When to Transplant Seedlings: Learn All You Need to Know

Gardening season is upon us. There isn’t a better time than this! The hours you spent planting seeds, waiting endless days for them to germinate and gently tending them as they grew led to you to this point. It is time to transfer them outside to your garden. But wait! How do you know when to transplant seedlings? No magical sign pops out letting you know when it is time.

Transplanting seedlings is a big deal. If you do this step wrong, all of your hard work will go down the drain. Plants can die if the process is messed up, and it starts by knowing when to transplant seedlings. The timing is important, so let’s take a look at what you need to know.

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Strawberry seedlings before planting

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Determine Your Last Frost Date

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Rows of young seedlings before planting

Your first step is to learn your last frost date. While the date will vary slightly, the USDA created a map and divided the United States into zones. Each zone has a date considered to be the last frost date based on the average of years prior. Of course, as you get closer to this date, you will need to keep an eye on the weather, but the information is needed to create your plan.

For example, I live in Ohio, in Zone 5b. Because of this, I know that my average last frost date is May 11. After this date, I can transplant seedlings that are unable to tolerate frost. Check out your zone and last frost date.

Understand the Needs of Your Plant

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Child’s hand holding a young seedling

Every plant has different needs and preferences. There are some plants that can tolerate frosts, so you will want to transplant those sooner than others. When you start the seeds, read the information on the back. It will give you an idea about when to plant the seedlings in your garden.

There are some plants that you will want to start two to four weeks before the last frost date. These frost hardy plants include peas, cabbage, lettuce, radishes, carrots, Brussel sprouts, broccoli, and kale.

On the other hand, many plants don’t like any frost at all. They need to be planted after the last frost date when you are sure the danger has passed. This spring, our last frost date was rather cold. I waited a few more days until I knew the temperatures were warm enough to support these plants. Tomatoes, peppers, zucchini, green beans, corn and eggplant all prefer warmer temperatures with no frost.

Wait for True Leaves

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Pepper seedlings with their true leaves

You don’t need to wait for your seedlings to be a specific size before transplanting them into your garden. Some of my tomato plants are huge this year, but some are rather small. All were planted at the same time! The general rule that you will want to follow is that a plant is ready for the garden once it has three to four true leaves.

The first leaves that develop after your seed sprouts are called cotyledons. They are not the permanent leaves and perform an important job. They feed the seedling for a small period until the true leaves grow. At this point, photosynthesis starts, and the job of feeding the plant for the rest of its life starts. So, watch the changes in the leaves. Photosynthesis needs to start before transplanting.

Harden the Seedlings

When you are making your plant to transplant, you need to account for a process called hardening off. If you take your seedlings from inside and put them directly into the garden, it will die. The sun is much stronger than the light you provide inside. Leaves and plants burn up or turn yellow.

The wind is another factor. Inside, the air is still, but the wind is constantly blowing outside. While you should brush your hand over the seedlings inside often, it doesn’t replicate the wind. The stem of the seedling needs time to learn to withstand the wind.

Via Youtube.com: How to Harden Off Seedlings

Start by putting the seedlings outside on a cloudy day, part in the shade. Over the following day, check location so that the seedlings get more direct sunlight each day. Hardening off typically takes one week, and then you could safely move the seedlings to the garden. Remember to account for this week in your plans.

Learning when to tran​splant seedlings takes time and experience, along with knowledge. Your first step is to figure out the last frost date for your area. Once you know that date, look at that type of plants you are growing. Mark on your calendar when each seedling should be transplanted outside. This date will range from four weeks before the last frost up to a week after the last frost date.

Then, check for true leaves. If they are nearing the time for transplanting, start the process of hardening off one week before you want to transplant. With a bit of planning and preparation, you will seamlessly transfer the plants outside with no problems.

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