Making a Simple Indoor Plant Compost Mix

Why a Bag of Multipurpose Compost Isn't Quite Enough

Walk into any garden centre in Britain and you will find a decent peat-free multipurpose compost that will grow a houseplant perfectly well for a season. The trouble starts around month six. Multipurpose compost is blended for a wide range of jobs, and it holds water rather too enthusiastically for a plant sitting in a warm, still living room with no wind and no sun to dry it out. Roots that stay wet in a sealed pot soon sulk, yellow, and rot.

The fix is not to hunt down something exotic. It is to take that same bag of compost and open it up with a few cheap, long-lasting additions so air and water can move through it freely. Once you have the ingredients in the shed or under the sink, mixing your own takes five minutes and costs pence per pot.

The Four Ingredients and What Each One Does

Every good houseplant mix is a conversation between something that holds water and nutrients, and something that creates gaps. Here is the cast list.

• Multipurpose or John Innes compost — the base. It supplies moisture retention and the initial feed. A peat-free multipurpose is fine, as is John Innes No. 2 if you prefer a slightly heavier, soil-based mix for palms and mature specimens.
• Perlite — those white, volcanic-looking granules. They are sterile, weigh almost nothing, and create permanent air pockets while improving drainage. They do not break down, so they keep working for years.
• Bark — use fine-grade orchid or horticultural bark chips rather than decorative landscape bark. Bark opens up the structure, adds a little acidity, and mimics the leaf-litter layer that many aroids and ferns root into naturally.
• Horticultural grit — roughly 3–5mm, not builder's sand and not the sharp sand sold for paving. Grit adds weight, improves drainage, and stops a light mix floating out of the pot when you water.

If you keep only one of these on the shelf, make it perlite. If you keep two, add grit.

A Reliable Basic Recipe

Work in parts by volume, not weight — a mug, a yoghurt pot or an old measuring jug will do, as long as you use the same one throughout. The standard mix that suits the majority of houseplants is:

• 4 parts multipurpose compost
• 2 parts perlite
• 1 part fine bark
• 1 part horticultural grit

That gives you roughly 60 per cent moisture-holding material and 40 per cent drainage and air. As a rough guide, this fills a 13cm pot about twice over, so a single batch goes a fair way. Tip everything into a clean trug or a big bucket and turn it over with your hands or a trowel until the white flecks and bark chips are evenly spread and no clumps of neat compost remain.

One small refinement: dampen the mix very slightly — think wrung-out sponge, not wet — before potting. Bone-dry peat-free compost can be stubborn about re-wetting, and a faintly moist mix settles around roots much more kindly.

Tweaking the Blend for the Plant in Front of You

The recipe above is a dependable all-rounder, but a few plants deserve their own ratio.

• Succulents, cacti and jade plants: go to 2 parts compost, 2 parts perlite, 1 part grit. Water should be gone almost as soon as it lands.
• Ferns, calatheas and peace lilies: 5 parts compost, 1.5 parts perlite, 1.5 parts bark, half a part grit. These like steady moisture with air, not a desert.
• Monsteras, philodendrons and other climbers: keep the basic recipe but push the bark to 2 parts. Their roots enjoy the chunky, airy feel of a forest floor.
• Palms, rubber plants and mature shrubs: swap the multipurpose for John Innes No. 2 and use 4 parts of it with 2 parts grit and 1 part perlite. The extra weight holds taller plants steady.

For orchids in bark, african violets and carnivorous plants, buy a dedicated mix. They sit outside this system and will not thank you for experimenting.

Mixing, Storing and Potting On

Make up more than you need and store the surplus in a lidded bucket or a tied bin bag in a cool, dry corner of the shed or garage. It keeps for a year or more, and having it ready means you will actually repot when the moment arrives rather than putting it off until next spring.

When you do pot on, work in spring or early summer if you can, when the plant is actively growing. Choose a pot only 2–3cm wider than the old one — jumping up several sizes leaves a wide collar of wet compost with no roots in it, which is exactly the rot risk you built this mix to avoid. Settle the plant so the base of the stem sits at the same level it did before, firm the mix gently with your fingertips rather than pressing hard, and water once to settle everything in.

Then leave it alone. A free-draining mix dries faster than neat compost, so you may find you water slightly more often but with far less worry. That trade — a little more frequent watering in exchange for healthy, breathing roots — is the whole point of the exercise. Get the blend right once and most of your houseplants will quietly reward you for years.