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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 exa