The first description of the inhibition of plant growth by root secretions of other plants was given by de Candolle already in 1832. However, it was only in the thirties of the twentieth century that the more serious interest in this kind of biochemical, invisible influence of plants (but also fungi and plants) was taken. They were scientifically called allelopathy. Allelopathy in the garden was introduced by Ch. Molisch in 1937 in Jena. At that time he described what the first farmers and gardeners who consciously started to grow plants organically were observing and using in practice. They did not use mass-produced chemistry and avoided monoculture. 

In the forest, in the meadow or in the old orchard there are whole characteristic plant groups. By analyzing their species composition, you can quite precisely determine the content of different components in the soil and the quality of the environment. This is the science called phytosociology. 

It has long been noticed that in nature, some plant species grow side by side, while others do not. On this basis, it has been concluded that certain plants “like” each other, i.e. they have a positive effect on each other. Others, in turn, are ‘foreign’ to each other. – so their mutual neighborhood is unfavorable and inhibits development. 

“In nature, there are numerous examples of insects choosing and multiplying better on weak, undernourished plants, crops and farm animals.

Leonard Hassman, Professor of Entomology at Missouri University

Invisible Influence 

Today we know that plants influence each other mainly through their roots and leaves. Fungi (especially mycorrhiza), algae and microorganisms also take part in these processes. Plant organisms secrete various volatile, liquid and solid substances, including essential oils, ethylene, glycosides. alkaloids, tannins. When the secretions reach the neighbors, they hinder, or on the contrary, help them to develop. 

On what does “Mutual Love of Plants” Depend? 

Allelopathy in the garden has a few dozen years of history and a lot of work has been published about it during this time. In 1992 S.A. Ostroumov’s publication  “Introduction to Biochemical Ecology” saw the light of day . There are also many popular science publications available in various guides to organic gardening. Nevertheless, the scope, role and strength of mutual neighborhood of plants are not fully understood. There are many reasons for this. Firstly, the mutual influences of individual plants become less pronounced in different soil and environmental conditions, especially when they are far from organic. Secondly, they also depend on the environment of other plants. Thus, the share of the third or fourth plant in the vicinity can significantly interfere with what only two plants reveal. For example, the favorable vicinity of the carrot season can be changed by growing next to, for example, tomato, lettuce, lavender or basil.

Thirdly, relationships depend on the condition of the plants, their growth stage, the fertilization applied (compost is the most appropriate), crop rotation, the presence of various animals, microorganisms and other factors. It was found that on fields, the described effects do not always occur, because soil microorganisms can break down the root secretions before their influence on neighboring plants is visible. A similar phenomenon can be observed in large gardens or with less intensive crops, more distant plants, etc. Also strong air movements cause the escape of allelopathic substances. Therefore, if you want to use this kind of dependence in your garden, it is advisable to use hedgerows from shrubs or tall plants at the edges. They will protect against strong air movement over beds. On the other hand, by hindering the air movement, the hedges will encourage the appearance of certain pests, such as carrot fly. 

Allelopathy in the Garden – Not a Prescription, but a Helpful Recommendation 

The above remarks make us reflect on the matter. So far known detailed allelopathic interdependencies between plants must not be treated as absolute prescriptions valid in all conditions. It is also not a miraculous measure that can protect plants from invasions of diseases and pests, especially in cases of poor soil tillage (e.g. excessive or inappropriate fertilization, faulty crop rotation, overgrowth, excessive shading). The neighborhood of plants should be appreciated as a helpful recommendation in ecological practice resulting from nature observation. It always needs to be checked and adjusted to the conditions prevailing in a given garden, orchard or field. 

Good Neighborhood Increases Yields, Strengthens and Protects

Skillfully selected plant neighborhood can contribute to a noticeable increase in yields, strengthen plants, protect them from diseases, pests (or at least reduce their negative impact). Numerous insects that threaten the crop are often strongly repelled by substances secreted by “good neighbors”. Better plant growth improves the quality and taste of vegetables and fruits. 

Allelopathy in the Garden among Herbs

The phenomenon of allelopathy in the garden is most easily exploited by introducing cultivation coordinates (i.e. in the form of different plants growing side by side), instead of monoculture (planting beds with only one species), which, after all, are usually not something naturally occurring in nature. Every diversity works well. 

It is definitely more beneficial to plant many plants in gardens, especially aromatic herbs, which secrete various volatile compounds, attracting useful insects and at the same time confusing many pests. The herbs can be planted in the form of rings on the edges of beds and fields (they are a valuable addition to any feed and can also be a great medicine for animals). 

It is Recommended, for example, to Plant Garlic, Horseradish, Peas, Broad Beans around Potatoes. 

The perimeter can be planted with a trestle, tarragon, corn and sainfoin (a butterfly plant which is also a valuable green fertilizer). The latter plant is “liked” with pumpkin, beans, peas, potatoes and wheat. Flowers and herbs, e.g. velvet, nasturtium, basil, marjoram, can also be the borders. 

Allelopathy in the Garden – Stimulate or Inhibit Sprouting 

allelopathy-in-the-garden

Analyzing the neighborhood, it was discovered that weeds and their seeds can hinder the growth and germination of useful plants, but the opposite can happen. Beet seeds secrete substances that inhibit the growth of the tares, and germinating barley seeds inhibit the germination of field mustard seeds. Germinating millet, wheat, oats, vetches, corn and buckwheat seeds stimulated the germination of mustard seeds. Lupine and corn most probably inhibit the growth of such weeds as white quinoa and Amaranthus retroflexus thanks to their root secretions. It is assumed that rye perfectly cleans the soil and plays an important phytosanitary role in the crop rotation, i.e. it cleanses the soil of pathogenic compounds. 

It was noticed that uncollected plant remains left on the surface of the soil or inside it emit toxic substances that affect the yield of the next crop. The observations of the cornfield were interesting. On one part the crop was harvested, on the other part it was left in its entirety. On the cleaned plot the biomass of herbaceous plants was higher during the next four years. The variety of plant and animal species was also bigger. 

Experiments also proved that extracts from wheat, oat and corn straw significantly inhibited the growth of wheat roots and shoots. 

The Wisdom of our Ancestors

There are many more of these types of relationships, sometimes different authors also provide conflicting information. It is worthwhile, however, to recall a lot of rules applied by our grandparents and grandmothers. They were not familiar with fashionable terms like “ecology” or allelopathy, but what they did, is often fully consistent with the rules of natural ecological cultivation. So, as you can see, you can rely on the centuries-old great wisdom and intuition of our ancestors often more than on scientific research. A fairly common rule was to plant celery and leeks alternately on the same beds next to each other. Sometimes celery, leeks, but also other plants from seedlings, including herbs, were planted in different free spaces on the beds, introducing great biodiversity. The celery was often planted alternately with tomatoes growing high on stakes. The rule was to plant beans and beans on dry seed near the potato. The potatoes were also accompanied by cabbage beds. Even pumpkins were planted in the ridges. The fields with onions were often located next to carrots. 

Allelopathy in the Garden – Dill in Carrot

Dill grew wild, mostly in carrots or cucumbers, but also in other fields. The rule was the rows made of tic beans. They were often located in places where strong, cold winds blew. The rows of corn or sunflower, also with regard to cucumbers and other cucumber plants, fulfilled a similar protective task. Poppy seeds were often left in beets, which were also sometimes allowed to grow wild. Between the rows of cucumbers, kohlrabi and dill were often grown. Various flowers were also planted in vegetable gardens, including marigolds, velvet and nasturtium, both in the form of rings and beds under trees. Today we know that they have strong protective properties.