What you’ll need

  • A 1.5-liter plastic bottle
  • A sharp knife
  • A cutting board
  • Two small plates
  • Cotton wool pads, loose cotton wool or several sheets of household paper
  • Cress seeds
  • Water

How to do it

1. Ask an adult to cut off the top of the plastic bottle with a knife.

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Well spotted!

The cress seeds under the plastic bottle germinate faster. They also grow into plants more quickly. The cress plants under the plastic bottle can be picked after approximately six days. The cress plants that did not grow under the plastic bottle need another one or two days before they can be harvested.

What's the secret? 

The bottle acts as a miniature greenhouse for the germinating plants, providing protection and better growing conditions. Two things are really important to bear in mind:

  1. The bottle retains moisture for the plants. It protects the seeds from drafts and stops the water from evaporating as quickly. If you put your greenhouse bottle outside on a cool, sunny day, you’ll notice condensation forming inside the bottle. This happens because the heat of the sun causes the water to evaporate and then gather on the inside of the plastic, which is cooler. This leads to the formation of droplets that fall onto the plant like rain.
  2. It is warmer inside the bottle than outside. This is also something you can observe outside on a sunny day: When the sun's rays penetrate the bottle greenhouse, they heat the air and soil below. This heat remains trapped inside the plastic bottle, while the space outside the bottle is kept cool by the constant air circulation. This means it cannot heat up as quickly.

Inside the bottle greenhouse, these effects can be difficult to spot – and even harder to measure. However, it is effective enough for the seeds under the bottle to germinate more quickly.

Tip: In spring you can move the seeds into your garden under a bottle greenhouse!