When And How to Transplant Seedlings into the Garden

By Heather Rhoades
Published: May 1, 2016 | Last updated: April 26, 2021
Key Takeaways

Transplanting tender, young seedlings into your outdoor garden too early or too late can have dire consequences for those plants you so lovingly started from seed in your growroom.

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Raising plants from seeds can be a rewarding and exciting way to add new varieties to your garden. Many of the best and unusual varieties of vegetables are simply not available in your local nursery and your only option is growing these plants from seeds. But to grow these unusual varieties, you must know something about planting seedlings.


How to Transplant Seedlings

One common question from people growing plants from seeds is, “How do I know when my seedlings are big enough to put out in my garden?” Planting seedlings out in the garden at the proper time is crucial to their development later on. If you put them out before they are ready, they may have a hard time surviving the elements. If you wait too long, your seedling may become root bound in its original container.

There is no hard and fast rule about how tall a plant should be before you put it out in the garden because different plants grow to different sizes. The amount of light a seedling gets can influence how quickly a plant grows when you are raising plants from seeds. If there is not enough light, a plant can grow tall very quickly, but this plant may or not be ready for planting out. The best way to judge if a plant is large enough to plant out in the garden is to look at the number of true leaves.


True Leaves on a Seedling

The general rule of thumb is that when a seedling has 3 to 4 true leaves, it is large enough to plant out in the garden (after it has been hardened off). When you plant a seed, the first leaves to emerge are the cotyledons. The purpose of these leaves is to provide stored food to the seedling for a short period of time.

True leaves grow shortly after the cotyledons. The true leave emerge and start generating energy through photosynthesis to help feed the plant for the rest of its life. Making sure that the plant has enough of these leaves to keep it sustained when planted out in your garden is important.

Just remember, it is not how tall, but how many true leaves your plant has that will determine when you should transplant seedlings. But even when your seeds are big enough to plant out, make sure you harden off yourplants first.


How to Harden Off Your Seedlings

When plants are grown from seed indoors, they are grown in a controlled environment. The temperature is usually maintained, the light is not as strong as full sunlight outside and environmental disturbances like wind and rain are non-existent.

Because a plant grown indoors has never been exposed to the harsher outdoor environment, they do not have defenses built up to help them deal with some of the environmental conditions found outside, similar to how someone who has spent all winter indoors will burn easily in summer sunlight.


The way to help your seedlings build up a resistance is to harden off your seedlings. Hardening off is an easy process and will make your plants grow better and stronger when you do plant them out into the garden.

Steps for Hardening off Seedlings

Hardening off is really just gradually introducing your baby plants to the great outdoors. Once your seedlings are big enough to plant out and temperatures are appropriate for planting outside, pack your seedling in an open-top box to make transporting the plants easier.

Place the box of plants outside in a sheltered, preferably shaded, area. Leave the box there for a few hours and then bring the box back indoors before the evening. Repeat this process over the next few days, leaving the box in its sheltered, shaded spot for a little longer each day.

Once the box is outside for the entire day, move it to a sunny area and repeat the same process. For a few hours each day, move the box from the shaded area to the sunny area, increasing the length of time each day until the box is in full sun all day.

During hardening off process, it is best to bring the box in every night. Once the plants spend the whole day outside, then you will be able to leave them out at night. At this time, it will also be safe for you to plant the seedlings in your garden.

This whole process should take just a little longer than one week. Taking this one week to help your plants get used to the outdoors will help ensure your plants have a much easier time growing outside.



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Written by Heather Rhoades

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Heather Rhoades is the founder of Gardening Know How, where she continues to write articles and answer questions relating to gardening.

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