Your neighbor does your shopping for you… and mine keeps pests away

Today we will learn what pests are and we will see that there are alternatives to the dangerous chemical products used in conventional agriculture.

One of these alternatives is co-planting; planting next to a crop that is attacked by a certain pest another crop that repels that same pest. In the co-plantation, the repulsive odor that certain crops present is used to repel pests. A well-known and striking case is that of the carrot and the leek (also garlic and onion); In this specific case, the benefit is mutual, since the carrot repels the leek moth and the leek repels the carrot fly.

In order to awaken the critical thinking of the students, necessary in a world with so much unverified information available, we will carry out a long-term experiment in our garden to verify the truth that sowing leeks and carrots close together is beneficial.


  • 6-12 years.


  • 3 sessions (in winter-spring).


  • Carrot seeds.
  • Leek seeds.
  • 15 sticks or branches of about 30 cm.
  • Indelible marker.
  • Wool or fine rope.
  • Recycled tetra brik.


  • Recognize pests as an agent that causes losses in crops.
  • Become familiar with a sustainable technique for pest control.
  • Learn to correctly sow small seeds.
  • Conduct a scientific field experiment.
  • Awaken the critical thinking of students.


This activity will help the students to experiment and verify, through a scientific experiment, data obtained from the Internet, thus helping to awaken their critical sense. In addition, we will see what pests are and we will know a simple way to prevent them from leaving us without a harvest.

Session I

To begin this activity, we will read the following text in class:

“What is a plague? A group of animals, usually what we call bugs, that feed on our crops, causing great losses in the harvest. Years ago, chemical products were used to control them, but today we know that these products also harm other living beings, including people, which is why we are stopping using them. So what can we do to prevent pests from eating our vegetables?

Well, there are many different ways to control pests without having to use dangerous products. Throughout this adventure we will get to know them (promote biodiversity, biological pests control, auxiliary plants, biopreparations, etc) , but today we will focus on a very simple one. It is about planting a crop that is attacked by a pest very close to another whose smell repels that pest.

In the case of carrots, there is a fly that can leave us without harvesting any of its rich roots. These flies hate the smell of leeks so much that they can’t even get near where leeks are growing. Thus, if we plant leeks near carrots, we will be able to harvest more healthy carrots (initial hypothesis).

To find out if it works in our garden (leeks and carrots are healthy and produce more when grow together), we will do an experiment that will tell us if from now on we should always sow the carrots next to the leeks or if it is not necessary”.

After reading, we will visit the garden with a notebook and pencil and we will carry out the planning of the experiment. To carry out this experiment, three rectangles will be needed in the growing areas (20 x 50 cm approx.) with a separation between them, if possible on different raised beds. We will delimit the three chosen areas with sticks in the corners and with wool or fine rope from stick to stick closing each rectangle. Within these delimited areas we will sow the seeds of carrots and leeks (for the latter we can also use small plants). In a rectangle we will sow only carrots (1), in another only leeks (2) and in another leeks together with carrots (3).

To end the session, each student must make a diagram of the garden and identify in it the different areas selected for our experiment.

Session II

The next day of class we will sow in our experimental areas. But before leaving the classroom we must make posters to identify the three testing areas. To do this, we will cut 3 rectangles (approx. 10 x 5 cm) from the tetra brik and we will make 2 holes the thickness of one of the sticks left over from the previous session in the middle area of each rectangle, one on top of the other, about 3 cm apart. On the posters we will write with an indelible marker CARROTS/LEEEKS/ CARROTS AND LEEKS as appropriate. We will have an ideal informative poster to identify the three test areas if we introduce the stick through the two holes.

With the posters made, some tools to loosen the soil from the garden and the two packets of seeds, we will move to the garden to start our experiment. First of all we must aerate and loosen the soil within the delimited rectangles. Then, after leveling the removed soil with the palm of the hand or with a board, we will make two very shallow parallel grooves (separated about 15 centimeters) inside each rectangle, in the direction of its longest length. We can make these grooves with a stick the thickness of a finger, or with the same finger, and they should not be deep, just half a fingernail deep. In each rectangle we will place its corresponding sign, sticking ⅓ of the stick into the ground.

In each groove, we will be placing the seeds two by two, separated by about 4-5 centimeters. Once all the seeds have been placed, we will cover them with a little garden soil without coarse materials. Finally, we will water with a watering can very carefully so as not to unearth the seeds.

In this way, our experiment will consist of an area to check if the leek repels the carrot fly, and two control areas to see what happens when these two crops grow separately.

To end the session we will check the expectations of each student for each area of experimentation. What do you expect to happen with the carrot seeds? And with the leek ones? Where do you think we will have problems with the carrot plague? Why?

Session III

This session will be held at the time the carrots and leeks are to be harvested. To do this, we must harvest the vegetables from our delimited areas and keep the crops separate. We will obtain a bunch of carrots (zone 1), a bunch of leeks (zone 2) and a bunch of carrots with leeks (zone 3). We will clean the soil of our roots very well and we will take them to class.

Once in class, the plants will be distributed, grouping as necessary so that all students have specimens to examine, but maintaining the separation of the three crops.

After explaining the symptoms of the carrot fly (black galleries that are easily seen on the root surface) and the leek moth (yellowish leaves with small perforations), each student should inspect their carrots/leeks for these symptoms. If there are any doubts, you can search for pictures of damage to carrots and leeks on the internet.

Each student should write down how many carrot/leek plants they have observed and how many of them had damage from pests (specifying if they had little or a lot of damage). The data of all will be unified in a table that will allow us to affirm if our starting hypothesis was true or not.

If we find damage, we can look for the cause by cutting the carrot with symptoms into slices. In the leek we should develop its leaves after cutting the first 2 or 3 centimeters from the base.

How do we interpret the results?

  • If there is no damage (or very little) in the carrots that have been sown together with the leeks and in those that have been sown alone there is damage, we can affirm that our initial hypothesis was true (the leek protects the carrot ).
  • If damage appears both in the roots of the carrots that have been sown together with the leeks and in those that have been sown alone, our initial hypothesis was wrong.
  • If we see root damage in leeks that have grown alone but not in those that have grown together with carrots, it is most likely because the carrot also repels the leek pest.
  • If no damage is observed on any plant, congratulations because these pests have not yet reached our garden!

Finally, we will be able to distribute the harvest, since we can take advantage of it if we remove the damaged pieces. All the plant material that the students do not take will be taken to the compost bin to recycle its nutrients.

— Dani Trigueros

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