House plants are normally grown in a nutrient containing "growing media" or "growing medium" which can be compost or soil, although it is often a peat or peat-free mix. You can often use these products straight from the bag and get good results, so why write an extensive article about the topic? A lot of indoor gardeners like to have some control over the "mixes" that they use, especially because not all house plants like the same thing. Others take enjoyment from creating their own "blends" from scratch so want to learn about what they can use or you may just want to understand the difference between Perlite and Vermiculite. Either way your house plants will only be as healthy as their roots so it's important to understand and encourage good root health and this starts with understanding the materials that surround and support them.
What makes a good indoor potting mix?
Most house plants will often be quiet happy in several different growing mediums types, so there is usually not one magic type for every plant. As a general rule all house plants need a growing medium that is sufficiently "open" and "loose" enough to allow it's roots to grow through quite easily, but not so open that the plant is unable to remain anchored in the pot.
It needs to be open and loose enough that water can flow through the soil but not so open that water literally just runs out the bottom and is not absorbed by the growing medium as it passes through. It also can't be too "closed" otherwise the plant will become waterlogged which will cause root rot. If a growing medium can hold water and also allow air into it, then it can support microorganisms and also hold nutrients which are essential for plant health and growth. A good indoor potting mix will allow the above to happen.
What goes into a good potting mix?
Generally speaking, most people will only use 100% Peat, Peat-free or Home Made Compost for their house plants (the first three in the list below). Everything else featured on this page are "extras", and while not essential can really boost or improve the root health of your house plants.
The use of Peat has become increasingly controversial in many countries in recent years. It's a truly exceptional growing medium for almost all house plants, providing ideal moisture and nutrient retention and is quite slow to break down. It's cheap and readily available in the majority of supermarkets and garden center's, it's also the most likely soil mix you will find in the pots of your recently brought plants. Peat is an all rounder and generally excellent for all house plants. So what's the problem?
The issue is that Peat comes from naturally forming peatlands which are the most effective carbon sinks in the world. Harvesting these peatlands destroys the surrounding environments and allows more C02 to exist in the atmosphere and ultimately influences climate change. It takes too long to form to be classed as truly renewable, so basically it's not sustainable because realistically we can't replace what we use.
However, we have to consider that Peat is a fantastic growing medium and will aid our house plant growth, which in turn results in photosynthesis and creates a better natural cycle than those who burn peat as an energy source. It's obviously still not brilliant however, which is why our stand point is that it's a better idea to use Peat free, or at least peat reduced products where practical. The rest of this article features growing mediums which can be added to 100% peat products to "dilute" and make the peat go further. So rather than only being able to repot 10 house plants with one bag of peat, if you mix it with other effective ingredients you might be able to repot 20 house plants and still get equally good results.
How to use it
Usually Peat can be used right out of the bag with nothing extra needed. It often comes with a level of nutrients already added and can feed a slow growing house plant for several months up to a year (depending on how quickly it grows). If you've read the previous section you will know we will recommend thinking about adding other materials to make your peat purchase go further, however ultimately this isn't essential to get good results.
Peat-free potting composts will contain mixtures of organic materials such as coir, green compost, shredded bark and then mixed with inorganic materials such as sharp sand and rock wool. This mix of coarse and fine particles is needed in order to form a compost that can hold water and nutrients but also air, which is essential for root growth. Peat-free products are becoming increasing popular, however as it's quite a new market there is not one main competitor or perfect "product". This means some perform better than others and there are lots of different material that could be in your Peat-free bag. If you can, get access to what's inside the bag and have a feel as you are looking for something which should be quite fine and smell reasonably pleasant.
How to use it
If you've chosen an unknown Peat-free compost you may want to experiment a little to check its suitability. Some mixes can be very bulky and have large components which make them harder to work with and less helpful to house plants which are very small or have very small roots. All being well however, just like a 100% peat mix, you should be able to use this straight out of the bag.
Combine it with other growing mediums if desired otherwise you should be pretty much good to go as soon as you open the pack. You should not use Peat-free products with carnivorous house plants such as the Venus FlyTrap because they originate from bogs and do require Peat.
Home made compost is very easy to make, fantastic for the environment and often free (or low cost). You will need a composter to get you started and a space in your garden to place it. It also needs to be sitting on soil so worms and other beneficial organisms can actually gain access to the material inside. Then it's just a case of adding green and brown waste and turning the "ingredients" every so often. After several months your waste will have been transformed into a compost which is a nectar of nutrient-rich humus.
How to use it
Home made compost can often be used to substitute a bag of 100% peat, and can be used in the same way a peat-free mix you buy from a store (in fact sometimes compost is what you're actually buying when it's labeled "peat-free" anyway!). You need to be make sure with home made compost that it's completely decomposed and has not taken on too much of the nature of the surrounding soil. If your composter is situated on top of clay soil for example the resulting compost will have elements of clay, fantastic for outdoor plants but possibly too heavy for indoor ones.
A lot of people tell us they use "dirt" scooped up from the yard or garden for their indoor plants. In some cases this is wonderful stuff, but often when someone says "dirt" they are talking about the stuff that sits on the surface of their soil. Exposed and weathered to the elements this makes it actually very poor quality, lacking in nutrients and any real ability to hold water. True topsoil sits just below this surface level and can be 8inches / 20cm deep and in ideal situations it's the cream of the crop. It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. This stuff is the business.
How to use it
Topsoil whilst bursting with life, and nutrients is not suitable as a sole ingredient in your growing medium for indoor plants. Not only is it expensive, it's incredibly heavy when compared to peat or peat-free products which makes it very difficult for plants in containers as their roots struggle to grow through it. Topsoil is heavy because its components are very small and thus compact easily meaning there is no air for the roots to breathe, this also means it holds water well, but by itself, too well. So to utilise the fantastic elements of topsoil you should mix it with other materials to keep it "open". It's common to combine it with peat or peat-free products, the total quantity of topsoil in an indoor potting mix will usually be anywhere between 0% - 30%.
Well rotten horse or farmyard manure is a fantastic way to improve or add to a chosen growing medium. It holds water well and adds a large number of different nutrients. In most cases a pot containing only manure is too rich for almost all house plants, the only exceptions being very heavy feeders such as the Banana. So it's far better to mix it with other types.
No matter how much you use it's imperative you use well rotten manure - the fresh stuff is absolutely bursting with biological activity which is basically burning itself out. The heat and strong biological activity will damage or even destroy roots. Plus it stinks.
So how do you know if its "well rotted"? That's easy to answer, what you are looking for, or rather smelling for, is a light earthy and perhaps sweet scent. It shouldn't smell foul or feel like mush. Basically if you're not comfortable holding it in your bare hands - it's not ready.
How to use it
You should not use manure in an overly high quantity when it comes to house plants as it is often too rich and holds water too well, which in turn will encourage overwatering and rotting. A mix containing 40% at the absolute most, and even then only for gross feeders, such as The Swiss Cheese Plant. You can use a small amount of manure in the potting mix (10%) for most house plants excluding those which thrive in nutrient poor soil such as the Venus Fly Trap and most cacti.
Sphagnum is often called Peat Moss and although it is linked to dried "Peat" it does not have the associated controversy because it is fairly renewable and Sphagnum is often used in it's live form. It may be best for the novice indoor gardner to give it a miss as Sphagnum is alive and thus needs care itself in order to survive and continue supporting the primary house plant.
How to use it
Both living and dead cells of Sphagnum Moss can hold large quantities of water inside their cells up to 25 times as much water as their dry weight, this makes it a great soilless material for certain house plants. You can't "blend" it with peat or compost if you want to keep it alive, but you can sit it on the surface of the growing medium to create an attractive appearance and aid in water retention.
It's commonly used to grow house plants which have large thick roots that are happy to be exposed to light such as those of the Moth Orchid, although you may also find it supporting various succulents like the House Leek.
White Perlite stays white, no matter how long it stays in the muck it keeps that bright appearance which can add some decorative appeal to your soil mixes, but it also has a much more practical application. In its natural form it's quite dense almost like a tiny stone, however once heated it expands and although at first glance you may think it's heavy it's actually really lightweight. The closest way to describe it is to think Polystyrene / Styrofoam (which is actually utterly horrible, but that's another story), perlite, although a non renewable resource is still very much abundant and in terms of gardening only small quantities are used.
How to use it
Perlite is a light weight product that does retain some water and keeps the growing medium "open". In general you use only small amounts in a soil mix as too much will make it very free draining and nutrient poor. Naturally that type of effect has it's place, perhaps for young seedlings or house plant cuttings where too much moisture saturation would cause rotting. You can also use Vermiculite (see below) as a similar comparable product, although perlite as a general rule will hold less water.
Vermiculite is used in many different ways, from house insulation to fireproofing, and as you might expect by us writing about it, it can also be used by indoor gardeners. It's somewhat free draining, however Vermiculite also holds several times it's own weight in water which makes it ideal in potting mixes. Just like Perlite it is very light weight and reasonably cheap, although you should always use freshly brought, rather than trying to utilise the old stuff from your attic!
How to use it
Vermiculite is very like Perlite in that it will help to keep your growing mediums "open", it's more discrete than it's rival's bright white appearance, however it tends to hold more water because of its sponge like absorbent nature. If you have the choice, use Vermiculite over Perlite when you have house plants which need a lot of watering. You can use 100% Vermiculite (or perlite) for cuttings but it's normal to reduce this drastically when the plant is up and growing (0% - 20%) because it does not hold nutrients well and without these your plant will suffer.
Gravel and Grit come in many colours, shapes and sizes, although you need to make sure you are using horticultural gravel or grit. Anything else will not be designed for potting mixes and could be contaminated with chemicals or simply have pieces which are too large. Prices can vary greatly but typically you won't be using a great deal so a little will go a long way.
How to use it
Unlike Vermiculite and Perlite, Gravel and Grit will hold no water at all, although it will still keep your potting mix "open" and free draining. It can also add weight to containers for top heavy plants in order to stop them toppling over. Applying a layer to the surface will also aid in water retention and stop the rest of your growing medium from drying out as quickly.
A composition featuring a reasonable amount of grit is great for house plants which dislike over watering such as cacti, but for typical house plants it should only make up a small quantity of the actual mix, ideally less than 5%. If your primary purpose for using it is to add weight, consider adding a layer at the very bottom of the pot or at the soil surface rather than where the roots will be growing.
There are lots of other organic and inorganic materials that can be added to potting mixes to improve drainage or improve the consistency of the end result. If people continue to move away from peat based products, alternatives will likely increase over time. Sharp Sand and Coconut Coir are two somewhat common substances which are used currently, both may already be present in the Peat-free products sold in your local shops, but you can of course actively add these to your own mixes.
How to use it
Sharp Sand improves drainage as the tiny grains are not capable of holding water and they prevent the other material from clumping together and compacting. Like Topsoil however this is heavy stuff and is not a good material to have in a great quantity, even cacti would struggle to thrive in a high concentration of Sharp Sand.
Coconut Coir is a natural fibre extracted from the husk of coconut which has excellent water retention properties while also maintaining structure and allowing air to circulate. It comes in two forms, roughly shredded or fine. The shredded version is good for combining with Peat, and compost etc, where as the fine version can sometimes be used as a complete Peat-free alternative by itself. You can of course combine it with many other materials, including those discussed above.
Photo credit of the Coconut Coir brick to MatiasMiika