Weeds for Wildlife: Rethinking Tidy Habits on Farm

What exactly is a weed? In basic terms, the term 'weed' refers to any plant growing in an unwanted location, whether that’s in a garden or a field where it competes with cultivated crops for light, water and nutrients.

Historically, weeds have been persecuted by governments and landowners alike, their unsightly appearance interrupting the tidy monocultures favoured by modern farming systems. The availability of relatively cheap and effective herbicides has made their control and removal all the easier. But what has been weed control's true cost to biodiversity? And what benefit does this control actually bring to the growing crop?

The legal landscape: which weeds must be controlled?

Under the Weeds Act 1959, certain species, such as creeping thistle, curled dock, and common ragwort, are classified as injurious weeds. Landowners and land managers can be reported to Defra if they fail to control these species. Ragwort's unfortunate reputation in particular is due to the compounds toxic to livestock, especially horses, if ingested in hay.

However, the situation is more nuanced than it first appears. Alsike clover, which contains similar compounds in lower concentrations, is celebrated in herbal leys and other seed mixes for its soil-improving properties, and groundsel also contains them. But we aren’t encouraged to dob in our neighbours if we see these plants growing on their land! In reality, horses will actively avoid fresh ragwort if they have sufficient other food in their pasture, problems usually arise when it's incorporated into preserved forage where the horse can’t identify it easily.

Ecological value of ragwort

Despite its bad press, ragwort attracts over 200 species of invertebrates, of which at least 30 are considered rare or endangered. It is a vital nectar and larval food source for species such as:

• Small Copper and Gatekeeper butterflies

• Cinnabar moth caterpillars

Likewise, spear thistle is a magnet for pollinators and provides seed for goldfinches and other farmland birds.

Likewise, spear thistle is an extremely popular source of nectar for pollinating insects, and provides valuable seed food source for goldfinches and other farmland birds.

The yield game: are all weeds worth removing?

Herbicide recommendations are often driven by the presence of weeds, but not all species or population numbers pose equal threats to yield in an arable situation. Consider the following comparisons:

Weed species	Plants/m² for 5% yield loss
Cleavers	1.7
Blackgrass	12
Thistles	17
Speedwell	62

Source: Pesticide availability for cereals and oilseeds following revision of Directive 91/414/EEC; effects of losses and new research priorities (HGCA) 2009

A field with a few speedwell or thistle plants is unlikely to justify the cost of herbicide application in the short term, especially when those same plants may offer valuable habitat or food sources for beneficial insects (think IPM!) and farmland birds. Although of course , while it may be ecologically beneficial to tolerate low densities of certain species in the short term, it’s important to balance this with a long-term view of weed ecology and population dynamics (e.g. seed return). A threshold-based approach, where herbicide applications are guided by species identity, density, timing and crop sensitivity, often delivers the best compromise between ecosystem services value and agronomic necessity. We need our farms to be resilient against outbreaks of problematic weeds, while sustaining biodiversity value by fostering suitably diverse weed communities.

Are we obsessed by tidy fields?

We have become increasingly conscious of the ‘appearance’ of our farms. Perhaps this is due to the ease with which we can tidy weeds with machinery and chemicals, as well as the speed at which hedgerows can be flailed compared to the labour-intensive task it would have once been. Many farms have margins and verges that golf course greenkeepers would be proud of, whilst others have the hedge cutter primed and ready for action on September 1st for the annual hedge flailing competition.

‘Scruffiness’ is equated with poor management, but this aesthetic tidiness comes at an ecological (and often unnecessary financial) cost.

Over-manicuring:

• Destroys overwintering habitat for beneficial insects

• Reduces cover for nesting birds

• Eliminates flowering plants that provide pollen and nectar

  
  

By contrast, tussocky grass margins and uncut hedgerows:

• Offer nesting and foraging habitat for yellowhammers, linnets, and other red-listed species

• Support predatory insects like ladybirds and carabid beetles that predate on crop pests such as aphids and slugs

Simple practice changes, like cutting hedges on a 2- or 3-year rotation or leaving margins uncut can make a huge difference for biodiversity. Cultivated margins can make use of the natural weed seedbank in the soil, providing all the benefits of weeds to farmland biodiversity but in a more controlled and tidy way than having them freely dispersed throughout our growing crops.

The garden no-mow May movement

This idea isn’t limited to farms. The “no mow May” movement (although not without its issues) encourages garden owners to let lawns grow, allowing flowering plants to flourish. Oakbank’s own Jamie Gunn embraced the concept and discovered over 50 orchids in a previously mown family lawn - a vivid example of the biodiversity we often suppress unknowingly!

Weeds as bio-indicators for soil

Traditionally, the presence of certain weeds and their condition was used as a diagnostic tool for soil health and nutrient excesses / deficiencies:

• Dandelions tend to flourish on compacted soil and may indicate compaction and calcium deficiency

• Docks thrive in in wet, poorly drained soil

• Groundsel, chickweed and bindweed growing healthily point to high nutrient levels in the soil

  
  

Understanding why these plants are thriving may reveal opportunities for improved soil management. If you are interested in finding out more, the "Weeds as Bioindicators: A Farmer's Field Guide", compiled from existing literature by Chris Maughan and Dominic Amos, Centre for Agroecology, Water and Resilience, University of Coventry, is available on ResearchGate.

Working with weeds

Our farmland biodiversity decline won’t be solved overnight, but small, thoughtful changes can help. By allowing some weeds to grow in managed areas we can:

• Support declining insect populations

• Feed farmland birds

• Boost natural pest control

• Improve soil health

Sometimes, the best thing we can do is leave the topper in the shed.

Frequently Asked Questions (FAQs)

Q: Are all weeds bad for crops?

A: No. Some, like blackgrass or cleavers, can be highly competitive. Others need to reach high densities to cause yield losses and may offer many ecological benefits at lower levels.

Q: What can I do to help farmland biodiversity?

A: Leave margins uncut, reduce herbicide use where economically viable, rotate hedge cutting and consider cultivated margins.

Q: What are cultivated margins?

A: These are strips of arable land left fallow or lightly cultivated to encourage naturally occurring weeds, offering biodiversity benefits without impacting the main crop.

Q: Can weeds indicate soil health?

A: Yes. Different weeds flourish in different soil conditions, making them useful indicators of compaction, drainage and nutrient status. If you are interested in finding out more, the "Weeds as Bioindicators: A Farmer's Field Guide" is available on ResearchGate.