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A guide to compost mixes for houseplants

If your indoor plants have struggled to thrive despite being in a good position and with the right watering and feeding regime, it might be time to look at the growing medium.

With the increasing variety of houseplants people grow, along with the move away from peat, a broader range of growing media is becoming available. Often when we think of indoor plant care, we think of access to light and how to water. However, there is also the question of not just where we grow them, but what we grow them in.

Indoor plant potting mixes

Make-your-own recipes are moving on from pre-made indoor plant mixes
Long gone are the days where houseplants had to make do with multi-purpose compost. For some time there has been more choice of houseplant potting mixes available to buy in bags at most garden centres, many specialised for plant types such as cacti, succulents, or orchids.

This is becoming a growing market, with houseplant companies supplying not only specialist mixes, but also the individual ingredients for gardeners to create their own bespoke growing medium for their plants.

Creating houseplant compost mixes

Glossary of terms

Growing medium: 
Essentially the material a plant is growing in, also known as the substrate. Houseplant growing medium could be the proprietary mix but it can also mean the individual ingredients such as coir, perlite, wormcasts etc that go into it.

Aerator: 
Some growing medium ingredients are described as aerators. This means they bring air into the substrate that plants are growing in. Roots need air – when the growing medium is too compacted and/or waterlogged, they cannot get air and they die. Aerators are made up of bulky and irregularly shaped materials (e.g. bark, pumice, clay pellets) that create air pockets and improve drainage.

Porosity: 
Refers to the spaces within the growing medium, which hold water and air. High porosity means more space for water and air, which can prevent waterlogging. Low porosity can lead to poor drainage and less oxygen available for roots.

CEC or Caton Exchange Capacity: 
Nutrients (cations) have a positive or negative charge – most are positive. Soils with a negative charge are able to hold onto these positive nutrients and make them available to the plant. A growing medium with a high CEC suggests higher fertility.

Hydroponics: 
There are different types of hydroponic systems, but essentially it is a way of growing without soil. Instead, nutrient-infused water feeds the plants. Semi-hydro systems use a soilless growing medium. Read more about hydroponics

Epiphytic:
Plants that grow on other plants for support rather than in soil or growing media, they get moisture and nutrients from the air and surrounding environment. This is not the same as parasitic plants, which take nutrients from their host plants. Orchids, ferns, Tillandsia and bromeliads are often epiphytic.

Read more about substrates

Plants have different growing requirements, some need high humidity, others need good drainage. Indoor growing means the plants are entirely dependent on the world that is created for them by the gardener, and this is where the market for bespoke growing medium comes in. By mixing different ingredients, gardeners can create the ideal cocktail for their houseplants to thrive. 

Key substrates

Soil Ninjas specialise in providing growing medium for indoor plants, and they take their soil bar to many RHS Shows. Customers can use their plant mix search to find the perfect medium for the plants they have at home, or they can learn about the properties of the ingredients used in the mixes to make their own recipes.

Clay pebbles

Mostly used in semi-hydroponics as a bulk ingredient. A great way of growing, especially for immediate root growth. It can be a bit tricky for beginners, especially when transporting from soil to semi-hydro, with a risk of root rot. Clay pebbles are also great aerators – you can add them to loosen the soil structure.
 

Vermiculite

Another aerator, but this one is highly water-retentive. It is a volcanic material that can absorb five to six times its weight in water, and slowly releases it back out to the soil as it dries. It’s great for plants like Alocasia or Calathea that don’t like drying out.

Coco coir

A by-product from the coconut industry, coir is ground up coconut husk that has been washed and milled. Coco will only absorb the amount of water that it can hold; the remaining water will drain through. It has an almost neutral pH and is often used as a substitute for peat.

Pumice

Pumice is volcanic rock which has expanded in high temperatures and then been ground down into small chunks. Its main function is as an aerator, and its porosity means it holds water for the plant. Good for thick-rooted plants and can be used as a propagation medium.

Worm castings

Worm casting is what worms excrete after eating organic matter. It is a natural fetiliser giving a nutritious boost to your plants. It is also good for improving growing media structure and enhancing water retention.
 

Zeolite

Zeolite is a made from a combination of metallic elements. It can be natural or man-made. Its porosity makes it good for both thirsty and dry plants as it releases water when required and makes over-watering less likely. It is also a good aerator, but is mainly prized for its high CEC.

Activated charcoal

Activated charcoal has long been used medically for its ability to absorb toxins. Similarly in soil it can filter toxins and harmful micro-organisms. Charcoal has a variable, but generally neutral pH and improves drainage.

 

Sphagnum moss

Sphagnum moss should only be obtained from sustainable sources. This highly water-retentive medium is especially good as a top dressing for plants that like humidity.

Bark

Commonly used as a substrate for orchids, bark is a great for the chunky roots of epiphytic plants. It’s also useful in mixes, where it adds air, improves drainage and lowers pH.

With an understanding of what each element provides, you can create a blend which meets each plants needs. For example, the Soil Ninjas’ mix for ferns contains coco coir, worm castings, horticultural sand, zeolite and activated charcoal. These create a mix that is:

  • High in nutrition
  • Well-drained
  • Moisture-retentive
  • A mix of larger and smaller air pockets for fine roots

Thicker roots require a chunky soil, so their Monstera & Philodendron Soil Mix contains coco coir, perlite, bark, coarse pumice, worm castings, zeolite and activated charcoal.

Armed with the knowledge of what each ingredient in the growing media provides to the plant, why not try out one of the prepared mixes widely available online and in garden centres – or even try creating your own bespoke compost.

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RHS Houseplants hub

RHS Houseplants hub

Guide to compost

Guide to compost

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