The mental image we all have of a root system is a taproot that anchors the plant firmly in the soil. The reality is that very few roots systems have a taproot – dandelions and carrots are two plants that do develop taproots. Most root systems, even those supporting large trees, are broad and shallow.
Rather than one main central taproot structure, most plants have a network of thick and finer roots that travel out horizontally in every direction from the main trunk or stem. The size and spread of a root system relates to the size of the plant so generally the larger the plant, the more extensive its root system.
The roots of a large, spreading tree may mirror the canopy of the branches while the roots of shrubs, perennials, annuals and grasses tend to be more fibrous forming a compact clump in the soil. Generally shrub roots are less likely to become invasive than tree roots.
What roots do
Roots absorb nutrients from the soil via tiny root hairs on feeder roots and also anchor the plant. Plants that develop a large system of feeder roots have a greater access to water and nutrients than plants with compact roots and so are more drought tolerant.
Some plants also develop fleshy roots that can store water and nutrients, which also helps them to survive when times are tough. Agapanthus roots are a good example of a plant with a good ability to withstand drought and poor growing conditions due to its large, fleshy roots.
New growth that sprouts from roots can also help a plant spread and reproduce itself. Roots that both spread and shoot are usually known as rhizomes or runners. Some are highly invasive including running bamboo.
Some plants may sucker – that is grow new plants from their roots. This may occur if the root system is damaged, for example by mowing surface roots or by excavation. Robinias, plum trees and many grafted trees can form suckers that are generally a problem to control and manage.
The root of plant selection & care
All this information about a plant’s root system helps a gardener understand how to select and care for different plants in a range of garden situations.
Consider whether the root system is large or compact, how it spreads, if it can become invasive and where to apply water and fertiliser. Remember all small and newly planted specimens need extra water and care as they settle in and develop their root system. As plants grow so do their roots, which not only spread but also become thicker.
Areas where large or invasive roots are a concern are where roots come into contact with structures such as foundations, retaining walls, fences, driveways, paths, swimming pools and drains. For a list of plants that are usually safe to grow near pools or other structures see our handy list.
Problem plants near structures include most trees (especially large trees such as ficus, jacaranda and gum trees) and running bamboo.
Where invasive plants are grown in confined spaces, their root systems may be contained by root barriers, containers or raised beds. Keeping plants pruned can also keep root systems smaller.
Roots that invade drains can usually be cleared with tools such as an electric eel. However it is important to replace old porous drains with modern plastic pipes to avoid invasive roots re-entering a drain as they continue to grow and search out water.
In areas where plants are close together, especially large trees, their root systems are competing for space in the soil to gather water and nutrients. Usually the plant with the biggest root system dominates which is one reason why it can be hard to grow perennials, shrubs and even grass under trees.
Some plants even exude chemicals to deter other plants growing or germinating nearby. This is known as allelopathy. Walnuts are trees that can have an allelopathic effect that deters other plants from growing or germinating nearby.