How to grow plants in containers
You have eyed that barren piece of concrete that is your excuse of a courtyard for a few years now. But you have seen better – those idyllic pictures in lifestyle magazines of colourful flowers bursting from rustic clay pots of every shape and size. You were properly inspired by this vision but your first and last attempt to plant something is still sitting in the corner and it is not pretty. That yellow-brown skeleton surrounded by dry red cracked soil is not what you were after.
But let’s forget about that failure and hammer down the basics of container growing in this post.
Good drainage is non-negotiable
If you grow a plant in a container, the excess water must be able to drain away. If it cannot, your plant will die; it’s just a matter of time.
If a container doesn’t have drainage holes at the bottom, it was not made to have a living thing planted in it. The beautiful pots sometimes sold by garden centres are containers for indoor plants in pots that do have drainage holes. Take the container with the plant outside for watering, and return it to the decorative pot once it has drained properly.
By the same token, make sure you empty saucers after every watering. Standing water is a breeding ground for all kinds of diseases that might attack your plant roots. Also, if your container is standing on a soft surface like soil or leaf mould, use something to lift it off the ground slightly to ensure that the drainage holes are not blocked.
Don’t waste your time adding rough objects like broken clay pot pieces to the bottom of the container to keep drainage holes open. This is a very old idea that needs to be left in all those 1960s gardening books. It has been shown that bigger pieces at the bottom of a container actually inhibit drainage because the surface tension of water doesn’t allow it to easily move from small particles like your growing medium onto bigger particles. You are in fact just bringing the container bottom closer to the surface and reducing the potential root volume.
The right kind of growing medium will never block your drainage holes.
The right growing medium
Topsoil or normal soil that you can get by the spade load from your own garden or the construction site down the road does not belong in a container that contains a live plant. It works just fine for the plants in the garden, you may protest, but that is because different forces are at play in the soil. Water can move up and down and sideways in the soil of the earth in a tug-of-war between gravity and water surface tension.
A container is not the earth. The main difference is that – even with drainage holes – the bottom of the container creates a saturated zone. The finer your soil or medium particles, the higher up this saturated zone will extend into the container volume, resulting in stagnant water which creates the anaerobic conditions that almost always lead to root rot and plant death.
A medium that is designed for containerised growing has an air-filled porosity that is significantly higher than normal soil. This ensures good drainage and allows enough oxygen into the root zone of the plant. Ask your local nursery for the best potting soil mix that they have.
How to fill the container
Never ever press or force your newly acquired quality potting soil mix into the container. The more you compress it, the more you are squeezing out the air-filled porosity – and unwittingly returning it to the properties of normal garden soil.
All you need to do is to tap the container lightly on the ground two or three times to settle the growing medium.
When and how much to water
A plant that is wilting due to a lack of water looks exactly like a plant that is wilting from root rot caused by over-watering. Even very experienced growers struggle to tell the difference. In addition, a growing medium may appear bone dry on top but be completely water-logged and soggy two-thirds down.
Experience will eventually be the best teacher, but a nice shortcut is to get a feel for the weight of a container when it is completely dry and completely wet. Then you can simply pick up the pot and judge by its weight whether your plant needs watering.
One golden rule is to keep watering until you see water coming out through the drainage holes. Apart from assuring you that the holes are not blocked, this prevents the build-up of excess fertiliser salts.
Nutrition is important
Having implemented what you have read on this post, you are finally managing to keep your plants alive. The courtyard is coming along nicely but you notice that the plants just don’t seem as happy as their magazine role models. Chances are that yours are a lot hungrier than their glossy friends.
A good quality, well-draining potting soil is absolutely essential for growing healthy containerised plants, but the very same properties that make for a successful potting medium, also make it very nutrient poor. If you want your plants to look fabulous, you need to feed them. They are stuck with what you give them, and if you don’t give them a certain nutrient, they will simply not get it.
A good way to fertilise containerised plants is to apply a controlled release fertiliser, and to supplement weekly with a complete water-soluble fertiliser solution that also contains calcium.
When to transplant into a bigger container
A lot has been written and said about root-bound or pot-bound plants. Unless you are starting a plant in a container in order to transplant it into the garden, this is rarely a problem – and even then, we can’t really see what the fuss is about. Most plants will be happy in the same pot for many years.
If you want to keep a plant containerised but let it grow bigger, you can pot it up as soon as the roots have captured the growing medium – that is when the root mass comes out holding all the growing medium if you lift the plant out of the container.
Conclusion
Growing plants in containers are not quite like growing plants in soil in the garden. But once you’ve mastered the basics it can be even more rewarding, and provide surprising opportunities for green spaces that can be very creative.
Happy growing
Pierre Marais
Growtip Specialty Fertilisers