To many gardeners, weeds are a recurrent annoyance that you have to put up with as a fact of life. They can interrupt your otherwise perfect lawn or stifle your crop yields, and at worst cost thousands of pounds in damage as in the case of the Japanese knotweed. The effects of weeds are extremely costly, and it has been estimated that up to 10% of agricultural production may be lost because of them. But, weeds play a key role in transforming inhospitable environments into new habitats, and without them we would not exist today.
Weeds are good for the ecosystem
Weeds are important as they play a key role in transforming barren earth into rich fertile soils. They are, in effect, pioneers as the first plants to colonise a piece of land and improve its soil for the development of more complex ecosystems. They do this in a number of ways.
Weeds act to shield the soil from the sun, protecting both insects and microscopic organisms from sunlight. Their roots stabilise the soil, creating a secure environment for life, while their stems trap organic matter, which breaks down in the soil and provides sustenance for insects. Weeds with long roots draw up nutrients from deep in the ground, improving the quality of the surface soil. When they finally die, they decompose into humus which increases the soil’s moisture and nutrient retention, and well its bulk density, which is important in the early stages of soil development.
Back hundreds of millions of years ago, the Earth was very different, as a barren rock with water running over the surface with no defined course. Key to transforming the Earth were plants that broke down rock into minerals and soil, which it then held in place with its roots. This led to the development of river banks that channeled water in a regular fashion. Periodically, such rivers would flood, depositing sediment over large areas, which allowed trees to take hold. Such larger plant life would produce even more debris that would block up rivers, causing more flooding, a process that would lead to the emergence of larger complex ecosystems.
The predecessors to the plants that we consider weeds today played a key part in all this as early pioneers that ensured soil stability in such flooded areas. Important to this were rhizomes that allowed plants to cope with severe disruptions in their environments. Rhizomes are branching stems that grow horizontally, often through the soil, and are the feature that makes weeds so durable, as even if you destroy a plant’s matter above ground any surviving rhizome in the soil will lead to its reemergence. Not only does the rhizome store energy, allowing a plant to reemerge in favourable conditions, the stems allow the plant to propagate vegetatively, producing a clonal plant.
One early example of a plant that helped stabilise the earth’s environment was the Drepanophycus from the Devonian Period, which was unearthed by a team from Peking University. It was discovered preserved in paleosols – fossil soils – that within were multiple sequences of sediment formed by river channels, which were periodically wiped out by floodplains. The plant grew continuously due to its rhizomes and trapped sediment, enabling stable soils to develop. And after the floods, the plant would reemerge, growing through the newly deposited layers of sediment. The team calculated that the plant had a modest, but significant role, in reducing soil erosion. It is believed to have carried out this function for centuries.
Weeds constitute an interesting case study in evolution and humanity’s effects on the environment
Today weeds constitute a fascinating area of study due to their phenotypic plasticity, or simply put, their ability to change in response to changes in their environment. An example of phenotypic plasticity may be a plant’s ability to utilise more or less water (in photosynthesis) depending on its availability. Phenotypic plasticity is especially important for plants that do not have the ability to change their environment (as in the case of many animals, such as humans), and weeds are especially adaptive as agricultural practices make it necessary to be highly responsive if they are to survive.
Weeds evolve quickly in three principal ways: through adapting to continuous habitat disturbance, emerging in part from agricultural practices; through reproducing with different cultivars (groupings of plants selected for certain characteristics) as to produce hybrids; and finally through returning to natural seed dispersal methods when certain domesticates (plants dependant on humans for survival) are abandoned. This has led to the survival of certain species that are extremely difficult to control as they have developed such traits as early germination, rapid growth from seedling to sexual maturity, and the ability to reproduce both sexually and asexually. Fascinatingly, a 2013 study carried out by Fudan University of Shanghai found that if genetically modified crops did crossbreed with their weedy cousins, the resultant weeds would have higher rates of photosynthesis, more stems and flowers, and significantly more seeds. So, in the future, weeds may become even more troublesome than they are now.
As such, the battle between farmers and weeds constitutes an interesting case study of evolution in action and the selection effect humans exert on plants. There are many examples of the latter. For example, tilling tends to favour annuals at the expense of perennials, while no till systems benefit perennials. Frequent mowing, on the other hand, tends to benefit weeds that grow horizontally. The grazing of livestock has led to an increase in noxious thistles and other inedible species on the rangeland. In some cases, weeds have even begun to replicate crops in their appearance and life cycle as in the case of barnyardgrass growing with rice.
Weeds perform an important signalling function
Weeds can tell you a lot about your garden, providing information about what is best to grow. If your weeds multiple rapidly it is likely that your soil is extremely fertile, and that you do not need fertiliser. If not, it may be wise to start growing forerunners such as onions before moving onto more difficult crops. If the amount of weeds is diverse, it is likely that you can grow a wide range of plants in your garden. If not, it will be worthwhile to ascertain the soil type. And weeds can do this too. Very acidic soil will produce sorrel and plantain but no charlock or poppy, while chickweeds is sign of neutral pH. High levels of nitrogen can be ascertained by nettles, ground elder, fat hen and chickweed. Compacted soil is noticeable for silverweed and greater plantain, while creeping buttercup, horsetail and silverweed may indicate wet soil with poor drainage.
Weeds constitute a good source of nutrients
Many weeds are edible and good for you. They are also effectively free and environmentally friendly. In the UK, nettle soup comes to mind as one famous example. Back in the Middle Ages, ground elder was grown as a crop and was believed to cure gout – hence its alternative name goutweed. It possesses a nutty flavour and can be added to salad. Many health blogs recommend dandelion as a superfood, which can be found everywhere. Sorrel and horseradish can both be made into sauce and the latter is often used with beef. There are many great blogs dedicated to eating and cooking wild food. Why not check them out for yourself?
A concluding thought
Perhaps, our obsession with weeds tell us more about ourselves than we think. Why are we pursuing them with such vigour? Instead of hastily striving for a perfect world without weeds, perhaps we should examine why they are there in the first place. After all, a weed is a plant whose virtues have yet to be discovered (Emerson, apparently).
Jorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!
His ideal afternoon would involve a long walk, before settling down for scones.
Jorge is a journeyman gardener with experience in growing crops.