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Technieken (Voor-)zaaien

Ice Saints and hardening off your seedlings

Hardening off, what the heck is that?

Yeah, it's a weird term. But that's what it's called when you gradually let your pre-sown plants get used to the outdoors. You do this before you move them into your garden box for good.

All seedlings that you pre-sow indoors need this special treatment. But it's especially true for summer vegetables.
Pre-sown plants and summer vegetables
Tomatoes, cucumbers, and zucchini absolutely cannot stand the cold. Just 1 night of frost and they're done for. 

It doesn't even need to freeze. If the temperature gets below 5°C, they can die. If it's below 10°C, they won't grow.

So: only move these plants outside when the weather gets warm enough.

What do Ice Saints have to do with it?

The Ice Saints are Catholic saints: Saint Pancras, Saint Servatius, and Saint Boniface. Their name days are May 12, 13, and 14.

Folklore tradition tells us that the very last frost of the spring should occur on these days.

There's little chance of frost after that, and temperatures rarely reach below zero.

Rare, but it still happens sometimes. So keep a close eye on the weather forecast mid-May.

Cold Sophie

De heilige Sophia van Rome - door Wil de Groot
Saint Sophia of Rome - by Wil de Groot
The Catholic Church is full of saints. May 15 is the name day of Saint Sophia of Rome. She died a horrible death in 305.  And she's the patron saint of late frost and thriving crops. As the saying goes:

"No summer before Boniface, no frost after Sophie."

But you know, that no frost part is never guaranteed. In 2020, it froze on the night of May 15-16. The ground temperature went down to 7°C in some parts of the Netherlands. Not common, but it happens. 

Hardening off seedlings

Plants you grew on the windowsill or in the greenhouse can't be transplanted into your garden box straight away.
Hardening off seedlings: getting used to being outside
You have to get them used to the outdoors: to the wind, the changing temperatures, and the direct sunlight. That's called hardening off.

Preparing your plants for the wind is equally important: plants need to be strong enough not to get blown over.

Think of your plants as bodybuilders. A bodybuilder can't lift 100 kg without training. If they try, they'll injure their muscles, back, and knees.

Step-by-step and little by little. That's how training works for people. Same goes for pre-sown plants.

How do you get your plants used to it?

Most plants are pretty tough. But if you put your pre-sown, frost-sensitive plants outside without getting them used to it first, they'll stop growing or even die.

So, on May 15 - or a few days later - put your plants outside for an hour. The next day, 2 hours. The next day, 3 hours. Keep going like that slowly: every day a little longer.
Vandaag mogen jullie een uurtje naar buiten
Today you guys get an hour outside

Hardening off with the Planty Gardening app

The app will help you through every step.

It waits until May 20, - just to be on the safe side - but it also depends on the weather. If it's been nice for a while and the weather forecast looks good, you can start earlier and harden off faster.

If it's still cold outside, wait a few more days until it warms up. 

The right spot

Choose a nice sheltered place outside to leave your plants while they harden off. A sunny spot, but not in the burning sun.

After about five days, most of your plants will be ready to stay outdoors.

As long as the temperature at night doesn't drop below 10 °C. If it does, then you should bring the plants inside in the evening. To a room that isn't heated.
Wit uitgelagen blad door te fel zonlicht

Transplanting to your garden box

At the end of May or beginning of June your plants will be completely accustomed to life outside. You can transplant them into your garden box: 
Voorgezaaide tomaat en klimcourgette in moestuinbak
Pre-sown tomatoes and zucchinis in the garden box
Enjoy pre-sowing - you'll be glad you did it 🙂
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