Whether you germinate your seeds indoors or buy seedlings already started, the time will eventually come to transplant them into soil outdoors. It’s a little bit like sending a child off into the world, where you do your best to prepare them for life outside. Thankfully, with some attention and planning, you can give your plants a greater chance of survival. But how do you know when that time is? Here are five essential tips for determining the perfect time to transplant your seedlings and starter plants outdoors.
Watch overnight temperatures
One of the first—and perhaps the most important—tip to consider when planting outdoors is to wait until after the danger of frost has passed. Few plants, particularly herbs and smaller vegetables, can endure freezing temperatures. This probably won’t be much of a concern for you If you live in an area that has milder winters. If you’re like me and live in a zone with potentially cold winters, however, paying attention to overnight temperatures once springtime rolls around is critical.
Spring weather here in Brooklyn is subject to wide temperature swings from day to day. I’ve seen snowfall as late as March in my neighborhood. Even early April can still feel like winter! Check your weekly and long-range forecasts. What you need to look for are consistent overnight temperatures above 50 degrees. Anything lower raises the possibility of stunted growth in your fragile seedlings. Once you are confident that the danger of frost has passed, you can evaluate your seedlings to see if they are ready to transplant outside.
This year was a particularly cool Spring and I've had to wait till early May to transplant my starter plants to my balcony. I'm waiting even longer for the seedlings that I've germinated from seeds using the paper towel method and peat plugs. I hope to get them transplanted into soil mid to late May.
Check the size of your seedling plants
First, take a look at the overall size of your seedlings. If you've purchased them at a store or farmer’s market, they may already be a good size for transplanting. But if you’ve started seedlings yourself from seeds, you'll need to be patient. They may not be ready if they're too small or if they have few leaves. The time it takes seeds to germinate and reach the proper size for transplanting varies greatly on plant variety and growing environment.
At the time of this blog post, I have Thai basil seeds that I started germinating in peat pellets 2-3 weeks ago, but they are still no more than an inch tall. The shiso seedlings that I started at the same time are several inches tall and fully ready to be planted outside.
Growing plants can be unpredictable, especially in their early stages, so it’s important to have patience and not rush them outside before they are ready.
Learn the difference between seed leaves and true leaves
The development of "true" leaves on your seedlings is a good indicator that plants are ready to be transplanted into your containers outside. When seeds first germinate, you’ll only see tiny sprouts pop up from under the growing medium. These are called cotyledons, which are embryonic or seed leaves. They are the first leaves of the developing plant.
It can be hard to distinguish one plant from another at this stage (markers are helpful and important!). Once the seedlings start to grow bigger, true leaves will form and you’ll start to notice the characteristics that make the plant recognizable.
You'll want to see at least three or four nicely developed leaves on each plant before moving them outside. Anything less means it needs more time inside. Remember the shiso I mentioned earlier? I had accidentally mislabeled the seedlings as another herb, but once more leaves appeared I immediately recognized the leaves.
Root development
Just as important as leaf development is the development of the plant's root system. To assess the roots, you’ll need to lift up your seedling and take a look at the underside. This is easy to do with substrates like peat pellets and rockwool. If your seedlings were started in small soil compartments, you'll need to carefully lift it out of the tray in order to assess the roots.
What you need to look for are signs of any roots making their way out of the bottom of your growing substrate. One of the benefits of starting seedlings in very small containers is so the root systems can develop in a focused manner. Once you spot roots outgrowing the substrates or pots, your seedling is ready to move to a larger container.
Hardening
Now that you’ve determined that your plants are ready to be moved outside, there’s only one more thing you might want to consider. It may be important to gradually acclimate your seedlings to the outside environment with a process known as “hardening.” I’ll fully admit that I was not aware of this process until somewhat recently and I probably did some damage to some of my plants.
It makes perfect sense that thrusting your baby plants into a possibly harsh outdoor environment can be stressful. That, coupled with the shock of being transplanted, can result in stunted growth—or worse. The process of hardening aims to create a gradual transition for your plants from inside to outside. It prepares seedlings for fluctuation in temperatures and wind conditions. It's pretty simple, but requires some attention and patience. We need so much patience for gardening, don’t we?
Find a calm, shady spot outside for your seedlings and leave them there for just a few hours each day. Bring them back inside afterwards. Increase the time they spend outside each day until they can eventually spend the entire night outdoors. Keep them watered during the process. By the time transplanting time arrives, they should be fully acclimated to the outdoor environment.
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