Types of weeds

Weeds: what they are and why they are not so terrible

A weed is any plant of a wild nature that grows in areas controlled by humans, such as crops, orchards or gardens, and whose presence is undesirable. Although they are associated with negative aspects, which we will review below, they also have some advantages, e.g. for the regeneration of biodiversity in cities.

Weeds are highly competitive, which in some cases can reduce the performance of other plants or crops.
Weeds are highly competitive, which in some cases can reduce the performance of other plants or crops.

Weeds have a bad reputation, sometimes unjustly so, because not all weeds are weeds. In fact, although their negative effects are undeniable, for example on certain crops, they are edible, medicinal and some even contribute to regenerating biodiversity in cities. In order not to be misled by prejudice, we will now take a closer look at these plants.

What are weeds

Weeds, also known as undergrowth, are simply plants that grow in places where humans do not want them to grow, usually fields, pastures, orchards and gardens. Therefore, a priori, their greatest sin is to be born at the wrong time and in the wrong place.

And if they are where they are not supposed to be, how do they emerge? The answer has a lot to do with their reproductive capacity, i.e. they themselves find a way to generate thousands and thousands of seeds that are dispersed, either by the action of the wind, animals, slurry — organic waste with the ability to ferment — or by the sowing itself.

In fact, weeds are characterised by the extreme ease with which they can emerge and multiply year after year in the same places thanks to their sophisticated reproductive systems and their enormous competitiveness, so much so that they sometimes reduce crop yields by stealing water, nutrients or light. They can also interfere with agricultural structures, such as water channels or harvesting processes.

How to eliminate weeds

Can weeds be killed? Although it is not easy, the answer is yes, at least temporarily. Preventive methods, according to the experts, are the most effective and for this it is essential to know the enemy, i.e. to know where, when, how and why they appear in order to be able to anticipate them. If prevention does not work, there are agronomic (ground cover), mechanical (harvesting), biological (grazing) and chemical (herbicides) methods to eliminate them. Some of the latter can be harmful to the environment or even to humans if they are misused.

The most common weeds

According to a guide, opens in a new window, on weeds produced by FMC Corporation, an American company specialising in agricultural solutions, there are two types of weeds:

  • Monocotyledons: narrow-leaved, they include 13 species of grasses and one species of liliaceae. Both are characterised by their single cotyledon, their pivotal roots, their leaves with long, parallel-veined sheaths and their inflorescences in panicles or spikes. They are also highly competitive in winter cereal crops.
  • Dicotyledons: broad-leaved, they include up to 49 species and also show a large presence as invasive species in winter cereal crops. They are characterised by their two visible cotyledons in the seedling stage, their fasciculate root system, their petiolate leaves of various shapes and palmate or pinnate veins, and their inflorescences of various morphology.


In addition to the aforementioned competition for water, nutrients and sunlight, which reduces crop yields and even crop quality, with consequent economic damage to the agricultural sector, weeds can cause the following negative effects:

 By their mere presence, they can hinder farmers' irrigation and harvesting operations, causing great damage.

 They can provide an ideal habitat for the proliferation of pests and diseases detrimental to the crop.

 Its excess can help create a microclimate favourable to the development of fungal diseases.

 Certain weeds are poisonous and their presence in pastures can pose a danger to grazing animals.

 Some, such as wild mint or poppy, can also affect the mycorrhiza (symbiosis between a fungus and the roots of a plant), deteriorating or destroying it.

 They can cause allelopathy, i.e. the production of toxic substances that inhibit the germination or growth of neighbouring plants.


As we mentioned at the beginning, not all weeds are weeds. In fact, for some experts, weeds bring more positive than negative benefits. Among the former, they point out the following: they protect the soil from erosion, improving its structure and providing organic matter; they generate, in many cases, a microclimate favourable to crops; and they improve biodiversity, harbouring beneficial fauna such as pollinators.

In addition, weeds are noted for their ability to regenerate urban environments, which is essential in parks and green corridors. Despite the fact that cities, especially large ones, are often inhospitable places for vegetation, these weeds find a place to grow, becoming a source of pollen and accumulating heavy metals to reduce pollution. Other points in their favour are that some of them are edible, e.g. nettles, nettles or wild chard, and others are medicinal, such as milk thistle or dandelion.