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potting up seedlings

When it’s Time for ‘Potting Up’

Artichoke seedlings that have been growing in long Rootrainers are well-rooted and ready for potting up.

With such a well-developed root system, this tomato seedling should be potted up so it can grow further in a larger pot.

If you start plants from seed, eventually you’ll have to master the technique of potting up. It’s what you do when a seedling gets too big for the pot or cell it’s growing in. Usually it’s too early to plant outdoors, and the seedling still needs a few more weeks of coddling indoors. Not all seedlings require potting up, but many do.

A good example is tomatoes, which are started in small pots about eight weeks before the last frost date in spring. After growing indoors for four to five weeks, the seedlings need to be potted up into something bigger, where they can continue to grow until it’s time to plant them outdoors. Choose a pot that has about three times the volume of the original. Avoid waiting too long, because root crowding can stunt plant growth and make it difficult for plants to recover after transplanting.

Why not just start with a big pot in the first place? You could, but most seedlings grow better in pots that are smaller, about 2″ square. With smaller pots, it’s easier to provide consistent moisture and fertilizer.

Lettuce growing in flats, which, in this example, are biodegradable containers from take-out food.

Another time you might do potting up is when you sow seeds in flats. These containers come in many sizes, but most are about 6×6″ and 2″ deep. They’re handy for small seeds, such as lettuce. You sprinkle the seeds onto the surface of the flat, cover with a thin layer of seed-starting soil (Germinating Mix) and let them germinate.

Once the seedlings have a couple sets of true leaves, you can gently take the soil out of the flat and dislodge the seedlings for potting up into individual pots. This process is called “pricking out.” Lee Reich, author of Weedless Gardening, takes you through the process in the video below. Watch and learn:

If you start plants from seed, eventually you'll have to master the technique of "potting up."

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.