Charcoal is charred wood produced by heating logs of wood at high temperatures (over 400 degrees Celsius) without oxygen. It’s a process known as pyrolysis.
You can easily recognise charcoal as a rough, black, lightweight material that produces red hot embers when ignited. It is commonly used as fuel for cooking, grilling, and apparently for gardening.
But before we start using charcoal in the garden, let’s look at the types of charcoal there are, and see which one is most suitable for plants.
Types of charcoal
I know of four types of charcoal commonly used for different purposes.
1. Horticultural charcoal
Horticultural charcoal is charcoal in its purest form. It does not contain additives, so it is the best type of charcoal to use for your plants. Luckily, it is also cheap to produce charcoal in Nigeria where I live.
The only material used in producing horticultural charcoal is wood obtained from cutting down trees. It’s a rather rudimentary process of setting on fire a pile of logs in a clearing in the forest, covering it up with some soil to create an ‘oven’, and allowing it to slowly bake or roast without oxygen for about seven days.
After cooling down, the large logs are broken up into smaller pieces, and the charcoal is ready for use.
2. Activated charcoal
Activated charcoal is produced by burning wood at a much higher temperature (than regular charcoal) and processing it further to increase the surface area. A higher surface area means the activated charcoal is very porous. It is often used in medicine to adsorb unwanted materials from the body, and in industrial processes to filter water.
You can find activated charcoal in its powder form or as compressed tablets, or moulds. It is safe to use activated charcoal in the garden, although it’s not cheap.
3. Grill or BBQ charcoal
Grill charcoal is produced by heating materials other than hardwood, also without oxygen. Such materials could include coconut husks, sawdust, and bamboo. Other substances are added to the resulting coal dust before it is compressed into blocks and sold as charcoal briquettes. These additives are there to make the briquettes burn longer for the grill.
Although the materials used in producing charcoal briquettes are byproducts of other industries, thereby making it a more sustainable form of charcoal, it contains additives. Therefore, it is not advisable to use charcoal briquettes for your plants.
4. Biochar
Biochar is also a type of charred material used in the garden, and sometimes ‘charcoal’ is referred to as ‘biochar’. It differs from charcoal in a strict sense, though, because the materials used to produce biochar are byproducts from agriculture and food processing, rather than wood. Materials could also include animal waste and animal remains.
And because these materials vary somewhat in their physicochemical properties, biochar production follows a more sophisticated process of controlling the temperature, pressure, and heating rate to get the most desirable qualities at the end.
This means that there are different types of biochar depending on what material is heated, how long it is heated, at what temperature it is heated, and how it is processed after heating.
It is typically sold as small 1cm to 2cm fragments that can be easily added to and mixed with the soil. It is safe to use biochar in the garden, though not cheap.
What are the benefits of charcoal to plants?
To clarify, we’ll be speaking of our regular cheap charcoal (or horticultural charcoal) from this point on.
Using charcoal in the garden has many benefits for your plants, and here are some:
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Prevents waterlogging in pots
Adding a layer of charcoal chunks at the bottom of your pot before planting decreases the risk of waterlogging as it prevents soil from clogging up the drainage holes. Some people add stones or broken terracotta pieces to achieve this purpose. But charcoal has a lighter weight than stones and clay. That’s something to consider if you plan on moving the pot later.
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Absorbs and stores moisture
Charcoal is a highly porous material, which means it has many tiny pores filled with air. These pores enable it to absorb and store moisture which is released to the plant’s roots gradually. So, when you water your plant, the charcoal stores up some of the water, but without making the soil soggy.
Tip: To prevent mold from growing in cupboards and other places in your home, place some charcoal around to absorb moisture and keep the air dry.
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Adsorbs and releases nutrients
The surface of charcoal has the ability to attract and later release nutrients necessary for plant growth. These nutrients include calcium, potassium, magnesium, phosphorous, and nitrogen. And this is why some orchid growers use just charcoal to grow their orchids. Using charcoal works out cheaper for me than using imported pine bark (also sold as orchid bark).
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Aerates soil
Mixing small pieces of charcoal with garden soil increases the spaces between soil particles. This solves the problem of soil compactness. When soil is less compact, air (oxygen) flows freely to the roots of your plant. On the other hand, if the soil becomes too compact, water also runs off at the sides of the pot when you water your plants.
Charcoal works out cheaper for me than perlite or pumice which do the same thing for the soil. Read more about how soil composition affects your plants.
Other benefits of charcoal I’ve come across include preventing fungal growth and adsorbing toxins from the soil. But it may be helpful to note that these applications use activated charcoal which has a higher surface area than ordinary charcoal.
Where to get charcoal in Nigeria
Charcoal can be fairly easily found in local markets in Nigeria. It is a cheap source of fuel used by many households in rural communities for cooking. It is also used by street vendors to grill chicken, fish, yams, corn, plantains, and just about anything that can be grilled. So, it’s not difficult to find if you look.
It is often sold in opaque black bags, but the tell-tale sooty signs are ever-present near the vendor. I found that the bigger chunks of charcoal cost more than the smaller pieces. Luckily, it’s the smaller pieces you need in the garden. Small pieces of charcoal to fill a 4-litre bucket would cost about N100, while the bigger pieces would cost N200. Larger sacks of charcoal are also available if you ask.
Will charcoal kill plants?
Sometimes, people confuse charcoal with ash. Ash is the greyish dust obtained from burning wood or other plant material in the presence of oxygen. Ash is very alkaline with a ph ranging from 9 to 13.
Now, most of our ornamental plants grow best in neutral or slightly acidic soils, and luckily most of our garden soils are slightly acidic. Charcoal is also slightly acidic to slightly alkaline (ph of 6.48 to 8.35), so it suits our plants just fine.
Fun fact: Rainwater, which contains dissolved carbon dioxide, is acidic with a ph of 5 to 5.5, and plants love rainwater.
What plants grow well in charcoal?
I know of people who have used charcoal in their gardens for years, but I began experimenting with charcoal myself just over a year ago. The results have been amazing.
I have successfully grown anthuriums in pots of charcoal mixed with cocopeat or coconut husks and top-dressed with compost. I also mix charcoal with compost and soil to grow begonias, alocasias, philodendrons, monsteras, hoyas, and pothos varieties.
Dendrobium, cattleya, and phalaenopsis orchids grow well in just charcoal too. So if you would like to save money on imported orchid bark, you can switch to locally-made charcoal.
Finally…
There you have it. As you can see, I am a big fan of using charcoal in the garden for its many benefits to my plants, and also because it’s more affordable than perlite. So, if you didn’t already know that charcoal could be used in the garden, now you know.
Thanks for reading.
