Weed management in the vegetable garden

Weeds, also known as unwanted plants (or wild grasses when they thrive in uncultivated areas such as roadsides), have long been considered undesirable wild plants that spontaneously grow in crops. Weeding is practised on every continent by billions of farmers. The need to control weeds has therefore long been recognised wherever agriculture has taken root. This essential plant protection technique is the origin of the saying “weeds are the family of bad farmers”. If we go further back in human history, weeds are mentioned, for example, in the Gospel of Saint Matthew (chapter 13, verses 24-30).

Nowadays, disregarding the millennia of experience of farmers around the world, some advocates of agroecology claim that weeding is unnecessary. Others believe that limiting weed destruction is beneficial for preserving biodiversity. Weeds are said to help maintain soil fertility and reduce pest populations. What do the latest scientific studies say that can help us make the right decisions?

Relationship between vegetable plants and weeds: a problem of food competition

The amount of assimilable mineral salts contained in arable land is not infinite. Any mineral salts taken from the soil by weeds will not benefit cultivated plants, resulting in lower yields and deficiencies that promote the emergence of diseases. Competition for water is also an important factor in the harmfulness of weeds, especially in regions where crops require irrigation. The more weeds there are, the more irrigation water is lost to weeds.

In many cases, in our latitudes, cultivated plants thrive in an environment that is different from their native habitat, and they need to be nourished and protected by farmers, whereas native weeds grow naturally.

In addition, cultivated plants have undergone selective breeding for thousands of years to meet specific objectives (feeding farmers, resisting disease), which in most cases do not include greater resistance to weeds, as this is more difficult to achieve (except for glyphosate-resistant GMOs, which facilitate chemical weed control). In fact, the opposite is often true.

However, competition between cultivated plants and weeds is more complex than it might appear at first glance. Many factors come into play to modulate the harmfulness of weeds to a greater or lesser extent. It is now accepted that crop yield losses are insignificant when the growth rate of certain weeds does not interfere with the growth of cultivated plants. However, in most cases, competition favours weeds when they have a high germination rate. When soil fertility is optimised by organic and/or mineral inputs, weeds that produce numerous seeds throughout the year are boosted. The population of these weeds can then become a serious concern. This situation is even more difficult to control when a reservoir of dormant weed seeds builds 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.

The gardener still needs to know how to diagnose what is going on in his garden. Some people are convinced that they have acceptable results when their crops are stunted. How many times have I noticed the existence of deficient and diseased vegetable plants caused by the presence of too many weeds, or even insufficient organic and mineral fertilisers to correct exhausted soils! What is misleading in agriculture is that the “all or nothing” principle rarely works. It is very often possible to obtain a harvest. But with what quality? Are the yields obtained in accordance with agronomic studies? With how much loss?

Changes in weed populations in cultivated soils

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.

The dormancy of weed seeds

Weeds and soil enrichment with nutrients

This is also the case with carrot rootlets, which can grow to a depth of 90 cm, a fact that has been known for a long time (1). Sunflower roots can grow to a depth of 3 m.

Most cereals have roots that extend to a depth of more than 50 cm. The root system of tomatoes spreads over 30 to 40 cm, and some branches can extend to a depth of 1 m (2).

When weeds become natural reservoirs for pests

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

• Weeds serve as natural reservoirs for polyphagous pests such as the South American leaf miner and the onion fly, which is fond of dandelions. Many aphids thrive in weeds. This is the case with the apple aphid, which migrates to plantain in summer. This is also the case with the black aphid, which easily migrates to field thistles. Certain viruses are transmitted by insects from one plant species to another, such as the cucumber mosaic virus, which is highly polyphagous; it can infect more than 700 different species representing 92 botanical families, belonging to both monocotyledons and dicotyledons (3). The role of intermediate host has also been observed for pathogenic bacteria and parasitic nematodes. The risk is even greater when weed species and cultivated plants are genetically similar.
• Verticillium wilt is one of the most well-known diseases in vegetable cultivation, affecting many plants, particularly tomatoes, potatoes, peppers, cucumbers, artichokes and, above all, aubergines. It is caused by a soil fungus (Verticillium) that enters the vascular system of plants through the roots. Verticillium also parasitises weeds such as black nightshade and amaranth, which then become vectors for the spread of this parasite, often via beneficial organisms (such as earthworms) or polyphagous pests.
• For cereals, field foxtail and other grasses are vectors of the barley, oat and wheat dwarf virus (BYDV). Certain weeds, such as common chickweed, can be healthy carriers of viruses that can be transmitted by aphids to cultivated plants (beet yellows virus or rapeseed yellows virus). These weeds show no symptoms and are vectors for the establishment and spread of viruses if they are left in place near crops.
• The transfer of diseases present on weeds to crops, as well as certain substances produced by pathogens, can become particularly dangerous to human health and that of domestic and farm animals. The prevalence of toxins from fungal diseases can affect crop quality rather than quantity. There is a correlation between weeds and the presence of several species of Fusarium on maize, which can produce more than 23 mycotoxins (4). Other examples include rye ergot, which affects grass weeds, and the particularly allergenic pollen of ragweed, which can be deposited on crops.

Weeds are also considered essential to maintaining biodiversity, especially when they encourage the establishment of beneficial organisms such as earthworms in the soil, aphid predators, etc. So how should weeds be treated? 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

It is therefore much more useful to eliminate as many weeds as possible within the crop perimeter, but to allow them to grow under control nearby, e.g. in paths and hedges, in order to benefit from ecosystem services where possible.

As the surface area of private vegetable gardens is often less than 500 m², periodic manual hoeing is more than sufficient to achieve this objective. Of course, while this work is still within the reach of amateur gardeners, the same cannot be said for large-scale farming, which requires specialised tools driven by tractors.

Weed control must therefore be undertaken using methods that incorporate management levers likely to hinder their development (e.g. sowing date, choice of variety and fertiliser, tillage periods, crop rotation diversification, stimulation of weed seed germination during intercrop periods, choice of cover crop for TCSL, precision chemical weeding, etc.); solutions to be considered when an amateur 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 gradually recruit at the expense of others, leading to a decline in weed communities and a reduction in their genetic reserve.

A significant change in cultivation techniques spread over several years can substantially reduce certain weeds, as the ecological conditions compatible with their existence no longer exist. However, some species possess a very high degree of adaptability to environmental changes. Here are some levers that can influence weed development in vegetable gardens and field crops. :

• An increase in nitrogen fertilisation has negative repercussions on weeds adapted to low-nutrient environments, such as lupidic alfalfa. On the other hand, excess nitrogen favours weeds that are greedy for this nutrient, such as bindweed. An invasive presence of bindweed in a cultivated plot is often a sign of nitrogen imbalance (for example, following the application of manure in the spring, producing an excess of nitrates that are not absorbed by vegetable plants).
• Precise adjustment of nitrogen input throughout the crop cycle is also a biological means of reducing weed growth. Certain crops, such as wheat, are highly competitive with weeds in nitrogen-deficient conditions, although yields are also reduced.
• Ploughing eliminates autumn weeds, but not spring weeds, which can be reduced by hoeing.
• Fake ploughing in summer and autumn is quite effective in eliminating annual weeds that germinate in spring, such as goosefoot and orache.
• In market gardening, nutrient-intensive crops with a very rapid growth cycle (such as turnips) significantly compete with certain weeds, or are little affected by their presence.
• Increased competition for light will be promoted by a denser sowing of a cultivar that will suppress weeds.
• Watering followed by the installation of transparent covers creates a greenhouse effect that promotes the germination of certain weeds. Weeding will destroy most of these weeds, which are sensitive to artificial temperature increases.
• Liming carried out to improve the structure of clay soils has an indirect effect on certain acidophilic weeds such as bracken, field bindweed and corn chrysanthemum. However, this technique can encourage other weeds such as field bindweed, which prefers soil with a basic to neutral pH.
• Certain weeds such as black nightshade, lamb’s quarters and chamomile secrete substances that reduce the germination of certain crops (5). This is also the case with certain productive crops such as rye and oats, which secrete toxic substances that harm weeds when they are vigorous. This lever should be used on a case-by-case basis, alternating with other control methods described above.
• The field thistle, which is very common in France, does not tolerate smothering crops such as rye, crimson clover, alfalfa and oats. Three to four successive stubble ploughings after harvesting are effective if winged tools are used, especially when weather conditions permit (dry periods).

Errors in agricultural practices can encourage the proliferation of certain weeds. For example, crops that are frequently grown on the same plot of land can cause a dramatic change in the weed population. The role of mutations selected by agricultural practices, such as resistance to selective or non-selective herbicides, must also be taken into account. It is therefore necessary to change cultivation techniques in order to break these forms of adaptation.

Another example of error: an excess of potassium and/or assimilable phosphorus in the soil promotes weeds that consume these nutrients in abundance. In wheat, rapeseed 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 details of their limitations):

• Cultivated plants that produce flowers usually provide a food source for pollinators over a very short period of time. This is particularly true of fruit trees and cereal crops. Weed flowers produce pollen for pollinators outside of the flowering season for crops.
• Crop pollination is facilitated by the presence of flowering weeds (such as wild oat, poppy, wild pansy, and cornflower).
• Weeds contribute to biomass production and CO² fixation in the soil.
• The rhizosphere of weeds is the site of multiple biological reactions such as atmospheric nitrogen fixation and the release of phosphorus from the soil. This characteristic is not necessarily very useful, as the rhizosphere of many cultivated plants produces the same benefits.
• Weeds produce food and shelter for invertebrates associated with crops. Weed seeds are a source of nutrition for certain insects, particularly carabid beetles that prey on various pests, and probably for earthworms.

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

Therefore, the destruction or preservation of weeds near crops must be assessed on a case-by-case basis. Maintaining weeds is incompatible with certain biocontrol techniques such as insect nets (see the chapter on biocontrol methods; insect nets).

Some books and articles on agroecology sometimes state that weeds should not be destroyed because they are a source of fresh organic matter. However, the production of humus by hoeing weeds is often mediocre. Weeds that reach physiological maturity contain more carbon than when green, but if weeds are allowed to mature, there is a risk of seriously reducing yields and causing deficiencies in exportable crops.

The limitations of mechanical weeding

Furthermore, manual or mechanical work is not effective against certain perennial weeds with rhizomes, such as creeping bentgrass, curly dock, field bindweed, thistle and horsetail. For example, the tuberised taproot of rumex is very resistant even after uprooting, as small pieces of the root collar left in the soil attached to rootlets are capable of regrowing.

It should be noted that, according to the Gironde Chamber of Agriculture, to cite one example, switching from chemical weed control to mechanical weed control under the rows generates an average additional cost of €500 per hectare (six to seven hours of extra labour, purchase of equipment, diesel consumption) and represents more than 17% of the cost price of bulk wines (6).

Advantages and disadvantages of mulching to reduce weed pressure

For large areas of vegetable crops, plastic mulching is used on certain crops (lettuce, shallots, melons, etc.) to limit weed growth; this is an easy process to implement at planting time and can also be used in vegetable gardens. However, it should be noted that this process has a disadvantage. It limits gas exchange between the soil and the atmosphere.

The active zone located in the top few centimetres of soil needs oxygen, and any obstacle that hinders gas exchange with the atmosphere has an impact on soil biodiversity. It is not only plants that are affected by the lack of light. Microorganisms such as cyanobacteria, which thrive on the soil surface, are also affected. On the other hand, mulching has the advantage of reducing water evaporation. In the Provence-Alpes-Côte d’Azur region, mulching is often practised in certain crops such as melons to reduce weed growth and limit water loss

While plastic mulch facilitates better weed control, it is also important to bear in mind that recycling it is problematic. There is another option. It is now possible to install compostable and biodegradable woven mulch with a lifespan of approximately 36 months. This mulch can be purchased by clicking here.

Is it normal for a vegetable garden to have a lot of weeds?

A properly prepared garden soil rich in nutrients inevitably encourages the growth of weeds, which appear mainly in summer. Birds and the wind constantly deposit weed seeds. The frequent regrowth 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 precipitation 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 all the weeds that have been cut within a few hours.

Herbicides approved for use in biocontrol plant protection products

Weed killers derived from living organisms, which are 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 broken down by microflora. Unfortunately, they are less effective than chemical weedkillers and are useless for permanently destroying tough weeds such as bindweed, horsetail and couch grass.

However, these weedkillers can still be useful. They are described in more detail by clicking here.