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When to Transplant Seedlings

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In order to thrive, many seedlings need to be moved to a larger pot while inside and then transplanted outside. Let’s discuss when to transplant seedlings to help your plants thrive by avoiding disease, malnutrition, and other problems.

Seedling being transplanted into garden with garden trowel and pair of gloves

Most of us start seeds indoors in small containers, which is great for getting seeds to germinate, but after they sprout, they can quickly outgrow their tiny seed starting container.

To avoid problems and establish a strong root system, the seedlings often need to be thinned and repotted into larger containers in order to thrive before being transplanted outside.

Do all seedlings need repotted?

The short answer is no. Some seedlings will not need to be repotted before being transplanted outside. Some stay small plants and are only indoors for a short time so it’s unnecessary. It also depends on how large the pot they’re started in is and the type of pot they’re started in since many plants can thrive in biodegradable pots or soil blocks without needing a bigger pot before they’re transplanted outdoors.

How big should seedlings be before repotting?

If your seedlings need repotted, the general rule of thumb is to repot in about 3 weeks, before the seedlings show any signs of stress. That said, there are a few signs that will help you decide if it’s time to repot your seedlings.

The cotyledons are falling off

Seedling tray with small seedlings with only cotyledons.
Cotyledons are the first leaves that appear when a seedling sprouts, these leaves provide the plant with nutrients until the first true leaves appear.

The first set of leaves that sprout on young seedlings are called cotyledons. These leaves will look different from the actual plant leaves, but have essential nutrients to get the seedling going.

Cotyledons naturally fall off once your new seedlings begin to develop true leaves, but if they’re beginning to yellow when you only have a set or two of true leaves, repot as soon as possible because things are getting cramped.

They have one to two sets of true leaves

True leaves will begin to emerge shortly after the plant sprouts. These leaves will look like the plants native leaves and provide energy via photosynthesis.

Sometimes, the seedlings have enough room to begin developing true leaves and the first leaves, or cotyledons, don’t start to yellow and fall off straight away. Once your seedlings have a couple sets of true leaves, not the cotyledons, it’s time to pot them up.

Yellowing leaves

If the true leaves are starting to yellow, your plant needs potted up, it’s starving for nutrients and space.

They are crowded

Thinning seedlings is essential. Lots of small seeds such as lettuce, basil, oregano and the like become crowded easily because the seeds to start are so tiny. But, this is also true when growing tomatoes, peppers and other larger seeded plants because you’re never too sure on the germination rate.

You can thin them and keep all of the seedlings or simply thin them and put the extras in the compost or throw them out to your flock, rabbits, or other green-loving critter.

They’re twice as tall as their container

Once a seedling has gained a bit of height, it’s time to repot so it doesn’t become too leggy. Leggy seedlings are a problem in and of themselves, so it’s best to repot when a seedling begins getting some height so it can get a bit more stability in a larger, taller container.

The roots are coming out of the cell

Once the roots begin to wind around the cell and sprout out the bottom of their small pot, you’re at the risk of the seedling becoming pot bound. If it’s not time to transplant it outside, it’s time to pot it up so it doesn’t become a big ball of knotted roots that can’t supply proper nutrition.

How to repot seedlings

Basil seedling being transplanted into a garden

Once you’ve determined your seedlings need potted up, the process is pretty straightforward. You just need a container about twice as large as the current container and potting soil.

What type of soil you use is up to you. You can use regular potting mix, leftover seed starting mix, or DIY your own seed starting mix.

Also, it’s worth noting that it’s best to water your seedlings a few hours before you plan to move them as a moist root ball will hold together best and will minimize the risk of transplant shock.

Step 1: Fill the Pot

Wet the soil before you add it to the pot to ensure even moisture throughout. Fill the new containers about halfway with the soil mixture.

Step 2: Remove Seedlings

You need to be gentle and slow when removing the seedlings from their cell. Gently grasp the stem of the plant and flip over the potting container and squeeze the bottom of the pot to loosen the soil.

Whatever you do, don’t grab the seedling by the stem and try to pull it out of the cell. If you can’t get the ball to loosen inside of the cell, I’ve had good luck using a spoon to loosen the soil and pop it out.

Step 3: Straighten the Roots

Do this gently, taking care not to break any of the roots. Just gently straighten the roots, this may take a few moments if your seedlings are root-bound. Just gently tease them loose and straighten them this will help maintain healthy root systems.

If you need to separate seedlings that were previously growing together in the same pot, you’ll want to separate them now by gently separating the roots from one another being careful not to break the roots.

Step 4: Seat the Seedlings

Gently seat the seedlings into the new pots at the same soil level it was planted in the original container. Continue filling the pot around the base of the plant and gently push down to give the seedling stability and remove any air pockets from the soil.

Step 5: Water & Label

Water the seedlings well and be sure to add a plant tag or other label to each pot before placing them back underneath the grow lights.

Transplanting Seedlings Outdoors

Whether you need to repot your seedlings or not, eventually you’ll have to transplant them out into the garden. This includes anything you’ve grown yourself as well as any seedlings purchased at your local nursery.

When to Transplant Seedlings Outdoors

This will vary a lot depending on the type of plant. Some plants are frost hardy, such as broccoli and onions, so they’ll be transplanted outdoors earlier. Whereas heat loving plants like tomatoes and peppers shouldn’t be transplanted outside until the threat of frost has passed.

Regardless of frost hardiness, you’ll want to make sure your plants have a minimum of four to five sets of true leaves before placing them out in the real world, aka your garden.

All of that said, you’ll want to be sure to harden off your seedlings before you transplant them to avoid transplant shock and death which would quickly put an end to all your hard work and effort.

How to Transplant Seedlings

After your seedlings have been properly hardened off and the weather is appropriate for their growth, you’ll want to get started getting them into the ground. It’s best to transplant seedlings on a cloudy day in the early morning hours to give them a bit of time to acclimate before the direct sunlight is hitting them full force.

You’ll also want to be sure to water them a couple of hours before transplanting to make it easier on them (and you as it’s easier to get them out of their cells) and avoid shock.

Step 1: Prepare the Garden

First, remove any weeds, rocks, or anything else that may have been left over or introduced to the garden bed over the winter.

The next part is optional, but some folks try to increase the soil temperature a little more rapidly, if you want to do this, you’ll put black plastic or similar over your garden bed about two weeks before planting time to allow the sunlight to heat the soil up more quickly than it will without the plastic.

You’ll want to add some organic matter such as compost to the top layer of your soil as well as work it in about 6 inches to loosen any compacted soil, provide adequate moisture retention, proper drainage, and nutrients.

When ready to plant, level the surface out with a rake.

Step 2: Water the Garden

A day before the planned transplant, be sure to water the garden to provide adequate moisture. If you get a nice, soaking spring rain you can obviously skip this step.

The goal is to make sure your soil is adequately moist but not soaking wet. Watering deeply the day prior should help you with this. If it rains the day of transplant and the soil is soaking, change your plans to a different day so you’re not trying to transplant in the mud which is no fun for you or the plant.

Step 3: Dig and Plant

Now it’s time to prepare the plants new home. Dig a hole that is slightly larger in diameter and about as deep as the plants root ball.

Carefully remove the seedling from its pot by turning it upside down. Then, plant the seedling as deep as it was in the pot in the planting hole. If you planted your starts in peat pots, you can plant the entire pot without removing the seedling as the pots will biodegrade in the ground.

Fill in around the seedling with fresh soil and gently tamp it down into the ground so that the roots make contact with the soil.

Step 4: Water In and Mulch

Watering the garden

Soak the soil surrounding the freshly planted seedlings to settle them in to their new environment and remove any air pockets from the soil. Mulch around the plants with natural wood mulch, straw, or spent hay to help retain moisture and control weeds.

Step 5: Water Regularly Until Established & Watch the Weather

Until the plants are well established, you’ll want to keep the soil moist never allowing it to completely dry out. This means you’ll likely be watering every day or so to keep it adequately moist.

When watering, be sure you do so at the soil surface, not from above as watering over plants can cause disease issues.

It’s a good idea to keep an eye on your weather forecast for the first week or two and be prepared to protect your plants from frost with a cold frame, row covers, or other means in the event of any off-season weather.

Whether you’re repotting crowded seedlings or moving your seedlings out to the garden to flourish, it’s all a task that is well worth taking your time and preparing properly so that you can have a successful garden.

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