Growing media: compost, soil, hydroculture, etc.

A wide variety of growing media are used by interior landscapers. They all have their advantages and faults and it is quite a complex science to work out which growing media work best in different situations.

Growing media is a broad term covering a multitude of mixtures that the plants grow in. Often called soil or compost (or dirt in North America), the materials used have to perform a surprisingly wide range of functions.

Functions of growing media

The growing medium has many functions. It must provide a suitable anchorage for the plant’s roots; it must act as a reservoir for water and nutrients; it acts as a buffer against sudden changes in the environment, especially changes in temperature and, for indoor plants in containers, it must be sufficiently heavy to provide stability to the plant display and reduce the risk of it toppling over.

Anchorage
Plant roots grow into the growing medium to extract nutrients and water, but they also serve to anchor the plant into the ground to prevent it from falling over. Often the texture of the growing medium determines how well the roots can ramify through it.

Water
A well-structured growing medium acts as a reservoir for the water a plant needs. Indoor plant displays are often fitted with subterranean irrigation systems that supply water to the growing medium, but it is the water in the growing medium around the plant roots that acts as the plant’s primary source of water.

The texture of the growing medium affects the rate at which water is absorbed and the volume that can be held. This is an important consideration for keepers of indoor plants where different species have very different requirements.

<< Well-structured compost - retains it shape after saturation and has plenty of air spaces

<< Poorly structured compost - collapses after saturation and has few air spaces

Nutrients
Fertilizer is usually applied to the growing medium whence it can be extracted by the plant roots. The chemical composition of the growing medium has a dramatic effect on the uptake of fertilizer nutrients. Media with a high clay content can lock up certain nutrients whereas loamless media, such as peat, are less able to bind nutrient chemicals resulting in greater availability of those nutrients to the plants.

Buffer against extremes of temperature
Indoor plants do not suffer the same extremes of temperature that plants in the wild experience. However, there may be occasions when the conditions inside a building suddenly change. For instance, the heating may break down in the winter or the air conditioning may fail on the hottest day of the year. Fortunately, the growing medium often has a high heat capacity which means that it is slow to warm up or cool down. This will protect the plant as it gives it time to adjust to new conditions.

Obviously, if the temperature changes are great or last for a long time, the plant will suffer. However, a well-structured growing medium gives the plant a chance to survive whilst the temperature is returned to normal.

Stability
Many indoor plants are displayed in freestanding containers. Often the height of the plant is more than four times the diameter of its container so, unless the container and the growing medium are sufficiently heavy, there is a risk that the plant display may be unstable.

Peat-based media

Peat forms the basis of many growing media used in interior landscapes. There are many reasons for its popularity, but there are also environmental concerns that need to be addressed.

Peat is a natural product formed by the partial decomposition of mosses and sedges. The decomposition occurs in acid, waterlogged conditions where the micro-organisms that would normally break down plants are absent. Very few interior landscape mixes are 100% peat. There are usually extra ingredients added to balance pH and to add weight.

There are many different types of peat. The differences are related to the environment under which the decomposition took place, the types of moss and sedge that decomposed and the time since the decomposition started. Whilst there are a multitude of ways to classify peat, many horticulturists refer to sedge peat (dark and very decomposed with a fine texture) and sphagnum peat (derived from the upper layers of peat bogs, paler in colour and with a discernible structure). Sphagnum peat is the type usually found in growing media used by interior landscapers.

Peat is a popular component for growing media used by interior landscapers for many reasons.

• It has good structure and texture, which encourages root development.

• It has good water holding capacity without getting too waterlogged which makes it ideal for use with subterranean irrigation systems.

• Peat has good chemical properties making fertilizer application easy. There are no minerals that will lock up nutrients, so fertilizer rates can be low. Soil pH can be adjusted so that nutrient uptake is always optimal.

• It is more or less sterile, so there is little risk of soil-borne plant diseases.

• It is lightweight, so plant displays are easy to transport and move once in situ.

• Most plants are already grown in peat-based mixtures, so there is little risk of transplant shock when plant displays are planted up into landscape containers. The plants are already adapted to their environment.

• Supplies from reputable companies means that there are good Quality Assurance procedures in place to ensure that the product is consistent.

• It is a natural product so very little product processing is required.

However, in many countries, especially the UK, there are concerns that the use of peat by gardeners and horticulturists is damaging the environment. This is because some peat is extracted from moors that form unique ecosystems and support a very special range of plants and animals.

Peat-based growing media usually contain other ingredients. Peat alone is not ideal. In the UK, peat-based growing media often include some grit to provide extra weight and some inert particles, such as perlite, to aid aeration. Ground limestone is often added to raise the pH to between 5.5 and 6.5.

Outside of the UK, peat-based media often contain additional components such as composted wood fibre and bark. A common mixture used in New Zealand also contains as much as 50% pumice extracted from the volcanic debris found there.

Loam and soil-based media

Loam or soil-based media are less common in interior landscapes than peat-based media. They are often heavy and difficult to handle and, as they are made from soil, consistency is difficult to maintain.

Some growing media are produced that contain a mixture of peat and soil. These include the famous John Innes compost mixtures that are formulated for specific horticultural purposes.

Many interior landscapes featuring large trees use media including some loam. This is to provide greater stability and anchorage for the trees.


Hydroculture

Hydroculture is a way of growing plants without soil of any kind. It is very popular in the Netherlands and Germany, but has never caught on in the UK.

Specially produced plants are grown in a medium of large clay granules and water. The clay granules (called LECA - Light Expanded Clay Aggregate) provide physical support for the plant, but the roots of the plant remain in water all the time. Fertilizer is provided in a soluble form that is added to the water.

Once installed, hydroculture plants are easy to look after. All that has to be done is to keep the water levels topped up and remember to add fertilizer (usually twice a year). However, hydroculture plants are more fragile and extra care has to be taken when handling or transporting them. The plants are also more expensive as they have to be specially grown so that they are adapted to the hydroculture environment. Specialist growers have now perfected the system and it is possible to get a very wide range of hydroculture plants, including such unlikely specimens as cacti.


Peat alternatives

There are now many alternatives to peat as components of growing media. Some are better suited for use in interior landscapes than others. The most common alternatives include bark, coir, wood-based products, organic waste and spent mushroom compost.

Bark
Composted bark is now a common component of many growing media. It has the advantage of being both a waste product from other processes and a completely renewable resource. However, there are some inherent difficulties that need to be overcome when using bark-based media.

The most important characteristic of bark-based mixtures is its decomposition by fungi in the mixture. This process uses a lot of nitrogen, which can result in deficiency for the plant. This decomposition can also result in elevated temperatures and the growth of mushrooms on the surface of the medium. This is unacceptable in indoor plant displays and is the main reason why bark-based media are not popular among interior landscapers.

Bark used in growing media comes from both softwood and hardwood sources. Softwood bark, from species such as the Scots pine and spruce, is rich in phosphorus, calcium and potassium.

Hardwood bark must be thoroughly composted before it is incorporated into growing media. This is because it is toxic to plants in its raw state. However, hardwood bark has been shown to be able to suppress some damaging soil fungi and root-destroying nematodes (eelworms).

Coir
Coir is a by-product of the coconut industry. It is made from the ground husks and fibrous shells of coconuts and is now very popular. Like bark, coir is a waste product and comes from a renewable source, so there are environmental benefits. However, coir has not yet proved popular amongst interior landscapers.

The benefits of coir include:

• Good structure and texture. Coir encourages root development and has good water holding capacity without getting too waterlogged. It is ideal for use with subterranean irrigation systems.

• Coir has good chemical properties making fertilizer application easy. There are no minerals that will lock up nutrients, so fertilizer application rates can be low. Soil pH can be adjusted so that nutrient uptake is always optimal.

• Coir is more or less sterile, so there is little risk of soil-borne plant diseases.

• Coir is lightweight, so plant displays are easy to transport and move once in situ.

• Coir is clean and sterile, so few risks of soil-borne plant diseases.

However, there are some disadvantages:

• Variable quality and consistency of product. This is because the coir has often been left exposed to the elements before being processed. This is less of a problem now than it was a few years ago when it was first introduced.

• Previous bad experiences by some growers mean that product is treated with some suspicion.

• Coir has to be transported from Sri Lanka, so the environmental benefits gained from using a renewable product have to be compared with the environmental disadvantages of shipping a product around the world.

• The raw material needs processing before the product can be used. The environmental benefits of using a renewable product have to be compared with the environmental disadvantages of additional industrial processing.

• Plants suitable for coir will have been grown in peat and will have peat compost in the root ball, so it will be impossible to guarantee a totally peat-free plant display.

Wood-based products
Some manufacturers now include wood-based products (other than bark) in growing media. These include sawdust and ground wood chips. These products have many of the disadvantages of bark with he additional risk that wood that has been treated with timber preservatives may be included. These may be toxic to plants.

Organic waste and spent mushroom compost
This category includes products such as treated sewage, composted sludge and by-products of the textile and mushroom industries. Their use in media used in interior landscapes is minimal as they seldom have all the necessary physical or chemical characteristics required for the maintenance of indoor plants.

Pests

Growing media, except hydroculture, make good habitats for sciarid flies and cockroaches especially when the soil is too wet. Fortunately, these pests are easily prevented by good management practices such as the use of subterranean irrigation systems or the use of approved pesticides.

Environmental issues

The most pressing environmental issue surrounding this subject is the use of peat in growing media. This is particularly the case in the United Kingdom where some peat moors are in ecologically sensitive areas. The debate on the use of peat is well rehearsed and further information can be obtained from organizations such as Friends of the Earth and from the Peat Producers Association. It is important to remember that many peat alternatives also have an environmental impact, especially those products that need to be transported long distances or require industrial processing to make them usable.

Author: Kenneth Freeman