Indoor plants grow in pots and containers and are almost entirely dependent on compost for their nutrition.
Even the best potting soil becomes depleted of nutrients as plants grow, so you need to re-introduce nutrients using compost so that the plant can continue to grow and protect itself against pests and diseases.
So, which compost works best for indoor plants? Unless it’s for a specific group of plants needing special conditions, peat-free multipurpose compost should be suitable for most indoor plants. However, it is not the only compost-containing product that is able to nourish plants, there are also others such as loam-based compost or potting mixes that should be able to achieve the same result.
To provide you with more insightful information, I’m going to breakdown each type of produce capable of nourishing your indoor plants so that you comprehend the differences between them and are able to make a more informed decision on which one is best.
What types of compost can you use on indoor plants?
After a year or two, plants may need repotting to maintain healthy growth, so it’s important to find a compost that will best benefit your plant.
According to the Royal Horticultural Society, there are three types of compost you can use for indoor plants: multipurpose compost, houseplant compost, and loam-based compost.
Peat-free multipurpose compost
Peat-free multipurpose compost (which should be available on Amazon) is the most suitable compost for indoor plants, but due to its high organic matter content, the compost sinks into the bottom of the pot as it breaks down, leading to fewer air pores around the roots.
This conditions roots and limits their growth, and also promotes the buildup of moisture, which can ultimately destroy them. Roots are like leaves and they also need oxygen, so you must create space for them to breathe.
The simplest way to go about it is by adding a drainage material like horticultural sand which can be easily found at an affordable price. Horticultural sand will improve water draining and also loosen compacted compost. For most houseplants, a 1:4 sand-to-compost ratio is sufficient, but for cacti or succulents, a 1:2 ratio is the most adequate.
If you can’t find sand or you think it’s too heavy to carry home, you can replace it with perlite (this one from Mother Earth is pretty good), a form of volcanic glass. Perlite is highly permeable but doesn’t hold on to water, making it ideal for balconies or patios, and you can apply the same 1:4 ratio.
Another probable option is vermiculite, which is also capable of increasing air porosity, but it holds on to water and releases it slowly back to the plant as necessary. This one can be applied on a 2:2 ratio, which means it can replace half of the compost. The final option you can consider is biochar, which can be used in the same quantities as sand or perlite, but it acts similarly to vermiculite, though it’s not as easy to find.
Loam-based compost is basically a combination of loam, sand or grit, and peat with increasing amounts of plant foods (or compost) added.
The term ‘loam’ refers to a soil containing the following three particles in equal amounts: sand, silt, and clay. Sand particles are the largest and tend to hold little water but provide good aeration. Clay particles are tiny in size but tend to get compacted and easily accumulate moisture. Silt particles are in-between sand and clay and help both particles mix more homogenously.
With loam-based compost, you don’t have to add drainage materials to improve aeration, but on the negative end, loam is an ingredient that is currently in short supply, and most will contain variable particles not consistent with what is commonly regarded as ‘good’ loam, which makes multi-purpose compost a more reliable option.
Potting mixes usually include peat moss, sphagnum moss, compost for moisture retention, vermiculite, or perlite for drainage. They’re formulas specifically made for plants in pots, which means they’re lightweight and blended to hold moisture and drain well.
You can find potting mixes that accommodate the needs of certain plants, for example, orchid mixes contain more bark for better aeration, while cacti and succulent mixes contain more sand for better drainage. Unlike pure compost, potting mixes usually contain small amounts of fertilizer that should disappear with two or three waterings, while others have additional fertilizer to provide plants with nutrients over time.
Potting mixes will typically contain wetting agents to reduce the surface tension of water so that the water can penetrate the mix and spread more evenly. This is meant to hold moisture near plant roots to reduce watering frequency, but there is also research suggesting this doesn’t ring true and it may actually condition plant roots.
Before you pot plants, you ought to moisten the potting mix with warm water. This ensures even moisture throughout the potting mix, which will prevent you from having extra work by having to moisten up the mixture inside the pot. You should look to replace the potting mix every 1-2 years, or at least consider replacing 50% of it with a fresh mix to prevent diseases or pests from taking over.
If you notice that the mix in your containers is suffering from a disease or insect problems, you should definitely swap it. A particularly popular mix you can get is the Miracle-Gro Indoor Potting Mix, but I still find that using peat-free multipurpose compost is better and less toxic to the environment. (Remember, potting mixes contain peat – which is emits carbon dioxide and methane when mined.)
Summary: The peat-free multi-purpose compost is ideal to keep your plants bursting with growth and vigor, but you will have to consider purchasing sand to increase its aeration. Loam-based compost is essentially compost with good aeration and drainage, but it contains peat, is short in supply, and it’s hard to find one that abides by good particle standards. Potting mixes are specifically made for plants in pots, but they tend to contain other agents that aren’t really natural, as well as peat whose mining releases greenhouse gases.
Which compost is best for indoor plants?
Peat-free multipurpose compost is optimal for indoor plants unless we’re talking about plants that require specific needs, like orchids. It’s not as complete as loam-based compost (since it lacks sand and clay), but you can easily add sand to the compost to give your roots a breather and also improve the compost’s drainage.
Unlike loam-based compost, peat-free multipurpose compost is widely available so you should be able to find it in gardening stores near you. Plus, multi-purpose compost is usually 100% natural and sustainable, something that can’t be said about potting mixes.
Even though potting mixes contain compost – they’re not pure compost and usually contain other additives like wetting agents and fertilizers, unless you opt for an organic alternative. Still, potting mixes are a pretty valid option to grow plants indoors.
Adding compost to indoor plants
The right amount of compost you should add to an indoor plant will depend on the size of the pot and plant. However, as we’ve mentioned earlier in this article, you will want to mix the compost and horticultural sand following a 1:4 ratio for most plants.
For small pots, you can opt for potting (or repotting) them outside on the pavement to avoid getting your carpet dirty. For bigger pots, gently scoop out the top couple of centimeters of compost and replace it. You may destroy a few roots, but the plant should quickly recover.
But before you go ahead place your plant inside a pot, make sure you have the following materials/tools readily available:
- A pot compatible with your plant’s size
- A fresh mixture of multi-purpose compost and sand (or a fresh potting mixture)
- Lava rocks (in case your pot doesn’t have a drainage hole)
If you have all the elements above, then it’s time to get hands-on. The video below will show you how to pot or repot indoor plants very easily:
If you’re not sure about whether or not you should be repotting your plants, look for the following signs for confirmation:
- Roots are pushing the plant out of the pot
- Roots are growing through the drainage hole located at the bottom of the planter
- The plant is growing slower than what’s intended
- The plant is top-heavy and falls over easily
- The plant dries out very quickly and needs frequent watering
- Noticeable salt and mineral build-up on the plant or pot
Some plants usually need to be repotted every 1-2 years, depending on how actively they’re growing, but you also have slow growers that won’t need repotting for longer than that and might only need a soil replenishment. Late winter or early spring are usually the best times to repot your indoor plants, but that may not be the case if they need it badly.
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