Transplanting Seedlings Outdoors: A How-To

Lizzi Brosseau transplanting seedlings into her community garden plot.

Lizzi Brosseau transplanting seedlings into her community garden plot.

Follow along with Modern Gardener’s Lizzi Brosseau as she tries gardening for her first time!

Hello, and thank you for coming along with me on this journey! In this week's post I'm going to be talking about how to transplant seedlings outdoors.

I’ve prepared our garden plot and made a garden plan, and now that we’re past the last frost date, my family and I are ready to transplant our seedlings outdoors.

However, here in Utah we’ve found ourselves in the midst of an unusually cold and wet spring. It’s June and we are still having overnight lows dipping into the 40s!

In this post I’ll talk about hardening off and transplanting seedlings, and what options you have to save them during atypical seasons when it seems like summer is never going to come. To get some help, I asked our resident expert Jess Kemper from the University of Utah’s Edible Campus Gardens, to give me guidance about determining when and how to transplant our seedlings.

Seedlings for transplanting into the Brosseau Family garden plot!


What is “Hardening Off” and Why Does it Matter?

First let’s talk about the term, “hardening off.”

Whether you grow your plants from seed or buy them at your local plant sale, most baby plants need to be acclimated from the safe indoor environment they started in, to the variable outdoor environment where they’ll grow into an adult plant. Jess recommends that seedlings spend their “awkward teenager phase” outside on the porch for a few days. This is called “hardening off,” and she recommends giving them half a day of direct sunlight with half a day of shade during this 3-5 day process. (They should still spend the nights indoors if gets below 50 degrees).

Plants need to be hardened off because they can be shocked by the relative harshness of the outdoors if all they’ve ever known is the warm comfort of a greenhouse or indoor shelf fitted with grow lights and heat pads. When plants aren’t hardened off, they could die when they first venture outside!

Tom Brosseau transplanting Dyer’s Chamomile seedlings.


What Are Ideal Conditions for Transplanting Your Seedlings?

Before we talk about ideal conditions for transplanting outdoors, I need to clarify something important—in this post, I’m talking about hardening and transplanting warm season crops, like tomatoes, peppers and squash. Cold weather crops, like lettuces and peas, can be transplanted before the last frost date and love cooler, wetter weather. Consequently, I’ve heard many gardeners say that their cold weather crops are thriving in this unusually wet and cold spring!

But when it comes to warm season crops, ideally you would...

  • Pay close attention to the weather forecast and look for a stretch of days where the overnight low doesn’t dip below 40 (or even better 50).
  • Harden them off and transplant them after the last frost date (which for Utah is around Mother’s Day).

However, if you have already transplanted your seedlings and can see cold temperatures in the forecast, don’t fret! There are simple ways to cover and keep your plants warm during cold snaps, which I’ll talk about in a later post. So with that in mind, let’s get our plant babies in the garden!

Closeup of Tom transplanting Dyer's Chamomile seedlings.


How to Transplant Your Seedlings in 6 Steps

  1. Dig a hole in your garden soil that is deep enough for the soil in your seedlings container to be fully immersed in the garden soil.
  2. Sprinkle into the hole any amendments, like compost or fertilizer, that your plant would need during their transition outdoors. I’m using humic acid and blood meal that is free and available at our community garden.
  3. Squeeze your plant’s container gently to loosen the root ball from the container. Be mindful of any roots that have grown out of the bottom, and untangle them or pinch them off if they can’t be untangled.
  4. Put your hand over the top of the container with the stem of the plant and leaves loose from your grip. Turn over the container slowly and allow the root ball to slide out of the container and into your hand. While your plant is upside down, take advantage of this opportunity to loosen any tightly wound roots!
  5. Place your plant into the hole and cover with soil. You’ll want to be sure that the soil around your plant stem isn’t shallower than the surrounding garden soil so that rain or drip water doesn’t collect around the plant, causing it to be over watered.
  6. When the soil is level, give your plant a good watering.

This is the most common method of transplanting seedlings, but there are other ways to put a plant in the ground. One that I’ve recently been learning about is trench planting, which I’ll briefly explain next.

San Marzano tomato plants that have been trench planted.


“Trench Planting” and Why it’s Great for Tomatoes

In our recent video about Growing Perfect Tomatoes, James Loomis from Wasatch Community Gardens talked about trench planting as a way to keep tomatoes from being shocked by deeper, cold soil. When you trench plant, you’re more likely to plant your warm season crop into shallower and therefore warmer soil, which is optimal for them. Trench planting is also a great route to go if your seedlings have grown substantially in their container.

To trench plant, you simply dig a shallow trench and lay the tomato plant on its side, rather than digging a deep hole to transplant your tomato upright. After you’ve covered the root ball and lower stem with soil, it looks a bit like your tomato plant is laying in its garden bed, but don’t worry! It should take a day or two, but the tomato plant will curve back upright as it searches for sunlight.

Wilted and yellowing leaves of this San Marzano seedling mean it needs to be planted!


When in Doubt, it's Better to Transplant Seedlings and Cover

One of the first things I asked Jess during my interview was why the leaves of our tomato seedlings are turning yellow. She said that it was probably a sign that our tomatoes want to be out in the garden. Jess suggests that new gardeners like me should be brave, and put their seedlings in the soil after threats of frost have passed. Tender seedlings can be given adequate cover on cold nights, which is preferable to keeping them inside in their growing containers for too long. Plants want to be outdoors in the sunlight and soil where they can spread and grow!

With that advice in our gardener’s tool bag, I’m glad that we’ve finally taken the plunge and transplanted our seedlings outdoors. I can see some low temperatures in the forecast, so I think I’ll have to experiment with covering my plants next! I’m looking forward to watching these plants grow, and am excited for the all the lessons I have awaiting me on this first-timer’s gardening journey.

Don’t forget to follow Modern Gardener on Instagram and Facebook to see my Stories about gardening, and make sure to chime in with your own! We love to learn from other gardeners, and look forward to see what you’re learning in the garden too!

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Lizzi Brosseau

Lizzi Brosseau, Digital Producer at KUED Channel 7.
Modern Gardener Host and Author

Lizzi works for PBS Utah as a digital producer and host of Modern Gardener. Read more