What You Need to Know About When to Plant Seedlings

Have you looked out at your garden and thought how much bigger and better you could make it if you had more plants? Adding plants to your garden can become an expensive proposition, which is why many gardeners learn to start their own plants from seeds early on their gardening journey. The trick comes in learning when to plant seedlings so that they are given every opportunity to thrive in your garden.

Hardening-off seedlings

seed starting

After learning about starting plants from seeds indoors, the next step is learning when to plant them outdoors. Understanding how delicate your seedlings are at this stage will save you a lot of grief and keep you from wasting time and money. Before seedlings can be planted in your container gardens and raised beds, you need to make sure they are prepared for life outside.

Seedlings started indoors under lights and in greenhouses have been coddled and they aren’t ready to experience the strength of sun and the wind. You need to start toughening them up little-by-little while they are still indoors. Some gardeners do this by placing a fan near the seedlings and allowing it to gently blow air across the plants. What this does is strengthen the stems and leaves and mimics the natural conditions they will experience outdoors. You can simulate this by regularly brushing the palms of your hands across the top of your flat of seedlings.

Don’t let your seedlings burn

You know how you easily sunburn after a winter of being bundled-up when you spend time outdoors in the early spring? Yes, plants get sunburned too! If you grew your seedlings under lights, they will not be prepared for the intensity of the natural sunlight outdoors.

One way to protect your seedlings is to place them in an enclosed porch, sun room, or window that faces south. In the week or two leading up to planting season in your area, take your seedlings outside and allow them to get slowly acclimated to the sun. Start off by placing them somewhere they will get dabbled sun or partial shade. Set them outside for an hour the first day, then increase that amount by an hour every day for a week. After a week of regular sun, your seedlings should be ready for regular sun.

Check your last frost date

The common advice is to plant your seedlings outdoors after all dangers of frost have passed. Here in Chicago, cautious gardeners will wait until after Memorial Day before putting plants in the ground. When all dangers of frost have passed can vary depending on where you live. It is a good idea to consult your County Extension for the last frost date in your area, and then keep an eye on the weather forecast to make sure temperatures don’t dip too far.


Just because it may be sleeveless shirt weather doesn’t mean that you should plant. I prefer to err on the side of caution when planting out any plants I started from seeds. Air temperature means little. What you should concern yourself with is soil temperature. That’s right. The temperature of the soil will be dramatically cooler than the temperature of the air above.

Good temperatures for tomato planting

I wouldn’t think of putting tomatoes in the ground unless the soil temperature was higher than 60 degrees F. I’d go so far as to wait until 70 degrees F before planting any of my summer crops, like tomatoes and peppers. Plant your seedlings at any cooler temperature, and their roots can be shocked and you will end up with stunted plants that never fully recover from sitting in cold, wet soil.

If you want to be a soil geek, buy a soil thermometer and take a reading for three days straight. For seedlings, measure the temperature of the first 2-3 inches. Go 4-6 inches for transplants you buy from garden centers. If the average temperature is over 60 degrees F for those three days, it is safe to plant your seedlings.

Before you even think about planting seedlings outdoors in the garden, get them into shape by creating movement that will strengthen their stems and leaves. Place a gentle fan near them, or sit them near an open window with a breeze. If you don’t want to deal with the price of electricity, brush the tops of the seedlings with the palm of your hand every time you are near them. Get them acclimated to the light outdoors by giving them a little bit of time in the sun before placing them out. A few hours a day will strengthen the seedlings and allow them to be able to sit in full sun. It may be a good idea to drape some shade cloth over them for a few days even after you’ve planted them. It’s better to be safe than sorry when your seedlings that your nurtured indoors are at stake.

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