08 Apr 2022 09:19

Cultivate tomatoes in the greenhouse


Text and photo: Spirekassen

You may be thinking about the greenhouse these days. Spring is here and the greenhouse is ready to be used. At my house all the windowsills in the living room are full of tomato plants. We tend to get excited when it comes to pre-germinating tomatoes. We fall in love with one sort after another without considering how many plants that fit in the greenhouse.


The boring basic rule

Usually, one plant takes up 1 m2. So, if your greenhouse is 10 m2 you should not have more than 10 tomato plants. Using this method, you ensure that the greenhouse has a healthy climate.

But real life is not like that 😉 One of my good friends loves 6 m2 Juliana greenhouses. Every year she cultivates 20 tomato plants in this small greenhouse. Most people actually do not follow this sensible rule. I hope this blog post will inspire you to use these cultivation methods and show you how to maximise the use of the greenhouse.


How do you use your greenhouse? 

Is the greenhouse for relaxation or cultivation? It is important to know what the purpose with growing tomatoes is. Maybe it is to harvest as many tomatoes as possible, or maybe the purpose is to relax in the greenhouse during summer while also cultivating some vegetables? How you choose to use the greenhouse will affect the way you cultivate your tomatoes.



Cultivating in pots

When you cultivate tomatoes in pots, it is important to choose a pot that is big enough and has a diameter of minimum 35 cm and is 35-40 cm tall. The roots will need at least 20 L of potting soil. Avoid planting more than one plant in each pot. You can for example choose to cultivate the orange tomato Lycopersicon lycopersicum L. Remember to use potting soil mixed with clay, which will prevent it from drying out.



Cultivating tomatoes in pots is a great choice if you like the flexibility. You can move the furniture and the pots around while the tomatoes are still growing. During the warm months you can also move the pots, so they provide some shade for a table or a sitting area.



Tomatoes need a lot of water especially high summer. When cultivating in pots, you should irrigate 1-2 times a day when it is midsummer. If you do not have an irrigation system, you may want to consider getting one. Tomatoes thrive well with drip irrigation.


If you have a base in your greenhouse, you can cultivate in growbags placed on top of the base. Before you transplant anything in the growbags, leave the bags in the greenhouse a few days, so they have the right temperature. Loosen the soil first and then cut a long hole at the bottom for the water to drain. Next you cut the holes for the plants at the top. I recommend choosing growbags with 50 L soil, or alternatively, you can put two 40 L bags on top of each other. Make sure that the soil in the two bags is in contact. When the tomato season is over, you can recycle the old soil in the growbags, and use it for winter cultivation in the greenhouse. The old soil can also be used to improve the soil in the garden.



Using new growbags every year, will reduce the risk of getting diseases. These diseases can be anything from fungal diseases, viruses and bacteria. New and fresh soil will also have an airy texture, which old soil will not have. Furthermore, your tomato season will start with the right pH-value in the soil and be full of nutrients for your plants.



Cultivating directly in the growbag is not easy. When cultivating in pots, the soil will dry out fast. The same goes for the soil in grow bags. Keep an eye on the plants and the soil on days where it is extra hot and consider buying an irrigation system to make watering easier for you.

Plant beds

Plant beds are a perfect match for tomato plants. The beds contain a lot of nutrients that the plant needs. But having your plants in a plant bed is different from having them in a growbag or a pot. In a plant bed it is necessary to add more organic matter every year. This can for instance be compost. Expect the plant bed to sink approx. 5 cm every year due to the process of decomposition.



Plant beds will have a healthy microbial life when the top soil are not replaced too much. When it comes to plant beds, you do not have to irrigate a lot and this cultivation method will not stress the plants as much as with other methods. If you in the past have had problems with blossom rot, then it will be less likely to happen using plant beds. Blossom rot is black spots on the tomatoes. It happens when a cell wall collapses due to the plant not being able to properly absorb nutrients.

When it is not tomato season, the beds can be used for winter cultivation or for flower bulbs.



In case of diseases in the soil, all the soil must be changed. Make sure that the soil is airy. In general, plant beds makes the climate in the greenhouse more humid. Therefore, if you prefer to use your greenhouse as a conservatory, you may want to not have plant beds. With plants beds, there are no flexibility to move things around.


Capillary boxes

Capillary boxes are an easy way to cultivate tomatoes. You do not need to irrigate every day and you can go on vacation without worrying. Capillary boxes are an ideal choice for the greenhouse because the plants can access water, which is at the bottom of the box – this is called the capillary action. You can buy different types of capillary boxes, but in my opinion, they are all ugly 🙊 Luckily, you can get some nice-looking box plantersthat hides the capillary boxes. 

During summer you must pour water in the boxes once a week. But of course, it depends on the temperature and the plant growth. My advice is to not only use rain water since it has not enough nutrition for the tomato plants. Instead use tap water or mix rain water and tap water half-and-half.

It is not necessary to use liquid fertiliser the first 14 days.



Capillary boxes have many advantages. It is easier and you do not have to do that much every day. If you want more flexibility, you can also get boxes with wheels, making it easy to move things around throughout the season.



Sadly, tomato plants are likely to get blossom rot when cultivated in capillary boxes. The disease often appears on the bigger tomatoes. Blossom rot happens when the plant needs calcium, and some tomato sorts are more likely to be infected than others. The lack of calcium usually happens in capillary boxes when the water evaporates. Especially in the summer when it is hot, the concentration of fertiliser will be high, affecting the roots and the rest of the plant. High levels of ammonium, potassium and magnesium can prevent the plant from getting the calcium it needs.

Hanging tomato plants

If you want to maximise the use of your greenhouse, you can consider cultivating vertically by having hanging tomato plants. There are many sorts that can be cultivated in plant hangers or in pots on a shelf in the greenhouse. You will not need a lot of soil for these plants. 5-10 L will do. Usually, you will get a higher yield this way. Also, consider getting an irrigation system for the hanging plants to prevent the soil from drying out.



By cultivating hanging tomatoes, they do not take up a lot of space in the greenhouse. Furthermore, hanging plants creates a jungle atmosphere. And children, dogs and rabbits cannot access the tomatoes that easily – making it safer.



You will need to irrigate more often. Irrigate twice a day when it is hot outside. Again, I recommend getting hold of an irrigation system, so you can relax and not worry about whether the plants are getting enough water.

Remember to tie up your plants

Tomatoes are a popular plant to cultivate in the greenhouse, but it is important to tie up the plant before it breaks or collapses. Check out the different solutions here.

Om Spirekassen

Christine Wiemann is a greenhouse grower and an agricultural technician and owner of the seed company Spirekassen. Christine is an author of several books about lifestyle, garden life and plant cultivation. Today she writes blogs and shares her knowledge and passion for greenhouses. Christine is a greenhouse expert and an ambassador for Juliana Drivhuse.

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