The aim of this article is to give an introduction to the various herbicides available to Irish foresters. most are used in the establishment of forests, principally to give that all-important weed-free spot of 1m diameter for the first few years after planting.

Some few may be used in forest nurseries, for the reduction of vigorous growth or even for the destruction of trees. As this article is an introduction, it will be necessary for the end user to identify and evaluate the particular silvicultural problem to be overcome and then select the herbicide to accomplish this.

The method of application also may determine to some extent the product chosen. Applicators are the subject of an article printed April 1996. The dose rates and time of application also are for the user to determine with the aid of the product label. No hard and fast rules can be given, as each situation to be assessed is different.

All pesticides which are permitted to be marketed in the Republic are contained in the publication, "Pesticides 1994" and its amendments and only those with approval for use in forestry (and also in general popular use in Ireland) will be discussed.

There are various jargon terms and technical phrases which sometimes seems daunting so these will be explained now


          a.i. Active ingredient - That part of the pesticide which actually performs the function. Many formulations contain inactive ingredients (the carrier) which help in the handling of the product. (Rather like mixing "weightless" tree seed such as spruce with sand to make it easier to handle). Can be expressed as ml per litre, g per kg, g/litre etc. or as a % weight/weight or weight/volume.

Chemical name - The full unique scientific name for the a.i. which is generally unpronounceable or unspellable. Rarely used, as an accepted short name is available.

Clearance No. - A product's number in the Pesticides Control Service list. No product can be sold in Ireland without such a listing and number. The listing also approves the packaging. No product may be re-packaged into a non-approved container displaying a non-approved label.

Common name - Generally accepted short name for an a.i. which may use parts of the chemical name in its construction.

Contact (herbicide) - One which kills the living plant at, or near the point of contact, but does not translocate.

Herbicide - A product used to kill plants, generally by disrupting growth patterns.

Product name - The name under which various manufacturers market their own formulation (generally trade marked). Even though two products may have the same percentage of a.i., they will each receive a unique Clearance/Notification No.

Residual (herbicide) - One which lies in the soil and continues to kill the target plants for a period after application.

Selective (herbicide) - Kills target plants and leaves crop trees and other plants unaffected.

Soil Active (herbicide) - One which is taken up through the roots. Another term for residual.

Total (herbicide) - Kills all plants with which it is in contact. Total herbicides can, however, be applied selectively (where the degree of care and skill necessary warrants it).

Translocated (herbicide) - One which enters the plant generally (but not exclusively) through the green parts and then travels throughout the plant to kill deep tap roots or rhizomous "runners".

The various herbicides will be summarised with regard to vegetation type controlled and type of action (foliar or residual).

Application rates vary according to the mixture of non-desirable plants on site and also soil type, water table and desirable trees to be retained.

The various formulations marketed also have differing concentrations of a.i. which will also affect application rates.

It is for herbicide specifier to take all of these factors and ecological considerations into account when selecting a herbicide regime.

Active Ingredient Site Type Vegetation Controlled Contact Action Translocated Residual Notes
Atrazine Soft Grasses, Coarse Grasses/Light Rushes   Not suitable on peat or waterlogged soils.On step slopes may run off and collect to high concentrations at base of slope.
Dalapon Grasses     Approved for aquatic use.
Dicamba Bracken   Can move sideways. Must be applied as narrow band furthest from tree rows.
Dichlobenil Grasses broadleaved weeds     Not suitable for peat soils.
Diquat Annual weeds     Rainfast in 15 minutes.
Fosamine Ammonium Scrub,woody weeds     Suitable for use near water.
Glyphosate Grass, rush, broadleaved, weeds, bracken, bramble, heather      
Impazapyr scrub, bracken and woody weeds, heather    
Isoxaben Annual broadleaved weeds     Not suitable for peaty soils.
Mecoprop Annual and perenial broadleaved weeds     Not suitable on peat.
Simazine Grasses, annual broadleaved weeds      
Terbuthylazine Soft grasses and annual broadleaved weeds some    
Triclopyr Perennial broadleaved weeds and woody weeds (bramble, furze, rhododendron) foliage stems root Application on hot days can lead to Volatisation
2, 4-D Ester Annual and perennial broadleaved weeds, heather   some  
Ammonium Sulpharmate All weeds especially rhododendrons, usually to prevent coppice regrowth (cut stump treatment)     Corrodes all metals.
Asulam Bracken & docis      
Diuron Grasses and annual broadleaved weeds      

Many of the above are formulated by the manufacturers as mixtures to give dual action and dual purpose herbicides.

As the chart above is provided to give an introduction to the various herbicides produced for forestry use, the various manufacturers must be contacted for detailed application rates and suitability for ground conditions and crop tolerances. Care must be taken in this decision process to ensure that the undoubted benefits of weed competition reduction by herbicides are not negated by crop tree death. The importance of this decision, sound properly maintained equipment and properly trained personnel cannot be over emphasized even to the extent of contradicting myself (para 2). There is a hard and fast rule;

          Read the label.

          Follow instructions given.

          Use herbicides carefully.





1) Guidelines for the Use of Herbicides in Forestry, Coillte, Declan Ward 1995.

2) F.C. Field Book 8 Use of Herbicides in Forest. Ian Willoughby and Jim Dewar 1995.

3) Availability, Suitability and Cost Effectiveness of Herbicides in Forestry, Diploma submission 1984.

4) Cost per Plantation Hectare of Different Weeding Methods 1984-5. F.C. Work Study 1986.

5) Pesticides 1999 HMSO.

6) Pesticides 1994 Dept. of Agriculture, Food and Forestry.

7) The UK Pesticide Guide 1999, British Crop Protection Council.