In the Garden With Susan: ‘Hardening’ off seedlings is the key to transplant success
Sat., March 18, 2023
Seed-starting is one of my favorite gardening activities. It’s a great skill to learn, you can grow varieties that are hard to find in garden centers and it’s quite economical.
We gardeners feel pretty proud of ourselves – and rightly so – when our seed-starting skills result in some nice-looking seedlings. Before we can transplant them out into the garden, there is one final step that is absolutely crucial to your plants’ success: the hardening-off process.
Whether you start your seedlings under grow lights or on a sunny windowsill, the fact is that neither of those sources of light are as intense as direct sunlight. If you transplant your seedlings directly into the garden rather than helping them slowly acclimate to the intensity of the sun, they will look like the plants in today’s photo.
Those crispy white patches on the leaves are a plant’s version of sunburn, which is also called leaf scorch. I don’t recall how I forgot to harden off those seedlings but they obviously weren’t ready to deal with bright sunshine.
If the sunburn is mild, the plants will recover after this setback. If it’s severe, the plants likely will die because this interferes with the leaves’ ability to conduct photosynthesis. That would be a waste of seeds as well as your time and effort, right?
The hardening-off process should be followed for about seven to 10 days, depending on how much time you have. Here’s how it works:
On the first day, move the seedlings outdoors for one hour in filtered sunlight, then bring them back inside. On the second day, increase the time to two hours.
On the third day, the plants get to stay outdoors for three hours but you can slowly start them edging into a bit more sunlight. Each subsequent day, add an hour to the seedlings’ outdoor excursion, and continue moving them into brighter light.
At the end of that time – provided the timing is right for transplanting them into the garden – the seedlings will be ready to go.
What if you work full time? That can make this process more challenging. Fortunately, there are some alternatives. I recommend starting the process on your days off, following the above routine. After that, choose a sheltered area that gets dappled sunlight, set them outside for the day and bring them back inside when you get home.
Be sure to keep an eye on the weather forecast and protect the seedlings with lightweight shade cloth (about 30% density) as you start moving them into a sunnier location. If you have some lightweight floating row cover on hand, that also will provide some protection. When the weather is overcast, you can leave the seedlings out for a longer period of time. Slowly but surely, the leaves will become a bit thicker and more able to handle full sun.
When you are calculating the date that you want to transplant the seedlings into the garden, you should factor in the time it takes for the hardening-off process.
For example, if you’ve started pole bean or pea seedlings indoors, you’ll want to plant them outside before they start getting tangled together while developing their vines. That can be a nightmare and risky to untangle since tender new vines can break easily. Because of this, I’d recommend beginning the hardening-off process as soon as the majority of those types of seedlings have germinated.
Susan Mulvihill is author of “The Vegetable Garden Problem Solver Handbook” and “The Vegetable Garden Pest Handbook.” She can be reached at [email protected]. Watch this week’s video at youtube.com/susansinthegarden.
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