Potting Up Seedlings: What, Why, When & How

If you started plants from seed, chances are you will need to pot up the baby seedlings before they get transplanted into the garden. Maybe even twice! Your individual potting up needs and timing will vary. It will depend on the size of the seedling containers you started with, how quickly your plants are growing, and how long it is from the time they sprouted until they need to go outside. The act of potting up, when done right, helps your plants thrive!

This article will discuss the what, why, when, and how of potting up. At the end of this post check out the demonstration video! It shows our process for potting up tomato and cucumber seedlings

If you’d rather skip the reasoning and timing behind potting up, click below to jump straight to the “how-to” and video.

Jump to the How To

What is ‘potting up’ seedlings?

Potting up couldn’t get any more literal. It is simply the act of transplanting seedlings “up” into larger containers than they were previously living in.

To be honest, potting up is a task I dread for some reason. It isn’t all that difficult, but does take a little time and effort. Over all the other things I need to do around the homestead, this is one chore that I consistently put off until it’s urgently due. Knowing this about myself, we try to combat this and set ourselves (and plants) up for success from the very beginning – by starting seeds in larger containers. This reduces the urgency to do it so soon after germination. We’ll talk more about container sizes shortly.

Why pot up?

Potting up seedlings as they grow provides them the best chance to grow stronger and bigger, feel less stressed, and live their best life!

1. By potting up seedlings into larger containers, it enables their roots to continue to grow without getting root-bound. A root bound-plant is not a happy plant. When a plants roots are being restricted to the point that they start to grow in circles around themselves, they become tangled and “bound up”. This can reduce the roots ability to spread out and flourish after they’re planted out in the garden. Plant health is directly tied to root health, so this means the plants are also less likely to flourish.

Plants with tight, bound root balls can be gently loosened during the time they’re transplanted. However, this could either help them, or harm them. Some plants don’t mind a little root-ruffling, and breaking up that ball can encourage the roots to spread as we want them to. However, some don’t take a liking to this treatment. They might even get a bit of transplant shock from it. Therefore, we try to prevent root binding in the first place, to reduce the amount we need to disturb the roots later.

2. Another reason to pot up seedlings is that as their roots grow larger, they drink more water, and thus dry out more quickly. You’ll notice that a small 6-pack full of soil and yet-to-sprout seeds will retain moisture much longer than a small 6-pack full of maturing, thirsty seedlings. Taking care of seedlings can be tedious enough, but especially so if they’re drying out on you every day!

3. Lastly, the potting up process feeds the seedlings! If you started seeds in straight seedling mix, or a mix with primarily seedling soil like we do, chances are they’re hungry. Seedling soil is very fluffy and pretty devoid of nutrients. Even if you have been feeding with an occasional dilute seaweed extract, the plants will definitely enjoy a slighter richer, heartier soil now!

When to pot up seedlings

The timing for when to pot up is going to vary from gardener to gardener, situation to situation, and plant to plant. The factors that influence the best time for potting up seedlings include their container size, the type of plant, when they’re intended to be planted outside, and how vigorously they are growing. There is no set rule like, “You must pot up within 33 days of germination”…

Container size

The best time to pot up a seedling greatly depends on the size of container you started it in. Smaller containers, like those trays with dozens of cells each, are going to require potting up sooner. Plants will feel cramped and overgrown in those fairly quickly. As I mentioned before, we usually avoid starting larger vegetable seedlings in tiny-hole trays. By starting them in slightly larger containers, like these reusable 4” nursery pots, we don’t need to pot up until about 6 to 8 weeks after germination. After that, we’ll move them into 6-inch or 8-inch pots.

Then why don’t we just start seeds in those larger 6 to 8 inch containers from the get-go, you ask? That way, we don’t need to pot up at all, right? Well, it’s not quite that simple. First of all, if you start seeds in huge containers, it’s going to take up a ton of space. You will not be able to fit nearly as many containers on your heat mats and under grow lights, which is pretty crucial during germination and the first few weeks of life. That means less plants, which is never a good thing.

Also, tiny seeds and seedlings don’t necessarily want to be swimming in a huge sea of soil. It is easier to overwater, and their roots might struggle to develop. They do like to be hugged, just a little. Plus, moving from a seedling start mix into richer soil is a great growth-encouraging step – one we’d miss out on if we started in big pots.

Type of plant

The timing for potting up also depends on the plant itself. Larger plants like tomatoes will more quickly outgrow their space than something smaller like herbs started in the same size container. Tomatoes grow much faster than peppers, so we always need to pot up our tomatoes earlier.

If you plan in advance, you could try to start certain vegetables, herbs, and flowers in appropriate size containers. For example, we start most of our flowers, herbs and leafy greens in 6-packs, and the other bigger veggies in 4” pots. We have found that by doing this, the flowers and herbs are usually okay in their 6-pack until the time they need to go outside, and may not need to be potted up at all. Squash grow very quickly and don’t like their roots disturbed. Therefore, we start those straight in larger 6” pots to give them plenty of space. We start them only about 3-6 weeks before they’ll be planted outside, so we don’t need to pot them up at all.

Timing to plant outside

Another variable that impacts potting up is when your target transplant date is. If you intend to plant out all your seedlings in the next week or two, then don’t bother! That is, unless they’re getting really really overgrown and bound, then it might be worth it. Especially if planting is still two weeks away. Yet if your plants are showing signs of being cramped in their containers and it is still several weeks or more until plant-out time, pot those babies up!

Growth

A few weeks after germination, start keeping an eye on the bottom of your seedling containers. Are roots starting to poke through the bottom drainage holes? How big is the plant looking? Does it still look happy? Has it still been growing steadily, or has it slowed?

When the roots start to poke through the bottom of the container a lot, it is time to pot up into a larger size. Can’t see the roots sticking through the bottom, but the seedling seems pretty large? Carefully take one out of the container and look at its root ball! Sometimes they’ll start to spiral around themselves before they come through the bottom. I should also note that you don’t have to wait until the roots are coming through the bottom, or until it is close to being root bound. You can pot up sooner too! I just always seem to wait until the last minute.

The moment we’ve all been waiting for…

How to Pot Up Seedlings

Find or obtain some containers that are slightly larger than the ones they’re already in. About twice as large is a good goal. If you’re going from super-tiny cells, even more than twice as large would be most efficient, reducing your need to do this again.

Now it’s time to give the seedlings some fresh, rich soil to play in! You could go two ways here. Again, this depends on your situation.

Soil for Potting Up

When we are potting up fairly large, established seedlings like the tomato in this example, we use a high-quality organic potting soil, straight from the bag. Potting soil is slightly more dense and nutrient-rich (often contains mild fertilizer) than the seed starting mix the plant was previously living in. At this stage in maturity, they can handle it! Not just handle it, but love it. Pre-moisten the soil if possible.

However, if you are potting up very small, less established seedlings that have thin fragile roots and no solidly formed root ball, they will probably like something a little more fluffy added in the mix still. In that case, it would be best to combine something like 60% organic potting soil and 40% of your favorite seed starting mix. That way, their tender roots meet less resistance when they’re trying to grow.

Add a little soil into the bottom of the new container, and gently ease the plant out of the smaller container without pulling on the seedling itself. Place its entire soil mass and roots into the new container. Then fill in around the sides with the new soil mix.

To Bury or Not to Bury? That is the question.

If the seedlings have gotten a bit tall and leggy, it is okay to plant most kinds of seedlings deep, filing soil up around the stem and burying it a bit. This is totally safe (and even preferred) for tomatoes. The portion of the buried tomato stem will actually shoot off new roots! You could also do this for other members of the nightshade family, including peppers, potatoes, and eggplant. Brassicas (the cabbage family, including kale, broccoli, and collard greens) and cannabis can also handle a little burying. Especially when they’re bigger and the stem is more firm. See the images below as an example.

Other types of plants may not like this practice. The now-buried stem could rot and kill the plant. This is particularly true for beans and trees, so keep the soil line about the same as it was previously. I would also avoid burying seedlings that are still very small and tender, regardless of their variety. I have read mixes messages about burying cucumbers. Overall, I think it is okay but not as commonly encouraged as with tomatoes. If you do, only bury them up to their first set of leaves, or just a couple inches.

To avoid the need to bury seedlings and therefore any risk of rotten stems, the best practice is to prevent leggy seedlings in first the place. To do this, provide ample light and other ideal seed-starting conditions. If you’d like to learn more about seed starting best practices along with ongoing seedling care, read here!

A Neat Potting Up Trick:

I learned a fun little trick a few years ago, which is demonstrated in the video and photos below. It can be used for potting up seedlings, especially ones that you do not want to bury the stems of). However, we most often exercise this trick when we are planting bigger plants like shrubs or small trees in to pots. The idea is to make a dummy hole, or a placeholder for the root ball, inside the container that the plant is being transferred in to.

Fill the new, larger container with the amount of soil you estimate should go below the plant. Then set the plants current container down inside the larger one. You can use an empty one, if you have something the same size on hand, or actually place the plant itself (still in the pot) down in there. Is it at the right level? Keep in mind things usually sink down a little after time and watering. If so, lightly pack soil in around the outside of the inner pot, creating a nest of soil. This trick works best with pre-moistened soil, reducing the likelihood it will cave in on you. Then you can pull out the dummy container, gently ease the plant out and into its new perfectly-sized hole!

By doing this, you are ensuring there are no air pockets and a nice amount of soil around the plant. Thus, it reduces the need to try to stuff soil in around it afterwards. Depending on the container or pot you’re working with, that can sometimes be difficult or awkward. It also reduces the jostling and possible shock to the plant being transplanted. Before we learned this trick, I don’t know how many times we accidentally overfilled containers and then had to pull or dig the plant back out to adjust the soil amount. If you’re working with a large, heavy plant, or one with a not-very-solid root ball, this can be a pain in the butt.

After Potting Up

Give them a good water! We prefer to water from below, allowing the soil to soak up water from the tray beneath it. The seedlings may also appreciate some dilute seaweed extract in their water, which can help to ward off transplant shock. To read more about fertilizing seedlings with seaweed extract, check out this post. We sometimes also feed our transplants with a fresh aloe vera soil drench, either alone or mixed along with seaweed. Aloe also reduces the risk of transplant shock, and encourages robust new root development!

That’s really all there is to it folks.

The freshly transplanted seedlings can now live happily in their new spot for several weeks, until they move in to their forever home – your garden. Don’t forget to harden off indoor seedlings before transplanting outside to prevent damage and transplant shock!

Check out the potting-up demo video: