A guide to compost mixes for houseplants

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

Can refer to either home-made garden compost or seed/potting compost: Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own.

. 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

A group of plants that have thick, fleshy leaves or stems that are able to store water. This allows them to survive in arid habitats with low moisture.

, 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

Plants have different growing requirements, some need high humidity

The concentration of water vapour in the atmosphere. This affects the rate at which plants transpire (lose water from their leaf pores), which in turn affects how much water their roots absorb from the soil. If air humidity is low, plants transpire quickly and become at risk of wilting if water is lost from the leaves faster than it can be absorbed by the roots. Different plants require different levels of humidity to thrive. Many tropical houseplants need high humidity and are best grown in a steamy bathroom, misted regularly or placed on a saucer of damp pebbles. In a greenhouse, humidity can be raised in hot weather by damping down (wetting) the floor or overhead misting.

, 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

Perlite is a naturally occurring volcanic rock which, when heated to high temperatures, expands to produce lightweight, micro-porous granules. It was traditionally added to compost to aid drainage and aeration, however, its manufacture, packaging and distribution has a big environmental impact, so locally-sourced grit or gravel is now preferred.

, bark

Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants.

, 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.