A question often asked is whether houseplants need a special compost. Take a look around the indoor plants area of a garden centre and you will often find bags of compost specially formulated for houseplants, or even particular types of houseplants. Are these really useful, or the result of clever marketing?

What is compost?

Compost is one of those words that has multiple, but related, meanings - all of which relate to decayed organic material used as, or added to, soils to improve their structure and fertility.

In this article, we are going to discuss growing media, or planting mixes that are commonly referred to as potting compost. We won’t be discussing how to produce or make garden compost, or the management of compost heaps.

houseplant compost bag

Properties of a good houseplant compost

Compost mixtures come in a wide variety of types. They are made from different components and they are formulated for specific purposes. Seedlings, for example, are often started in very fine compost that encourages tiny roots to grow and which will provide good levels of moisture. General purpose compost mixes are often coarser in texture and can be for outdoor potted plants as well as to improve the structure of garden soil, and some compost mixtures have different chemical properties that might make them suitable for lime-hating plants or to provide nutrients for specific types of plant.

For houseplants, the most important characteristics are its physical attributes, which effectively means its texture.

We have discussed in previous posts about the importance of root health for the survival of houseplants. Without healthy roots, most indoor plants are destined to an early death.

Healthy roots need four things: anchorage, oxygen, water and a supply of nutrients.

Anchorage

Plant roots have a very important role in holding a plant in place. They either grow through a substrate, such as soil, to provide grip and mass to stop the plant being moved or, in the case of epiphytes and lithophytes, they grow around their support structure (other plants, or rocks, respectively) and grip on tightly. Some houseplants, notably ferns, do not have true roots, but they do have structures that help anchor the plants in place.

For good anchorage, a substrate needs to be quite dense, to provide the mass needed to weigh the plant down, but also be sufficiently open-textured to allows the roots to grow without obstruction

Air

Plant roots are very active parts of a plant. They grow and they need energy to carry out some very complex processes, such as the extraction of water and nutrients. As such, they need oxygen to fuel these physiological activities, and that means the growing medium needs to have plenty of air-filled spaces, even when it is watered to its full capacity.

Water

Water is also critically important. A good compost needs to be able to hold moisture, which can be extracted by the plant over time, but not be so saturated that the water displaces all the air. Water inside a plant is maintained under high pressure to provide structural rigidity (especially for those plants without any woody tissue) and to transport nutrients around the plant to its various tissues and organs. Nutrients extracted from the soil (whether natural, or in the form of fertilizers) are soluble in water, so are only available to the plant when there is water present.

This photograph shows a comparison between two different types of compost after they had both been saturated with water and allowed to drain.

example of two types of houseplant soil

This is an easy test to perform at home.

  1. Take a plastic flower pot, at least 15cm in diameter, and fill with compost. Lightly firm it as if it had a plant in it.
  2. Fill a bucket of water to the depth of the pot and put the pot in. You will need to weigh down the pot to stop it floating.
  3. Leave for about thirty minutes and then remove. Allow the water to drain from the pot for about 10 minutes.
  4. Turn out the soil onto a plate, as if making a sandcastle.
  5. A good houseplant compost will look more like the sample on the right. The one on the left is completely saturated and has collapsed, whereas the sample on the right has retained its structure as well as being able to hold onto water.

Nutrient availability

In natural soils, especially those with a high clay content or the presence of other minerals, plant nutrients can be chemically very tightly bound to the minerals, which makes them less available to the roots of plants. The acidity (pH) of the soil can also affect the availability of those nutrients.

Formulated compost mixtures are made in such a way as to ensure the pH is balanced and that nutrients are readily available. This is why so many compost mixtures are made from inert materials, such as peat and coir.

Durability

This very much relates to structure and texture. Compost mixtures that are prone to saturation will compact, shrink and are more likely to decompose, so that will mean having to top-up planters more frequently and even re-potting more often than would otherwise be necessary.

Different types of compost

Peat-based substrates

Peat is a very controversial subject. Its use in horticulture, especially in the Northern Hemisphere, is being questioned due to significant concerns about the environmental impact of its extraction and use. There are plans to eliminate its use in horticulture altogether and make more use of alternatives, which I will explain later.

Peat has been used in horticulture for generations, but it became very popular during the second half of the 20th century, when lightweight plastic pots were introduced to the nursery trade and the mass production of plants began in earnest. The main advantages of using peat stem from its physical and chemical properties:

  • Its quality is consistent. Peat from the same source and supplier will be the same year in, year out.
  • It has no natural nutritional value, so fertilizer and other plant nutrients can be added in precisely the right amounts.It has an open structure that encourages good root development.
  • It is lightweight, so it is easy to handle and move.
  • Its pH is close to that required by most species, so only slight adjustments need to be made.
  • It is naturally sterile, so there is little risk of pests or diseases.
  • It is widely available and cheap, at least in the Northern hemisphere.

With all these advantages, you can see why peat is such a popular material, and why finding good alternatives is proving tricky - especially for indoor plants.

Much of the peat used in Europe, the UK in particular, is taken from areas that support a unique range of plants and animals. These habitats are under threat from peat extraction and many have now been protected by law.

Extraction of peat from other sources, such as from around the Baltic, has less of an impact on fragile ecosystems, but does result in the release of large amounts of previously stored carbon dioxide into the atmosphere.

Carbon dioxide emissions due to extraction are supplemented by CO2 released into the air as peat naturally decomposes as it dries out and is exposed to oxygen. You might notice that over time, peat-based potting compost shrinks in volume. This is partly due to it compacting and settling under its own weight, and the weight of water, but it is also significantly due to natural oxidation - the peat decomposes and a product of that decomposition is CO2, adding to global heating and affecting, albeit in a relatively small way, climate change.

Coir

Coir is a fantastic material to use as a compost base. It shares many of the properties of peat, in that it is inert, lightweight and has ideal air and water capacities and does not compact easily. It also seems to encourage really excellent root growth and it is less attractive to soil-dwelling pests such as fungus gnats (sciarid flies). It also does not decompose at anything like the same rate as peat, so needs much less topping up.

Coir has other benefits as well. It is a waste product from the coconut harvest, so is completely natural and renewable. It is also the base material in the foli8 coir pot that our plants are supplied in.

Composted green waste

A lot of new peat-free and peat-reduced composts are made with composted green waste that has been sterilized, graded and standardized to ensure consistent quality. Some of these materials are really very good, but they are almost all formulated for garden use rather than houseplants. In my experience, these substrates can be a little too prone to saturation and also seem to be more attractive to fungus gnats.

However, this category of compost mixtures is developing very fast and it is likely that there will be mixtures that are more appropriate for houseplants on the market soon.

Loam-based substrates

Some compost mixtures have a proportion of loam (essentially a mixture of carefully selected natural soils) in them. These add some density to the mixture and are especially useful for really large plants where the extra heft of the mixture can provide better stability for the plant. If you have seen large indoor trees in places like shopping centres or office atriums, they are usually planted in a loam-based substrate.

However, for houseplants, such a mixture is unnecessary and, as they contain mineral soils, managing fertilizers is more difficult.

Hydroponics, vulcaponics and other materials

Hydroponics and hydroculture are ways of growing plants in nutrient-enriched water, with the plant supported by an inert material such as expanded clay granules or rockwool. This system has been used in commercial horticulture for decades and is used extensively for the production of salad crops.

Hydroculture is also used by many commercial interior landscapers and many green wall systems are also hydroponic. There are many advantages to hydroculture, the main ones being that watering is very easy and fertilizers are added precisely.

It may seem odd to grow plants in water (especially once you have read earlier about the perils of over-wet composts), but many plants (including cacti and succulents) can be grown in such a way.

The reason is that the plant roots can be persuaded to adapt over time to extract dissolved oxygen from the water.

This picture shows the root of a ZZ plant (Zamioculcas zamiifolia) grown under hydroculture conditions.

zz plant roots with root hairs

 

What you can see is very dense growth of root hairs that act almost like gills to extract as much dissolved oxygen as possible.

A more recent innovation is the practice of vulcaponics. This is when a soil-grown plant is potted into a planter which has a layer of crushed lava rock, or pumice, in the base and around the plant’s rootball. The pumice layer acts as a water reservoir and the roots eventually start to grow into it, adapting to the new conditions as they grow. A similar process occurs when using a material called Seramis, which is a very fine expanded clay granular product which encourages excellent root growth.

These methods are sometimes called semi-hydro systems as they slowly convert a soil-grown plant into a hydroculture plant, with all the benefits such a system confers.

In fact, the foli8 coir pot can be used in the same way.

This picture shows the roots of a Beaucarnea recurvata (ponytail palm) growing through the coir pot (which acts as a moist jacket to the rootball) into the gap between the pot and its planter. This gap can be filled with compost, a vulcaponic medium or expanded clay granules.

roots growing through a coir pot

Specialist compost mixes

Some plants really do benefit from a specialist substrate. Many orchids are epiphytes that cannot be persuaded to grow in normal houseplant compost mixtures (they need a very open, free-draining granular substrate that allows light to penetrate to the roots, which is why orchids are often sold in transparent pots). Some cacti and succulents also do well in substrates that are very free-draining. You can buy specialist cactus composts, or make one yourself using a mixture of standard compost, grit and perlite. You will be able to find recipes for such mixtures on specialist cactus-enthusiast websites.

Is it necessary to buy special houseplant compost mixes?

Apart from the examples given above, most houseplants do not really need a special growing medium. Most indoor plants are very adaptable and will manage well in a standard general-purpose compost mixture - ideally a peat-free medium.

You can buy bags of compost from garden centres and online that are packaged as houseplant mixtures, but they tend to be very expensive (as much as 10 times the price, litre-or-litre, as a general purpose compost). I have never seen the need to use such substrates myself and would be surprised if they perform 10 times as well as a good general purpose mixture, or products such as coir, but you must be the judge of that.

 

By Kenneth Freeman

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