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Containers, soil and drainage for rooftop gardens


After you have you figured out where you wanted to put your containers and flower pots, it is finally time to get your hands dirty!


To be honest, unless the ground soil you want to use is polluted or suffers from other types of degradation, like compaction, gardening in containers can be harder than in the ground. In containers, plants roots suffer more easily from temperature change and drought. Also, if there is an insufficient drainage or an inadequate soil texture, their roots can rot. Therefore, giving extra thought and care to containers, soil and drainage is one of the key elements for a successful rooftop garden.


1. Choose your container


When it comes to containers, the most important element to consider is the size. Size will mostly depend on the plants you choose to have, so before planting anything, check the minimum size required for what you want to grow. To give you a few examples, the average depth required for lettuce is 15 cm, for tomatoes 45 cm and some small trees will prefer a 60 cm deep planter.


Please note that the information below only focuses on the depth of containers. However, width is an important element as well. Container width usually increases proportionally with depth, which suits a plant with a deeper root system. However, there are some planters where the width is more important than the depth, so several plants can grow together in them, this is also the case with raised beds.


So, should we go for smaller pots or large raised beds?

Beside having an impact on the overall aesthetic of your rooftop garden, using both actually has benefits.


Pro smaller pots arguments:

Smaller pots are convenient to move when there is a typhoon or when something needs to be fixed on the balcony or rooftop. It is also very useful if you want your plants to follow the sun path and stay in direct sunlight or get more shade during the summer season. As they can be moved easily, smaller pots are also recommended for beginner gardeners who are not 100% sure of their design. Indeed, if you set up a raised bed in a wrong spot and cannot move it anymore; well, you will regret it...


Pro raised beds arguments:


Plants most sensitive part are the roots, so the more stable the in soil temperature, the better it will be for the plants. As one of the main benefits of raised beds is to grow different kinds of plants together (with roots of different sizes), they are usually quite deep which means that the soil will remain cooler and moist for a longer period of time and often will need less watering. One more argument that I watched in permaculture video was that plants can connect with each other through their root systems, a common phenomenon occurring in nature which seems to work well for them, so why not trying to do the same in a container.


My rooftop containers: a mix of both movable pots and raised beds

I personally use both. I have two raised beds made of reused wooden boards that I had from previous art projects in which I first placed a waterproof plastic layer before setting up my drainage and soil. The other containers are way smaller except for one: a wooden Japanese bathtub that I found last year on the street (on the night of my birthday!). I only fill it up to half so it doesn't get too heavy.


When I am considering the size of my containers, I also try to see how I can reuse pots that I already have so I don't buy new ones. This is a personal challenge but the idea is to prevent the use of virgin materials for a more eco-friendly gardening. You might not start with the most attractive materials but you can make them pretty by customizing them or hiding them with the foliage of your beloved plants.

2. Drainage


If I had to give advice for drainage, it would be do not put saucers under your pots. If there is stagnant water in the saucers that means there is a lot of water remaining at the bottom of your containers and so a higher risk for plant root rot. Also, mosquitoes will lay eggs in the water. Instead, elevate your containers by placing them on shelves or bricks. If you still prefer to use a saucer, think about adding more pebbles or stones for drainage at the bottom of the containers so the soil doesn't remain soaked after watering.

Basic drainage consists of: - A container with holes at the bottom (unless you choose to use a wicking bed),

- Stones or clay pebbles to prevent the soil or the roots escaping through the holes at the bottom of the pot,

- A geo-textile to separate the soil and roots from the drainage material (pebbles or stones). You can use an old fabric from your home. Please note that if you use natural fabric, like cotton, it will break down faster, though.


I use this drainage system especially for smaller containers. As most of my pots don't have saucers, I try not to put a too thick a layer of pebbles so there is more room for the soil; hence, more space for the plants to grow roots. When the holes are too big or bigger than the pebbles themselves, I also usually add a piece of woven plastic bag between the holes of the container and the pebbles.

3. Soil


Here is one of my favorite parts of gardening. Basically, soil is the foundation of all forms of life in your garden. Some lucky gardeners already have a healthy loamy textured soil in their backyard where plants can grow easily if they get the right amount of sunlight. For many other gardeners, the situation is less idyllic. They might have to deal with a degraded soil. The most common degradations being compaction and pollution.


However for us, rooftop gardeners, we start with no soil at all, and although this might sound like a curse, I see an opportunity here to make my own soil! Of course, you can also buy soil at the flower market but guess which one is cheaper and more beneficial for your plants? The soil that you make.

Here is how to make it:


Lasagna gardening, a soil made of greens and browns

Lasagna gardening


Soil is made of different layers. The layer in which plant roots usually evolve is right under the surface and is called topsoil. It is composed of humus, minerals, roots and soil organisms. Humus refers to all the organic matter which is decomposed by soil organisms, or in other words, bug poo! It is THE BEST nutrients that your plants can ever wish for. Two other crucial elements present in the soil are water and air.


Lasagna gardening is a technique to create a beneficial environment for soil organisms to live in, with food, air and drink (meaning water). They will break down the material and through this process will make humus. The result will be a fluffy and very healthy dark soil in which plants thrive well.


List of different composting materials for lasagna gardening

To set up a lasagna gardening, add a

succession of layers of browns and greens. Browns are carbon-rich composting materials while Greens refers to nitrogen-rich composting material. Add the layers, bottom to top, from thicker to thinner but also according to how long each layer will take to break down. In the bottom, start with dry branches or old pieces of wood (old shelves, old crates for example). These are great life generators and I have noticed earthworms and fungi particularly love them.


The motto is again to use whatever you already have at home or materials that you can find near your place. For example, we all have in our home some cardboard or shredded paper (non glossy and with plant-based ink are preferable). They can be used as browns while kitchen waste like vegetable and fruit peel will do perfectly for greens.


Near my place, there is a park where every 3 months, they cut the grass which I collect whenever I have time. If you collect it straight after being cut, it will be rich in nitrogen (greens) and will need to be used quickly. If you collect it after a couple of sunny days, the grass will be dry and can be considered a carbon-rich material. I prefer it that way as I can collect more and store it all year long for whenever I need some (for mulching for instance).


I have to say there are always a few people who seem to be concerned about my mental health when they see me collecting dry grass in the middle of the loan in the park. If they are brave enough and come to ask me what I am doing, I am always happy to explain the purpose of my madness. The relief on their face when they understand it is for gardening is always a great source of fun for me.


On a daily basis, you can also talk to garden and park maintainers and ask them if they can give you a bunch of dead leaves. In Taipei, they are usually happy to give you straight away a huge bag but some also like to set up an appointment at their convenience so they can offer you a bag of dead leaves without other trash like packets of cigarettes or plastic.


I also like to use weeds like the mighty Bidens pilosa as a nitrogen-rich composting material. I carefully get rid off the seeds if there are any and trim the stems and leaves to add to a layer of greens. However, I don't uproot it as when it grows back it can be used as a regular mulching material while its flowers are great pollinator attractants.


Note: The common ratio between carbon and nitrogen for lasagna gardening is 2 to 1. However, in tropical and subtropical climates, it is advised to add more carbon (3 to 1).

Basically, the layer of browns will be 2 to 3 times thicker than the layer of greens and the thickness of each layer will vary according to the size of your container.


Also, you can add one layer of soil between the greens and browns. I usually add from lower quality soil in the bottom to richer soil and then to compost in the top layers. Finally, don't forget to water thoroughly each layer when you are setting up the lasagna gardening soil. It will help to start the decomposition process.


4. Mulch

Remember that word. Get it tattooed on your arm next to your beloved's name because mulch is essential for a living soil and living soil is...? That's right! The KEY ELEMENT for a healthy garden. Not to mention, it is also the best way to keep the soil moist for a longer period of time and prevent water impact; hence, soil compaction is one of the simplest ways to keep carbon in the soil.





A living soil is a soil in which plenty of little friends live and will break down decaying organic matter, feed your plants with their poo and protect them from diseases.

They represent the soil fauna: earthworms, potworms, springtails, isopods. soil mites and centipedes can be seen with the naked eye, while nematodes, fungi, and bacteria are too small and can only be observed through a microscope. They are your plants' best friends and are not a threat to humans. Instead of trying to get rid of them, welcome them with a cosy home, a nice roof and food. How? That is what mulch is for. Any carbon-rich composting material will work. It can be dead leaves like in a forest or dry cut grass. Again, a composting material that you can find locally is preferable. It can vary through the year (for example, dead leaves during cooler seasons, cut grass during summer). I like to add a little bit of nitrogen-rich material like a few banana peels or fresh cut leaves once in a while. When mulch has almost disappeared, it means your friends in the have eaten it all so add a new layer and ... that's it! Simple, isn't it? With all these techniques, you will have the most ideal environment for your plants. The next step is to choose which plants you want to grow.


Enjoy with music!




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