Allelopathic Plants:

All plants need to compete for resources, like nutrients, water and light. Plants have evolved many different methods to get their share, even in crowded ecosystems. Sometimes these methods are aggressive - some plants win by playing dirty and poisoning their competitors! These unsociable plants are allelopathic.

Old growth Kauri - only the tough survive in this soil.

What is Allelopathy?

The term “allelopathy” comes from the Greek words allelon (meaning “one another”) and pathos (meaning “suffering”). So basically, it describes plants that make each other miserable!

Allelopathy can be used as a broad term, to describe when an organism (of any type) produces chemicals to compete with another organism. Most often though, it’s used to describe how plants produce biochemicals that affect the growth, germination, or root development of other plants. These chemicals can be exuded into the soil through roots, released from leaf litter, or from decaying plant material.

Allelopathy isn’t inherently good or bad: some plants produce toxins that attack nearby plants to snatch limited resources, but others can release substances that actively promote plant growth. An example would be when plants exude allelochemicals from their roots to encourage Bacillus bacteria to fix nitrogen, which helps feed them and their neighbours.

Regenerating Kauri forest; note how bare the understory is.

Other plants can produce allelochemicals to protect themselves, by targeting specific bacterial and fungal pathogens, or insect pests. These natural chemical defences have been used as sustainable insecticides for countless generations. Examples are azadirachtin (from the neem tree) and citronella (from citronella grass or lemongrass).

Some plants even release allelochemicals into soil to reduce nutrient availability, to deliberately make their environment less fertile and unpleasant to competitors! Kauri trees are a fascinating local example; you can read more about them below.

If gardeners understand the nature of these plants, it’s easier to work with them to get the best advantage. Plants with potentially harmful effects can help create healthy gardens when thoughtfully planted; even aggressively phytotoxic plants can be useful to gardeners, to help with weed suppression.

As you might expect, some invasive weeds in NZ are allelopathic, which gives them an unfair advantage. This explains how these weeds are so quick to exploit space, at the expense of desirable plants. We’ve pointed out some of the worst offenders below, so you can make a special effort to keep them under control.

Allelopathic plants in New Zealand gardens

Kauri (Agathis australis)

Did you know that the iconic kauri tree ruthlessly controls the soil environment around it, to keep competitor trees out of its patch? Kauri release allelochemicals from their roots and leaf litter to create a highly acidic type of soil called podzol, that inhibits the growth of many other plants. Acidic podzol denies access to critical nitrogen and encourages anaerobic soil conditions. This explains how kauri forest came to dominate large areas of the North Island; kauri transforms soil to make it inhospitable for other trees, but it can happily thrive on the reduced nutrients that are still available.

If you have kauri in your garden, it’s useful to know these trees can suppress the growth of nearby plants, or lawn grass. The best companions for kauri are the shade-tolerant native plants naturally found in the understory of kauri forests. Plants that are naturally adapted to cope with kauri-influenced soil include mosses, ferns, epiphytes, ponga and nikau, plus shrubs like coprosma, lancewood, kawakawa, mingimingi and mapou.

Manuka (Leptospermum scoparium) and Kānuka (Kunzea ericoides)

Although Mānuka and Kānuka are much loved for their role in honey production, they also have allelopathic tendencies. Mānuka and kānuka roots and leaf litter exude a plant toxin called grandiflorone, a natural weedkiller that allows them to secure space and nutrients at the expense of other plants, especially seedlings. This is why these two species are usually the dominant trees in regenerating bush areas: they jealously guard this ecological niche from competing species.

Keep this in mind when planting new mānuka or kānuka (or planting around or under established trees). It’s best to give them plenty of personal space, so they won’t affect nearby plants.

Black Walnut (Juglans nigra)

The black walnut tree is one of the best examples of allelopathy we know of! Black walnuts produce a phytotoxin called juglone, which is toxic to a wide range of other plants. It’s especially deadly to tomatoes, potatoes, and other Solanaceae species. Acid-lovers like gardenias, azaleas and rhododendrons are also very sensitive. Juglone is found in the roots, leaves, and nut husks; it can become concentrated in the soil around the tree and inhibit the germination and growth of other plants.

When planting a black walnut tree, think carefully about the plants you plan to grow close to it. Grasses and some fern species can cope with juglone, so siting a walnut tree in a grassed area is a good option.

Eucalyptus Trees (Eucalyptus spp.)

The leaves, bark, and roots of eucalyptus trees release strong biochemicals that can inhibit seed germination and stunt the growth of nearby plants. In a garden, eucalyptus trees can suppress the growth of groundcovers and plants growing underneath them.

Eucalyptus trees also release volatile oils containing allelochemicals into the air; these can affect nearby plants. This can make them a bit of a handful in a garden, so it's wise to choose low-maintenance plants (like succulents or culinary herbs) that can tolerate the effects of eucalyptus.

Allelopathic Weeds to watch out for

Woolly Nightshade (Solanum mauritianum)

A common weed pest in NZ, woolly nightshade (AKA tobacco weed, kerosene plant) produces water-soluble compounds that leach into soil and prevent other plants from getting started. Woolly nightshade is very hardy and fast-growing, which allows it to form dense stands that crowd out native or desirable plants.

The large, grey- green felty-looking leaves of this unpleasant plant are quite distinctive. Purple and white flowers form into bunches of berries, which are poisonous to humans and livestock. The fine hairs covering its leaves and stems can irritate skin and cause breathing problems, so always wear gloves and a mask when tackling woolly nightshade.

Sedges (Cyperus spp.)

Weedy and invasive sedge or nutgrass species are very challenging to control with weedkillers. To make matters worse, they exude allelochemicals from their roots that are toxic to other plants, allowing them to out-compete native plants and form large, spreading clumps.

Tree of Heaven (Ailanthus altissima)

This highly invasive weed produces a phytotoxic compound called ailanthone in its roots and bark, which builds up in the soil to keep other plants out of its space. The root system is very vigorous, spreading sideways and shooting up suckers that rapidly take over a wide area, forcing out native plants. Tree of heaven will quickly regrow from stumps, or any small chunks of root left behind.

Purple Loosestrife (Lythrum salicaria)

This hardy weed rapidly spreads in moist ground or shallow water; moving water helps it to disperse seed widely. The flowers may be pretty, but purple loosestrife is a disaster in wetlands, riversides and creeks, as it forms dense clumps that fill in and block up waterways. It doesn’t only choke out plants; it also removes food sources for birds and fish species. Purple loosestrife probably owes its success as an ecosystem invader to the fact that it exudes allelochemicals from its roots and foliage. These substances remain in soil or leach into the surrounding water, to inhibit the germination and growth of other wetland plants.

Himalayan Honeysuckle (Leycesteria formosa)

This invasive weed produces chemical compounds in its roots, which also remain as residue in its decaying foliage. This inhibits the growth of other plants in its vicinity, especially seedlings. It forms berries, so seed is widely dispersed by birds; when Himalayan honeysuckle germinates in a new spot, its allelopathic nature helps it to spread aggressively. It’s a particular problem in bushland or scrub environments, as after it dies out, other invasive weeds are first to follow in its footsteps.

Gorse (Ulex europaeus)

Gorse produces a complex range of organic compounds that inhibit the germination of other plants, giving it a competitive advantage. It's likely this assists this notorious weed to rapidly dominate disturbed ground and invade pasture.

Fortunately, gorse does have a part to play in native bush regeneration. Gorse can act as a 'nurse crop' for native trees to become established, although it takes many years for the trees to take over.

Broom (Cytisus scoparius)

Broom secretes chemicals into the soil that can kill or interfere with the mycorrhizae fungi that most other plants depend on. It also fixes nitrogen in the soil, which is a great outcome for most introduced species, but doesn’t suit native plants that prefer nutrient-poor soils. If left uncontrolled, broom can form dense stands that choke out other vegetation.

How to manage allelopathy in the garden

Tips for making it work:

1. Plant Prudently: When planting allelopathic trees or shrubs, carefully consider their location. Make sure their root zone is far enough away from other plants that might be affected by them. Allelopathic species are often more practical as a garden centrepiece or focal point, rather than in a border planting.
2. Choose Allelopathy-Tolerant Plants: If you’re planting near an allelopathic species, choose plants known to be tolerant to their effects. Many native ferns, mosses and epiphytes can thrive under or around a difficult allelopathic plant.
3. Boost Soil Health: Fertile garden soil can help plants to be more resilient and tolerate the effects of allelopathy. Adding generous amounts of nutritious organic matter to soil improves aeration, drainage (and water-holding capacity), nutrient availability, and overall plant health. Organic matter in the form of compost and mulch can also help soil to ‘buffer’ or resist the acidifying effect some allelopathic plants have on soil pH.
4. Control Invasive Allelopathic Weeds: For the weed species we’ve pointed out, regular checks and prompt removal will prevent them from dominating your garden, pasture or your local slice of native bush. Keep an eye out for seedlings and nip them in the bud.