Self-watering flower box DIY & volume calculator

Create a self-watering flower box - with calculator to calculate volume (soil etc.)

How to plant in a self-watering flower box? How to make a self-watering flower box? Calculate volume and feel free to follow our step-by-step instructions below as these questions are not completely obvious. You have the option of buying an inner pot from us in stainless steel with a self-watering part or making your own self-watering pot. Plants are expensive and therefore it may be wise to think about how best to fill your flower box in order to create the best possible conditions for your plants when you plant. Water supply and insulation then become two key components together with good soil.


Below is both a guide and shopping list to help when you plant. You will also find a calculator so you can easily calculate volume (how many bags of soil and leca you need).

1. Insulation of the planter (exclude this step if you use the planter indoors)

To even out the temperature in the pot both in summer and winter, it may be wise to either use bubble wrap or insulation board. There is a blue version that is normally used for house foundations as it does not absorb water. It is the best option but the white version is also good. The white is normally cheaper than the blue.


For our planters, we recommend that you cover the entire interior with 3-5 cm insulation board before you start planting. However, make sure to make a hole in the bottom so that excess rainwater can drain out via the drainage holes in the planter.

Trendsetter planteringskärl i Corten från Cortenfabriken. Trendsetter är vår Cortenkruka (en rostig kruka) / balkonglåda / planteringskärl / blomlåda som andas design & kvalitet. Självbevattning i rostfritt & dekorram i Corten som tillval. Om Cortenstål (COR-TEN), tips gällande materialtjocklek, hur man påskyndar rostprocess, husmorstips för rostfläckar & ledbelysning av Corten.

How to plant in a self-watering flower box? Decorative frame protects and hides insulation

Cut your boards for your self-watering flower box

Below you will find how many insulation boards in the standard size 120 cm x 60 cm are needed to cover our respective planters. You buy these in regular hardware stores. You can cut them as below to minimize spillage. We have divided the plates so that you can thread in each part. Whole plates do not fit as the upper edge of the planter is bent. Below we have counted on 3 cm thick plates in our calculation. The easiest way is to use an ordinary handsaw to cut to the right size.

Size chart for insulation for your self-watering planters

Start by covering the bottom with insulation boards and making sure to create at least one hole for water drainage. Then you assemble the long sides and finally the short sides.

2. How to make a self-watering flower box?

If you want to create a water reservoir for hot summers (self-watering system), it is now wise to place a plastic box that covers almost the entire bottom. Keep in mind that it can be good if one of the drainage holes is not completely covered. This is so that excess rainwater can drain out.

The plastic tub should be at least 10 cm high and thus need not cover the entire interior of the planter or its bottom. Place the tub as close to one short side as possible. Think about where you want the water pipe to come up so you can fill the water reservoir once all the soil is in place. Hiding the pipe in one corner behind plants can be an idea so it won’t be seen. If you choose a box that is higher than 10 cm, you should make sure that water can drain by making a hole between 5-10 cm up in the pot. Otherwise, you risk the rainwater forming a pool and that the plants are dying.

If you don’t have a self-watering part, an old housewife’s trick is to put coffee filters over the holes to preserve some moisture in the pot for the plants.

Trendsetter Hur plantera i självvattnande blomlåda Vad behövs för att bygga egen, fylla den och hur gör man Här finns både instruktioner & inköpslista. Trendsetter planter


3. Fill 10% of the pot with leca

Then pour in approx. 10% of the planters’s volume with lightweight clinker before planting. These are usually cheaper than the small bags of lightweight expanded clay aggregate (LECA) you find with toys in various stores and work just as well. You calculate how much is needed for your particular pot by calculating the volume in the calculator above. Everything to answer up to how to plant in a self-watering flower box?


4. How to make a self-watering flower box? Install water pipes!

Push down a tube between 30 to 40 mm in diameter that runs from the bottom of the pot up to the top edge of the pot to allow you to easily fill water reservoirs. You can either take a plastic pipe or a metal one. Be sure to cut the lower part diagonally so that the water flows easily into the water reservoir. Here you will find, among other things, transparent plastic pipes. It is fine to use others with such as drain pipes for your self-watering system.


5. Push the cotton twine down to direct water to your newly planted plants

How to plant in a self-watering flower box? Push down the cotton twine at the bottom of the water reservoir in order to later lead water from the water reservoir to about 5 cm below the ground surface. A good size is around 6 mm and these are pushed down at about 20 centimeter intervals.


6. How to plant in a self-watering flower box? Use anti grass cloth to separate soil from water reservoir

Now is the time to cover the leca balls with anti grass cloth before you plant so that the soil does not later flow into the leca balls and clog the water reservoir. Make sure that there are small holes to thread the strings through which are to be led up to about 5 cm below the soil surface.

7. Fill with the right amount of soil for your self-watering flower box

How to plant in a self-watering flower box? Regarding soil, we recommend you google to find high quality soil. You calculate how much is needed for your particular pot by calculating the volume in the calculator above.


Shopping list to fill our planters from the Trendsetter series

Below is a compiled list for you if you do not want to calculate volume above. The links show suggestions of where to buy these accessories and how much to answer the question – how to plant in self-watering flower box? The headings below contain links that take you to suggested places of purchase.

Size of planter (cm)
Number of insulation boards
120 x 60 cm
Size of water reservoir
(LxWxH cm)
70x26x29 162x19x7 
62x62x58 357x57x10 
Size of planter (cm)
Liter of leca Height pipe cm
70x26x29 329
62x62x58 1758
Size of planter (cm)
Meters of 6 mm cotton twine40-liter bags of soil
70x26x29 0.871
62x62x58 2.94

* In addition to the above, anti grass cloth corresponding to the pot’s bottom size (LxW) is also needed.