Integrated Weed Management: The Pros and Cons of using herbicides

All of our members use herbicides as a front-line tool for invasive weed control. The targeted use of approved chemicals offers an effective, low cost option (for control) which enables property owners to achieve their personal, legal and/or environmental objectives.

But it’s not the only way to deal with invasive weeds. Indeed, all professional users of herbicides must ensure that each and every contract is preceded by a formal assessment of the options available, a so-called Integrated Weed Management Plan (IWMP), and this has been ‘best practice’ as reflected in the Plant Protection Products Code of Practice since 2012. But the HSE/Defra didn’t give direct guidance on what such a plan should look like…

How do you do an Integrated Weed Management Plan?

Now, under the Government’s ongoing (Pesticides) National Action Plan Review, it is clear that our members will need to formalise their arrangements and, since the Amenity Forum recently published some guidelines for its members, we thought it would be useful to run through an example for an invasive weed species like Giant Hogweed – which seems to be ‘in the news’ all the time at the moment!

1. Justification/project objectives

Before starting we need a clear justification for controlling the target weed and establish objectives (level of control desired).

This could be the desire of the client to have less (or no!) dandelions in their lawn but obviously, if the target is both listed as a Schedule 9 invasive species AND is harmful by skin contact (via photo-sensitisation which can result in a severe rash or even burns) this makes the decisions more firmly based on practical needs rather than aesthetics. If the plants are in an area of public access the land owner also has a duty-of-care to ensure public safety.

2. Review available control methods

There are only a finite number of ways in which weeds can be controlled. In each case we need to consider both what is practical/feasible/possible and the cost, in financial terms and environmental.

Lets look at a situation where the Giant Hogweed is (as typical) next to a river with fairly steep banks. These are the options:

Physical removal (cutting, pulling):

  • Access is difficult, safety considerations (slope, river)
  • Physical contact with the plants will require care and PPE (no volunteers) as will disposal of the green waste
  • Soil disturbance may risk putting sediments in to the river
  • Physical disturbance of the area (trampling) increases disruption of the native flora/fauna
  • Trampling (many technicians) on/off site can increase the risk of spreading seeds (unless good bio-security protocols are in place)
  • Overall efficacy likely good if a minimum 3-4 year program + monitoring is put in place

Heat/electricity:

As above plus:

  • Access/safety (bulky power source needs to be close by, high voltage and water are not compatible!)
  • Application equipment (hoses, cables) may damage surrounding vegetation
  • Overall efficacy likely fair but the size of Giant Hogweed plants may limit impact; unknown effect on tap roots
  • Some risk of hot foams etc. scorching non-target plants
  • Costs very high for small areas
  • Not suitable for volunteers

Cultural techniques etc.,:

  • Giant Hogweed easily out-competes natives so habitat management is unlikely to succeed
  • Mulching/Turfing after removal may suppress the seed bank but also prevent normal riparian natives from re-establishing
  • Biological control (species-specific pathogens and or herbivorous insects, mites etc.)
  • None currently available

Herbicide

  • Low disturbance (access for spraying via extension lances) also reduces risks of slips/trips
  • Glyphosate products are approved for use near water (when used as directed and, in England, with EA permit)
  • Soil half-times very short (non-residual)
  • Efficient use of manpower; still not suitable for volunteers though!
  • Efficacy very good (systemic) but, as above, a 3-4 year programme is required + monitoring
  • One spray per annum possible, especially after first year
  • Usually possible to mix chemicals off-site (avoiding the potential for spillage)
  • Some risk of overspray (careful nozzle selection can minimise)
  • Overall cost-benefit due to lower manpower, ease of access (less disturbance), efficacy

3. Waste minimisation & energy efficiency (cradle-to-grave)

A guiding principle here is to seek to avoid landfill use (soil scrapes only when necessary and consider stockpile management to exhaust the seed bank). Ideally, green wastes should be managed on site but the hazards associated with handling Giant Hogweed are not inconsiderable (mulching of green waste is usually precluded on safety grounds). The IWMP should also include a desk-top review of energy use (carbon cost).

Finally, herbicide wastes should be avoided altogether – as would be expected following normal best practice, i.e., herbicide spray volume mixed should = calculated dose/water volume required = zero waste for disposal.

Conclusion

So, that was just a quick run-through to give some idea what we think an invasive weed management plan might look like (for just one plant/situation).

The key is to ensure the plan is always specific to the site and the client’s needs and/or instructions. Put simply, the optimum IWMP for an specific site (SSSI) is likely to be different to that for a domestic/residential garden and remember:

  • IWMPs are an important part of the Amenity Standard
  • Client’s needs/objectives must be clear including their budget limits (the cost-benefits of herbicides may not be their priority)
  • As PCA members you have the knowledge, experience and skills to be able to weigh-up pros and cons and give ‘best advice’ to all your clients
  • As a desk-top exercise you can develop your own IWMPs for each species and be prepared to share it with your clients – help them understand that every approach, chemical or otherwise, has pros and cons

All-in-all, a great opportunity to demonstrate your professionalism!

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