18 Jul 2022 11:41

The pests are coming!

Midsummer is here. What’s not to like? Summer holiday, delicious grilled food and the tomatoes are ripe in the greenhouse. Everything breathes peace and you may be enjoying your plants in the greenhouse with great satisfaction.

However, July also brings pests. We cannot avoid them, but this article will give you the tools to prevent and fight diseases and pests.



Why are there pests in the greenhouse?

Especially in July, the pests appear. What all greenhouse pests have in common is that they multiply very fast when it is hot. Some years are worse than others. This year I have observed that there are more aphids than usual. If you have a brand new greenhouse, that recently was put up, you will not see that many pests in the first year. But when you get to year two and three, that is when the problems start.  


The most commons pests in the greenhouse

  • Lice
  • Scale insects
  • Whiteflies
  • Thrips
  • Spider mites
  • Leaf miners

Where do pests come from?

  • Most pests that visit your greenhouse come from the surrounding nature in the area. Every time we air out in the greenhouse to keep a healthy climate, you also risk letting in pests.
  • Garden exchange can also lead to pests in your greenhouse. Exchanging plants can be fun, but always make sure it has no pests before you place it in your greenhouse.
  • The pests may also come from the plants that you buy from a plant nursery, a supermarket and so on.
  • If you pre-germinated in the windowsill indoor, you may also risk that your houseplants have transferred pests to your pre-germinated plants.
  • In the greenhouse, pests can survive the winter in the aluminium construction, in the soil etc. and thereby, return every year and harm your plants.


How do I find the pests?

Many of the pests in the greenhouse have sucking mouth parts. They make a hole in the plant cells of the leaves and suck out the content of the cell. All plants are more vulnerable on the underside of the leaves and where the new shoots are. Therefore, the pests will also appear on the underside of the leaves and on new shoots. When they suck out the plant cells, the leaves and the top of the plant start to curl. This is a clear sign of that the attack has begun.

Multiple pests such as aphids and scale insects excrete a sugary and sticky liquid which is called honeydew. It leaves a glistening layer on the leaves of the plant. The honeydew is a bigger problem than the pests itself. The honeydew is the reason why that the plant is often attacked by fungus diseases – especially sooty mould. Sooty mould appears on the leaves and looks like a brown or black layer of powder.

Cucumbers are often infested by spider mites, who are visible on the underside of the leaves. Here you will see something similar to a spiderweb.



Five important signs of pests

  • Look for pests on the underside of the leaves and on the shoots
  • Look for curled and deformed leaves and curled shoots
  • Look for glistening and sticky leaves. They may also have powdery-looking grey or black growth
  • Look for something looking like a spider’s web on cucumbers
  • Check ALL your plants for pests once a week

The best pest control tips

Water is your friend

What all pests have in common is that they hate cold water. Daily showering your plants and especially under the leaves are a very effective way to get rid of pests. A humid climate often makes the pests infested with fungus diseases and they will no longer thrive and eventually die.

Also clean your greenhouse a couple of times throughout the year. Preferably with only water and a brush.



Ventilate and enjoy the surrounding nature

You can easily ventilate the greenhouse all day and night even though you may also let in some pests. All animals have natural enemies – even the pests. By opening the windows, you also invite many useful animals inside the greenhouse. Animals who feed on pests.



How to control pests?

Almost all pests have natural enemies. Natural enemies are considered useful animals. You can even order useful animals online and get them send directly to your mailbox. It is important to move the animals to the greenhouse shortly after you have received them.

A lot of people have asked me whether they should be worried of the useful animals escaping the greenhouse. The answer is “No”. Useful animals will stay where the food is.

After moving the useful animals to the greenhouse, you should not expect that all pests will disappear completely. You will see that there will be a natural balance between pests and useful animals, who won’t harm you nor your plants.



The myth of marigolds

”I just need to plant marigolds next to my greenhouse plant then they won’t be infested by pests”

  • It is true that marigolds contain substances, that have a suppressing effect on nematodes in the soil. However, marigolds do not distinguish between the beneficial nematodes and the non-beneficial nematodes and will therefore also affect the beneficial ones. The suppressing effect will only be visible if the marigolds only are cultivated before the main crops are.
  • If marigolds are cultivated together with tomatoes or another crop, the nematodes will pull away from the marigolds and then attack the tomatoes or another plant. You do not want this to happen.
  • Some marigold variants affect aphids, but only if you use the whole plant and mixes it with water and sprays it on the aphids. So, a couple of marigolds in the greenhouse has no effect.
  • Marigolds attract slugs. If you already are struggling with slugs, you risk making it worse by planting marigolds.