up into).
  2. Fill it 1/3 of the way up with worm casting seedling mix. Moisten the seedling mix with 2 turkey bastes of water.
  3. Remove the seedling from the smaller seed cell and gently massage the roots to loosen them. If some break that is totally normal.
  4. Once loosened, place the seedling into the larger pot and let it rest on the seedling mix you just put in it. 
  5. Fill the pot to 1cm from the top with worm casting seedling mix. Gently compact this soil.
  6. Moisten the seedling mix with 5x turkey bastes of water. 

Once that is complete you can place it back into its seed starting station and allow it to continue to grow just as it was before potting it up!

Image of a potted up tomato plant in worm casting seedling mix

This tomato plant now potted up into a 6" seed cell has all the space it needs to grow big and strong. 

Do I need to fertilize the seedling? 

If you're using our worm casting seedling mix, you don't need to add any additional fertilizer! The plant has all of the nutrients and microbes it needs to continue growing while indoors.

Furthermore, if you still have several weeks until the outdoor temperatures are warm enough to transplant then you actually don't want the plant growing too large as they will quickly take up all the space in the seed starting station! 

What happens next?

We pot up seedlings to allow them to continue growing without stress until it is warm enough in the outdoor garden to transplant them. As such, you next want to be looking at the average night-time low temperatures to determine when you can begin hardening them off. 

  • Cool Weather Crops: For cool weather crops (i.e. brassicas, leafy greens), you want to wait until the average nighttime low temperature is at least 8C (46F) before transplanting them.
  • Warm Weather Crops: For warm weather crops (i.e. tomatoes, peppers, squahes, cukes, etc.), you want to wait until the average night-time low temperature is at least 10C (50F) before transplating them

Additionally, you will want to harden the seedlings off over a multi-day process. Once those criteria are met, then rather than potting up your seedling you will now transplant it into its forever home! 

Seedlings 14 days after being potted up and thriving in the worm casting seedling mix

Only 14 days after being potted up and all the seedlings have put on tons of new foliage growth. 

By following this process for potting up your seedlings while indoors you will ensure that they have all the space they need to grow in a natural manner and make their way to the garden healthy, lush, and ready to thrive under the warm summer sun!

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