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Technieken (Voor-)zaaien

How do you keep your pre-sown plants healthy?

Pre-sown plants need 4 things: lots of sunlight, the right temperature, moisture, and enough nutrients - but not too much. How to care for your windowsill plants.
Voorgezaaide tomaten op de vensterbank
Pre-sown tomatoes enjoying the view
Now I use large MM air pots for tomatoes and summer vegetables. They grow just as well as in the former larger pots I used, but take up less space. In most of the photos, you can still see the old pots.

Windowsill pre-sowing

Summer vegetables need to be pre-sown indoors. Then they have to get strong before they can survive the outdoors. That takes time. For tomatoes, it's 7 weeks or so.

Other than time, they need 4 things to grow well indoors:
  • lots of light - sunlight direct overhead is best
  • the right temperature - around 15°C
  • enough nutrients - but not too much
  • consistent watering

How do you make sure they get all these things?

Why do you raise summer vegetables indoors?

For tomatoes, it takes about 4 months to grow from a seed to a plant with ripe fruit. They need warmth and sunlight during this time.

If you start sowing seeds outdoors when it's warm enough - around late May - it's too late. By September, days are shorter and it cools down quickly, so you won't harvest much.

But if you sow seeds indoors in late March or better, mid-April, you'll have a nice tomato plant by late May. Then you'll harvest lots of tomatoes from late July to mid-October.

Other summer veggies like cucumbers, zucchini, and pumpkins also need warmth to germinate, so it's better to do that indoors. Since they grow fast, sow them indoors in late April or early May and plant them outside in your container by the end of May.

This also prevents birds from stealing seeds and slugs from eating the delicate seedlings.

1. Seedlings want as much light as possible

Your plants need lots of sunlight. They even create their own nutrients from it. If they don't get enough light, they'll go looking for it. Then their stems grow long and get floppy.
Zaallingen van tomaten: te weinig licht en veel te dicht op elkaar
Tomato seedlings with too little sunlight and too close together
So choose the spot with the most possible light. Place your plants as close to the window you can. And give the pots a quarter turn regularly, so they get light from all sides.

This is how Laura does it:
afeltje verhuist van links naar rechts om in het licht te blijven staan
The table moves from left to right to stay in the sun
She puts them on a table that she can easily move. The table follows the sun during the day and the plants get as much light as possible. 

A south-facing window is perfect for sunlight. But beware: temperature is also important. It can be tricky to get the balance right.

2. Not too warm - around 15°C

In order to sprout, summer vegetables need a lot of warmth, around 20°C. But once the seeds become plants, that's too warm. Not because they can't handle it, but at lower temperatures, they grow slower and become stronger. And that's what we're going for.

So put your young pre-sown plants somewhere with direct sunlight, but not too warm. Experienced vegetable gardeners put their seedlings in a light, unheated bedroom or in a cold greenhouse after they've germinated.

Mochi puts them under a skylight in the spare room:
Zaailingen op de logeerkamer
Tomatoes in the guest room
Xanadu keeps seedlings in the stairwell under a skylight. When it gets too hot, they open the door to the roof terrace for a cross breeze:
Voorgezaaide planten onder een lichtkoepel
Tomatoes under the skylight
If you don't have an ideal spot, or if it's endlessly sunny, there are a couple options.

Take your plants out of the window during the hottest part of the day. Or put something in between like a sheet of white paper or some netting for example.

Tinus uses gauzy curtains:
Vitrage beschermt courgette tegen te felle zon
Gauze fabric protects the zucchini plant from full sun
For Silvia, it's very simple: she just places them in front of her sliding glass door.
Pre-sown plants in front of a sliding glass door

Do you have a greenhouse or unheated conservatory?

You're lucky 😄

Because starting from early May, you can move your plants there. Just make sure to regularly check if the potting mix is still moist, as it can dry out quickly in sunny weather.

If it's going to freeze at night, bring the plants inside in the evening, just to be safe. 🙂
Pre-sown summer vegetables in a a greenhouse

3. Enough nutrients, but not too much

When your seedlings are small they don't need a lot of nutrients. The MM-Mix is full of them, so your seedlings will grow too fast if you only use that. I use equal parts MM-Mix and vermiculite for just the right balance. 
Voorzaaien op een mengsel van half MM-mix en half vermiculiet
Combine the MM-Mix and MM-Vermiculite for pre-sowing
As the plants get bigger, they'll need more nutrients. At the end of April, transfer your tomato seedlings to a pot with just MM-Mix. 
A yellomato plant in a pot with just MM-Mix
Pumpkins, cucumbers, and zucchinis have larger seeds and should be pre-sown later: from late April to half May. You sow those seeds in the same half-vermiculite-half-MM-Mix mixture in a pot or MM-Airpot
Climbing zucchini seedling that's just come up
Since these plants are inside for a much shorter time, you don't have to move them into a larger pot.

4. Water regularly

Keep the mix moist, from the very beginning until the moment they can go outside. The bigger the plants, the more water they need.

If they are in an MM air pot, it's best to water them from below. So, in the saucer or tray that the pots are placed in.

Getting them used to the outdoors

If you've done all this - and everything went well - by the second half of May, you'll have beautiful plants ready for the outdoors. There's little chance of a hard frost after that. 

But before you can leave them outside day and night, you have to get them used to the fresh air first.
Afharden: laten wennen aan de buitenlucht
Hardening off: getting them used to the outside world
This is called hardening off: you put your plants outside a little longer every day. After about 5 days, you can leave them out all night. If the temperature will stay above 10°C for the next few days, you can transfer them to their permanent home in the garden box.

They need some time to get used to that too. But after a week, they'll start to grow again.

The rest is a piece of cake

Keeping the plants in good shape in the windowsill is the trickiest part of growing summer vegetables.

But once they're outside, nature takes over and you don't have to do too much. Just some extra attention once in a while. But more on that next time.

Happy pre-sowing!

PS: Tips van ervaren MM-ers


Hot tip. Start pre-sowing as late as possible, so you spend less time on these steps. Pre-sow tomatoes in the two weeks before April 15, and the other summer vegetables in early May. The Planty Gardening app is here to help 😉

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