Weed management in the vegetable garden.

weeds, also known as Wrong grass (or crazy grass when they thrive in uncultivated areas such as roadsides), have long been considered as undesirable wild plants that spontaneously introduce themselves into crops. Weed control is practised on all continents by billions of farmers. The need to control weeds has therefore long been recognised wherever agriculture originated. This indispensable plant protection technique is the origin of the saying "weeds are the family of bad farmers". Going back further in human history, weeds are mentioned in the Gospel of St. Matthew (chapter 13, verses 24-30).

Nowadays, ignoring the thousand-year-old experience of farmers all over the world, some agro-ecologists claim that weeding is unnecessary. Others believe that it is beneficial to limit the destruction of weeds to preserve biodiversity. Weeds would contribute to maintaining soil fertility and reducing pest populations. What do the latest scientific studies say that can help us make the right decisions?

Relationship between vegetables and weeds ; a problem of food competition.

The amount of available minerals in topsoil is not infinite. Any mineral salts that are removed from the soil by weeds will not benefit the crop, resulting in lower yields and deficiencies that can lead to disease. Competition for water is also an important factor in weed damage, especially in areas requiring crop irrigation. The more weeds there are, the more irrigation water is lost to weeds.

In many cases, in our latitudes crop plants thrive in a different environment than where they come from, and they need to be nurtured and protected by the farmer while native weeds grow naturally. Moreover, cultivated plants have been transformed by directed selections for thousands of years to meet objectives (feeding farmers, resisting diseases) which most often do not include stronger resistance to weeds, which is more difficult to obtain (except for PGM resistant to glyphosate, which facilitate chemical weeding). It is even often the opposite.

However, competition between crops and weeds is more complex than it may seem at first sight. Many factors intervene to modulate to a greater or lesser extent the harmfulness of weeds. It is now accepted that crop yield loss is not significant when the rate of development of certain weeds does not interfere with crop growth. But, in most cases, the competition turns in favour of the weeds when they have a high germination power. When the fertility of a soil is optimised by organic and/or mineral inputs, weeds that produce many seeds throughout the year get a boost. The population of these weeds can then become very worrying. This situation is even more difficult to control when a reservoir of dormant weeds seeds is built up in the soil.

A reverse competition to the benefit of cultivated plants can occur. This phenomenon is encountered in particular in viticulture and arboriculture due to the root mass and rooting depth of cultivated plants, which results in a dominant food competition on weeds. Quantifying and specifying the conditions of competition between cultivated plants and weeds has proven to be very useful in recent years for environmental balances and to benefit from ecosystem services.

Some vegetable crops are very affected by the presence of weeds in their growing area. This is the case for carrots which, at the beginning of their vegetation, do not tolerate competition from weeds very well. This is also the case for peppers, shallots, celeriac and others that do not tolerate competition from weeds. This is why it is essential to control weeds throughout the crop cycle in order to avoid yield losses that can be significant. In addition, weeds can bring opportunistic diseases caused by nutritional deficiencies.

Evolution of weeds populations in crop soils.

The majority of weeds are dependent on crop soils and are selected by weed control methods, tillage, the development of herbicide resistance and farmer interventions to modify fertility such as organic and/or mineral fertilisers. When the farmer manages to reduce the populations of certain weeds, or even to eliminate them, other weeds take advantage of the places freed up. Species have thus become dominant on plots commonly treated with selective weedkillers or having been weeded mechanically or manually. Organic fertilisers can also cause weeds to increase in periods when the nitrates produced by these inputs are in excess.

Conversely, some species have disappeared. This is notably the case for acidophilic species when calcareous amendments are applied, or for biennial weeds that do not resist several weeding operations undertaken during the growing seasons. Cornflower (Centaurea cyanus) has been steadily declining since the practice of chemical weed control. It is considered that the best adapted species are annual weeds whose vegetative cycle and environmental conditions correspond to the crops, or when they have organs such as rhizomes that are more resistant to mechanical weeding.

At least two types of plants can be distinguished by their mode of reproduction. The first group includes geophytic and hemicryptophytic weeds that reproduce in the soil using specific organs such as rhizomes (horsetail, etc.). The second group consists of therophytes that reproduce by means of seeds. The vast majority of therophytes present in crops produce more or less dormant seeds that germinate when certain conditions are present. Some weeds, such as perennial sow-thistle, reproduce by both methods, making them more difficult to control.

weeds seeds can be resistant to burial, to enzymes of herbivorous vertebrates or beneficial soil organisms, and to drought.

Seeds often germinate at different times of the year depending on the date of tillage, sowing, or even the type and method of cultivation (e.g. speedwells, mustards are difficult to control in organic farming, which is not the case in conventional farming). All species that reproduce in the soil have the disadvantage of multiplying when the farmer unintentionally breaks them up with a tillage tool. Some species have such deep roots, such as equisetum arvense (field horsetail), that they cannot be eradicated by tillage alone.

The harmfulness of a therophyte is also characterised by its ability to produce more or less abundant seeds. This harmfulness is even more obvious when the seeds are very small and light, which favours their dispersion by strong winds. The poppy, so appreciated by the activists of the movement in french that bears the same name, can produce 60,000 seeds. These activists are probably unaware of the threat posed by a colony of poppies in a field, which will not fail to take advantage of the fertilisers provided by the farmer. Chickweed is known to germinate all year round. Most therophytes disperse their seeds around them. When germination conditions are favourable, the seedlings can choke out a semi of vegetable plants.

Many seeds are stored in the soil waiting for favourable conditions to produce seedlings, such as a return to the surface after ploughing or timely weather conditions or the end of a drought. It is therefore important to reduce the production of these seeds before tillage (e.g. by false seeding) if you do not want to have an invasive weeds population at the next tillage.

Market gardeners are confronted with problems linked to the rapid succession of crops on certain plots, selecting species with a short life cycle and seed production throughout the growing season, such as chickweed, wild purslane and many grasses.

seed dormancy of adventitious flora.

weeds and soil nutrient enrichment.

Some weeds are said to bring nutrients taken from the surface to the surface. The idea is not false, but the maintenance of these weeds in agriculture is often not very useful, because it is mainly the weed that will benefit from the substances taken from the surface, unless it is destroyed and its residues are left in place to be decomposed. Some of the substances captured deep down could end up in the soil as a result of exchanges between plants and micro-organisms, but to my knowledge there are no serious studies showing a measurable and acceptable benefit for crops.

A weeds is no more efficient than some vegetables or cereals in taking nutrients from deep in the bedrock. Some root vegetables such as chicory and beetroot have branches down to 90 cm. This is also the case for carrot rootlets, which go down to 90 cm, which has been known for a long time (1). Sunflower roots go down into the soil to a depth of 3 m. Most cereals have roots that go down to a depth of more than 50 cm. The root system of tomatoes extends 30-40 cm and some branches can reach down to 1 m (2).

When weeds become natural reservoirs of bio-aggressor.

Grass strips are often kept between rows of fruit trees and vines. Because of their root development, trees and shrubs are effective competitors against weeds. Permanent grass strips have the advantage of facilitating atmospheric nitrogen fixation in the soil. But grass strips near crops can have disadvantages, the most well-known of which are described below:

Weeds are also considered essential for maintaining biodiversity, especially when they favour the establishment of useful auxiliaries such as earthworms in the soil, aphid predators, etc. So how should we treat weeds? How can we reconcile the imperatives of agriculture with respect for the environment, consumer health and ensuring the sustainability of agricultural systems?

Biological weed control.

Thus, it is much more useful to eliminate as many weeds as possible within the crop perimeter, but to let them grow under control nearby, e.g. in paths and hedges, to take advantage of ecosystem services where possible.

As the surface area of home gardens is often less than 500 m², periodic manual hoeing is more than sufficient to ensure this objective. Of course, if this work is still within the reach of the amateur gardener, it is not the same for large-scale crops which require specialised tools driven by tractors. The fight against weeds must then be undertaken according to modalities integrating management levers likely to counteract their development (e.g. sowing date, choice of variety and fertilizers, tillage periods, diversification of rotations, stimulation of weed seed germination during intercropping periods, choice of cultivated cover for No-Tillage farming , precision chemical weeding...); solutions to be considered when a home gardener manages an area of 1000 m² or more.

A weed community evolves according to environmental conditions. When these conditions become less favourable, certain species are gradually recruited to the detriment of others, which leads to a depletion of weed communities with a decrease in their genetic reserve. A consequent change in cultivation techniques spread over several years is capable of significantly reducing certain weeds, as the ecological conditions compatible with their existence no longer exist. However, some species have a very wide plasticity of adaptation to environmental changes. Here are some levers capable of influencing the development of weeds in the vegetable garden and in field crops:

Errors in agricultural practices can encourage the spread of certain weeds. For example, crops that are frequently grown on the same plot can cause a dramatic change in a weed population. There is also the role of mutations selected by agricultural practices, such as resistance to selective or non-selective herbicides. It is then necessary to change the cultivation technique to break these forms of adaptation.

Another example of a mistake: an excess of potassium and/or assimilable phosphorus in the soil favours weeds that consume these nutrients in abundance. In wheat, oilseed rape and maize crops, this phenomenon is observed to the benefit of weeds when the supply of these nutrients is poorly regulated.

Weeds and biodiversity corridors.

The presence of weeds in biodiversity corridors is considered beneficial for the following reasons (non-exhaustive list with precise limits):

If biodiversity areas become reservoirs of particularly virulent and uncontrollable pests, then action must be taken to sanitise these areas by removing the weeds that protect these pests.

Thus, the destruction or preservation of weeds in the vicinity of crops must be assessed on a case-by-case basis. The maintenance of weeds is incompatible with certain biocontrol techniques such as anti insect net (see chapter on biocontrol methods; anti insect net).

In some books or press articles about agroecology, it is sometimes written that weeds should not be destroyed because they would be a source of fresh organic matter. However, the production of humus by hoeing weeds is often poor. Physiologically mature weeds have more carbon than when they are green, but allowing weeds to mature can seriously reduce yields and produce deficiencies in exportable crops.

The limits of mechanical weeding.

In market gardening on small areas, manual weeding with a specialised tool, supplemented by hand pulling, is within everyone's reach. But this technique cannot be considered for large areas. It is difficult to imagine that the solution to today's environmental problems would be to return to the methods of the Middle Ages by employing an army of farm workers receiving a derisory salary. The use of harrows, rotary wheels or hoes is still one of the most reasonable ways to weed large areas. But these techniques are moderately effective, as it is difficult to remove weeds located too close to the crop.

Moreover, manual or mechanical work is not effective against certain perennial weeds with rhizomes such as creeping quackgrass, curled dock, field bindweed, thistle, horsetail. To give an example, the tuberised taproot of dock is very resistant even after pulling up, small pieces of rootlets remaining in the soil being able to start again. It should be noted that according to the Gironde Chamber of Agriculture and to cite one example, abandoning chemical weeding for mechanical weeding under the row generates an average additional cost of 500 euros per hectare (six to seven hours of additional work, purchase of equipment, consumption of diesel) and represents more than 17% of the cost price of bulk wines (6).

Advantages and disadvantages of mulches to reduce weed pressure.

For large areas of market gardening, plastic mulching is used on certain crops (lettuce, shallots, melons, etc.) to limit the development of weeds; a process that is easy to set up at planting time and that can also be installed in the vegetable garden. However, this process has a disadvantage. It limits gas exchange between the soil and the atmosphere. The active zone in the first few centimetres of the soil needs oxygen and any obstacle that hinders gas exchange with the atmosphere has consequences for soil biodiversity. It is not only plants that are affected by the lack of light. Microorganisms such as cyanobacteria that thrive on the soil surface are also affected. On the other hand, mulches have the advantage of reducing water evaporation. In the PACA region, mulches are often used in certain crops such as melons to reduce the proliferation of weeds and limit water loss.

Although plastic mulch facilitates better weed control, it should also be taken into account that its recycling is problematic. There is another possibility. It is now possible to lay compostable and biodegradable woven mulch with a life span of about 36 months. This mulch can be purchased by clicking here.

Is it normal for a vegetable garden to contain many weeds?

A properly prepared and nutrient-rich garden soil is bound to encourage the departure of weeds, especially in summer. Birds and wind continually deposit weed seeds. Frequent re-growth of various weeds can be considered a good test of the soil's fertility potential. In summer, periodic hoeing prevents weed seedlings from spreading. Hoeing should be done in the morning when no rain is forecast for the day. In Provence, due to the intense heat of the afternoon, hoeing in the morning is particularly effective in drying out any weeds that have been cut within a few hours.

Registered weedkillers in biocontrol crop protection products.

Weedkillers derived from living organisms that are said to be more environmentally friendly are included in the list of biocontrol plant protection products. These are preparations based on acetic acid, pelargonic acid and other substances. These products are rapidly degraded by the microflora. Unfortunately, they are less effective than chemical weed killers and are useless for the permanent control of tough weeds such as bindweed, horsetail and couch grass. These weed killers can still be useful. They are presented in more detail by clicking here.

Can weeds reduce the proliferation of gastropods?

Leaving weeds near crops would have the advantage of attracting slugs and snails, and the vegetable plants would be more protected. I have never noticed that all the snails and slugs go to the weeds as if the weeds would attract them like a magnet. Moreover, it is to offer these predators enough food to reproduce. In the end, you end up with more slugs and snails that will eventually invade the garden anyway. Most gastropods have a very large diet and do not distinguish between a green grass and a tasty salad.

Some home gardeners believe that snails and slugs are involved in weed control. If weeds are to be managed by biological control, much more effective predators must be chosen, including granivorous organisms. Many arthropods, such as carabid beetles, ants and birds consume significant quantities of weed seeds. It is therefore important to encourage the establishment of useful beetles in and around a vegetable garden, just as it is useful not to systematically destroy all ant nests. However, one should not expect a complete eradication of weeds by promoting the establishment of granivores. The few studies show that the predation rate is about 40%, but can be quite variable depending on the agronomic context (type of weeds, climate, protection by plant cover, biodiversity corridor, type of crop...).

Weed control with thermal lances.

In order to replace chemical weed killers, processes considered more respectful of ecosystems are available in garden centres. But are these processes relevant? The fear of "chemicals" sometimes leads to surprising practices with harmful consequences for the environment.

I take as an example the very recent craze for gas-powered thermal machines, which are adulated by certain spheres of the ecological movement. The city of Lyon, for example, which claims to be a model of ecological practices, requires its gardeners to use this method of weeding. However, weeding with gas requires powerful equipment that uses a fossil resource that produces CO², a greenhouse gas as many people know. It takes 5 to 8 passes to obtain an acceptable result. And even then, the roots of some perennials are not reached. So don't expect to get rid of brambles, hemp, horsetail, bindweed, etc., which easily produce new shoots like in a forest after a fire. Moreover, you cannot use these thermal lances in the vegetable garden if you do not want to incinerate the vegetables at the same time. It is not easy to incinerate plants in fire-prone areas, which are quite numerous in the Provence-Alpes-Cote d'Azur region.

Boiling water; another environmentally friendly weeding method.

There is also the advice given in small leaflets distributed in certain specialised organic shops where you learn that you can weed with boiling water which, of course, but this is not specified on the leaflet, requires large quantities of water and energy to heat it to 100°, thus releasing CO² into the atmosphere, not to mention the risks of accidents during handling and spreading.

Is it possible to weed without using environmentally harmful chemicals?

Some websites have become tools of misinformation for those who are not careful. For example, it would be possible to get rid of weeds with a simple and cheap trick that can be found on many "alternative" gardening websites, for example by clicking here.

How do you do it? With the following solution for one square metre using a spray bottle:

- 480 ml white vinegar
- 140 g of salt
- A drop of washing-up liquid
- A large spray bottle

Note the precision of the quantities of the ingredients, but it is not clear why.

Of course, the mixture would not be "aggressive at all - except for the removal of weeds". Clearly, the genius who invented this trick has a big problem with orders of magnitude, and the properties of cooking salt:

This miracle solution still represents 1.4 tons of salt per hectare (to be compared with the doses of "chemical" herbicides often expressed in grams/hectare).

As a reminder, cooking salt (NaCl) is a sodium atom linked to a chlorine atom. Sodium chloride is known for its considerable impact on the environment, which can lead to the disappearance of plant and animal species and the destruction of agricultural soils. Sodium chloride accumulates in groundwater over time and is released into watercourses even during the summer, when groundwater plays a crucial role in water supply.

1)Répertoire universel et raisonné d’agriculture carotte et panais, en pleine terre – François de Neufchateau – An XIII, 1804
2) La fertilisation des cultures légumières – Ctifl ; H Zuang – Edition 1982
3) http://ephytia.inra.fr/fr/C/16181/Hypp-encyclopedie-en-protection-des-plantes-Biologie-epidemiologie
4) Reboud X., Eychenne N., Délos M., Folcher L., 2016. Withdrawal of maize protection by herbicides and insecticides increases mycotoxins contamination near maximum thresholds. Agronomy for Sustainable Development, 36 (3), 1-10. https://link.springer.com/article/10.1007/s13593-016-0376-8
5) Kadioglu et al., 2005 Allelopathic effects of weeds extracts against seed germi- nation of some plants. Journal of environmental biologie, Academy of Environmental Biology, India, 26 (2), 169-173. http://europepmc.org/abstract/med/16161968
6) Vitisphère 14-10-2019 https://www.vitisphere.com/actualite-90426-Bordeaux-plaide-pour-un-delai-de-sortie-du-glyphosate.htm