23 Mar 2023 09:14

Make your own self-watering plant box for your greenhouse

Get a good start this spring, by building your own self-watering boxes, also called capillary boxes. You will then be ready when it's time to sow or plant your plants in your very own self-watering system. It might be a good solution, if you have flooring in your greenhouse, so you won’t be able to plant directly in the ground, or if you just wish to water a little more.  

In this article, your find our guide on how to make your very own self-watering plant box out of recycled materials. TagTomat started with five of these plant boxes on the roof of a bin storage, as shown above. That was the beginning of TagTomat. Mads, the founder of TagTomat, learned to make plant boxes from his parents, and Mads' dad learned it from his dad.  


Guide: Make your own self-watering plant box 

The plant box consists of a mortar tub and a self-watering system made of recycled materials, which instead of becoming waste, can now be used to create green cultivation experiences inside or outside the greenhouse. Although you can easily buy self-watering capillary boxes made of polystyrene at most hardware stores, it is both fun to build your own, and your homemade plant boxes last longer and can easily be moved around in the greenhouse.


This is what you need 
  • A plastic tub or a tub of your choice, we use 14-gallon mortar tubs. 
  • A cut-up fibre cloth, to put soil on.  
  • Two  bottles.  
  • A large polystyrene box or 1 ½ of the smaller ones.  
  • A knife.  
  • A drill and a screwdriver to create a slotted pipe, alternatively cut it with a knife.  
  • Plants and/or seeds.  
  • A 40-50L bag with soil or correspondingly plant soil filled with nutrients.

You may be able to find a polystyrene box in your local supermarket and simply recycle a couple of 1½ bottles you have lying around at home. You might even have a mortar tub or other container left, you are no longer using, that you can revert into a plant box.  

Note that the principle of this guide can also be transferred to other containers if you do not feel comfortable using plastic tubs which are made from recycled plastic. Recycled plastic is not necessarily approved for food and may contain PFAS.

This is how to 
  1. Place the polystyrene box in the mortar tub, and cut a hole in the two bottles, and adjust the height.  
  2. Cut three A-shaped slits in the bottom of each bottle, fold them towards the middle, and fill the bottles with soil.  
  3. At the upper edge of the polystyrene boxes, cut or drill a couple of holes in the box for a slotted pipe.  
  4. Place a fibre cloth on the polystyrene, and cut two holes, to match the placement of the bottles, and fill in the soil. Alternatively, place your bag of soil directly on top of the polystyrene and cut holes in the bottom of the bag, above the placement of the bottles, and on the top, where your plants are placed.  
  5. Dampen the soil with some water (about 40-70 ounces), put in your plants or sow your seeds in the soil. You don’t need to put your plants directly on top of the bottles. Fill up one of the corners with water, by pulling the fibre cloth a side.  

Watch this video to see, step by step, how to build your own capillary box. 

Note that the video is in Danish, but it is possible to choose English subtitles in the settings. (1) Open the video using the link above. (2) Click settings. (3) Click on 'Subtitles/CC' and 'Auto-translate' to choose your preferred language.

We grow a lot of tomatoes in the mortar tubs, on top of the bin storage. But you can grow everything from tomatoes, cucumbers, beans, lettuce, spinach, chillies, herbs, flowers and much more.  

The plant boxes can contain 6.5-gallons of water, so it is only necessary to fill them with water, once a week, during the summer months. You water the box in one of the corners, by drawing the fibre cloth to the side. This ensures the soil not collapsing because of watering from above. However, in the first week or two, you should water the box from above, until the roots get a hold.  

If you think the black mortar tubs are ugly, make a plant box cover. For example, out of wood or old bricks.  

Remember to add fertilizer during the season, to get beautiful and healthy greenhouse plants.  


The best green wishes,

Team TagTomat  


Om TagTomat

Behind the Danish company TagTomat (in English ‘Roof Tomato’) is a skilled team whose passion is to create green communities and inspire to green do-it-yourself and do-it-together projects. Today TagTomat sells organic flower seeds and vegetable seeds, which are packaged with our seed packaging machines from 1895. It all began in 2011 in the heart of the neighbourhood Nørrebro in Copenhagen with just five self-watering plant boxes on a wheelie bin storage. You can read more about TagTomat at our website TagTomat.

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