Natural herbicide: discover allelopathic plants!

What is an allelopathic plant?

Since antiquity, humans have described the behaviour of certain plants that have the ability to inhibit the growth of other vegetation. The term “allelopathy” was first used in 1937 to describe the phenomenon of competition between plants. It comes from the Greek ‘allêlon’ meaning “reciprocal” and ‘pathos’ meaning “suffering”: it refers to the entire range of chemical interactions of one plant species on others.

Allelopathic plants naturally release chemical compounds that inhibit the germination and development of competing species. This is a great advantage for those who prefer gardening with minimal effort! Taking advantage of the allelopathic power of plants can indeed reduce weeding in the garden as weeds become scarce near these plants. Discover through these tips which plants possess these properties and the principles of allelopathic planting.

Allelopathy: a clever method to reduce the weeding chore between plants

Trees under which nothing grows

• Eucalyptus are champions of non-competition: very few plants manage to grow beneath their foliage.
• Walnuts release juglone into the soil, inhibiting the growth of other plants.

Walnut and eucalyptus

One should not confuse the properties of these trees with the root competition exerted by other species, such as pines, which also prevent the growth of other plants. The result is the same, but the mechanisms at play are different! By planting these trees, you will choose a space cleared at their base, where only a few resilient plant species may manage to settle.

Friendly allelopathic bushes

Large shrubs – 2 to 4 m

Some large-growing shrubs, such as Laurus nobilis or bay laurel, common myrtles, Pistacia lentiscus, or Atriplex halimus and Atriplex canescens prevent the growth of other plants at their base. Associating them in a free hedge or at the back of a flowerbed will save you a lot of gardening work: limited pruning and weeding guaranteed! These shrubs are very drought-resistant and thrive in poor, well-drained soil. They can tolerate very short frosts of -10°C, down to -12°C or even lower for the ‘Tarentina’ subspecies of Myrtle.

Laurus nobilis, Myrtus communis, Pistacia lentiscus, and Atriplex halimus

Small shrubs and bushes – 1 to 2 m

Cistus are the quintessential allelopathic plants. When their leaves fall to the ground, they release chemical compounds that prevent the germination of other plants. Phlomis, also known as Jerusalem sages, possess similar properties. These sun-loving shrubs create stunning floral displays in the garden from spring onwards. Both are hardy down to -10°C and prefer well-drained soil. Dot your flowerbeds with these moderately growing bushes: their evergreen foliage will provide an interesting base throughout the year.

Cistus and Phlomis fruticosa

Perennials and herbs that do the cleaning

Perennials

Most lavenders possess naturally weeding properties. Your low-maintenance beds can combine them with the magnificent Clary Sage – Salvia sclarea and Artemisia abrotanum, also known as lemon balm. The grey foliage of the Silver Santolina Santolina chamaecyparissus will be enhanced all summer by the golden yellow flowering of Achillea filipendulina ‘Parker’s Variety’, while Perovskia will point its countless lavender spikes to the hungry pollinating insects alongside the curry plant Helichrysum italicum.

Perovskia, Artemisia abrotanum, Salvia sclarea, Santolina chamaecyparissus and Helichrysum italicum.

Aromatic plants

Your displays can be enhanced by the scents of a few aromatic plants. Many of them exhibit the properties we are interested in and unhesitatingly tidy up around them:

• The rosemaries
• The thymes
• the Common Sage ‘Albiflora’ and Common Sage ‘Berggarten’
• The oregano
• The Hyssop – Hyssopus officinalis

Rosemary, thyme, common sage and oregano

Groundcovers that will weed for you

In the foreground of your borders, the soft and fluffy cushions of Bear’s Ears Stachys byzantina will spread over fifty centimetres, to the detriment of any adventive plants that might try to sneak in! In the same range of grey, Ballota pseudodictamnus can stand out against the clumps of Nepeta faassenii ‘Six Hills Giant’ in bloom all summer until October. Some yarrow, such as Achillea millefolium and Achillea crithmifolia, are just as easy to grow as the previous groundcovers, provided you avoid excess moisture. They also release phyto-toxic compounds that inhibit the development of other plants: your borders will be filled with easy, floriferous plants that require almost no maintenance, even at the edges.

Stachys byzantina, Achillea crithmifolia, Nepeta faassenii ‘Six Hills Giant’ and Ballota pseudodictamnus

How to create a bed of allelopathic plants? The main principles

• If it’s a new bed, start by marking the outline on the ground (using stakes and string, or a hose laid on the ground for sinuous shapes).
• Prepare the soil to your liking by removing the lawn or existing plants (using hand tools or a tiller for the less adventurous). Refine and rake. Carefully remove all plant debris, especially the roots of invasive adventives, such as couch grass, bindweed, or horsetail.
• Arrange the chosen plants, spacing them according to their future size. Place the larger ones at the back and the smaller ones at the front. Respect planting distances to prevent competition between the plants.
• For the first two years on average, weed the uncovered soil manually. You can apply a mineral mulch or a organic mulch to promote soil life and the development of your plants. Moisture will be better retained, and “weeds” will grow less during the development phase of your allelopathic plants.
• Once the plants reach their mature size and cover the entire area, weeding will become much less necessary. Gradually, the “weeding” effect will become apparent, limited to the ground area of the plants. Allow the natural mulch, made up of fallen leaves from your allelopathic plants, to remain on the soil. Their decomposition also releases anti-germinative compounds.

Thanks to allelopathic plants, such as Phlomis or lavender: beautiful flowering beds with minimal weeding in sight.

For a tailored selection of plants, feel free to use our Plantfit web application!

Sophie’s advice: For allelopathic plants with anti-germinative effects, the impact will be null on undesirable plants that reproduce vegetatively through their roots… couch grass and horsetail, for example, will continue to thrive. Therefore, the preparation phase is crucial to eliminate as many of these adventive roots present in the soil as possible.