"If a site has any potential for the production of trees, it will already be colonised by the plants we call weeds. These plants will utilise the available supplies of nutrient, water, light and space to the detriment of the plantation."


Irish Timber Forestry asked David Conroy, owner of Contracked Lands for information on weed control. David, who holds Diploma in Forestry, D.M.S. and is a BASIS advisor on herbicides, entered forestry in 1974 and worked for the British Forestry Commision in Coniferous Harvesting. He was subsequently transferred to Development Corporations and worked in establishing amenity plantings of up to 650,000 plants per annum. In 2001 he set up his own company which sells consultancy, surveys and also are herbicide application contractors. 


David writes:

The usefulness of herbicides in reducing the competitive effects of weeds on forestry plantings is not in doubt. What then of the various methods of application utilised to apply the herbicide? These are many and various and, although sometimes dictated by the product used, are generally adaptable to various methods of application. In writing this article, I have assumed that machine applicators, whether wheeled or tracked, are unsuitable for reasons of topography, tractability, availability or cost.

As with species choice, to maximise the site potential in relation to the preferred planting species, so the method of application needs to be fine tuned to suit the site location and ground conditions.



When droplets are created by ejecting herbicide through a nozzle, a wide range of droplet sizes are created. The larger droplets fall to the ground rapidly and the smaller ones can drift beyond the target species to waste and possibly cause damage. The spinning disc of the CDA creates uniform droplets of a desired size to optimise chemical uptake. By using small droplet sizes, incremental spraying can be adopted whereby the drift effect is utilised to create swathes or bands of spraying, which overlap across an area to fully treat the whole area. Different spinning heads can create larger/heavier droplets, which can produce swaths down to 10cm wide.

Pros and Cons: Specifically formulated herbicides allow the operator to avoid diluting or mixing concentrates. Light weight is specified on the uneven terrain experienced by pedestrian operators. Accurate calibration is required and the sprayer may be fragile in the forest environment.



These range from the very simple 1kg screw-top container with holes drilled in it and the purpose made "fishtail" 5kg container to the gravity hopper, which discharges a fixed quantity of herbicide, and the 2 stroke engine powered applicator.

Pros and Cons: First two methods simple and cheap. Calibration difficult to maintain, however. With personal experience of herbicide to be applied at 3g/m2, I found this quantity was invisible to the operator. On applying the product until it became visible, operators have later found the trees had become invisible! The fixed quantity hopper type delivers 3.8g of specific product with calibration not required and overdosing not possible. The motor powered applicator type ensures even application but can be heavy on poor terrain and requires calibration. It also needs fuel, tools for maintenance and mechanical skills.



These are more accurately termed hydraulic nozzle sprayers and there are many brands and models. They are simple and robust in construction and used widely throughout the world. Generally they hold 20 litres of diluted herbicide, although most sprayers also come in the more manageable 15 litre size. Knapsack sprayers can be used for the complete swath or, using a suitable guard, individual application.

Pros and Cons: Must be supplied with clean water in large quantities, which may be a problem in remote sites. 20 litre capacity leads to operator imbalance on rough terrain and fatigue on all terrain. Can also be used for cut stump treatment. Accurate calibration required.



This is another form of hydraulic nozzle sprayer, but uses low volumes of pesticide creating individual spots of up to 1.2m in diameter. The 5 litre back-pack avoids operator fatigue and imbalance and calibration is easily undertaken in depot as the measured dose is not dependent on operator walking speed.

Pros and Cons: Easily used in rough terrain. Not designed for overall or band spraying. Can be used for cut stump treatment or chemical thinning. Can also be used for tree marking using household paint diluted with thinners, thus avoiding expensive aerosols and their propellants. Ecologically sound as only minimum of herbicide is used.



These can be used for selective or non-selective application, selective being used in the physical rather than chemical sense. In general appearance, the weedwiper is like a floor sponge but with a special head and hollow handle to contain the pesticide. When the desired species is overtopped by weed, the weedwiper is used to apply a total herbicide to the non-desired species, ie. selectively diluted herbicide is applied via a rope wick, which is fed from a small reservoir, and usually a red dye is included to aid identification of treated plants.

Pros and Cons: Lightweight in use. No calibration required. Cannot use wetable powders as capillary action of wick also acts as filter. May not be robust enough for arduous forest conditions. Ecologically sound as herbicide is only applied to weeds with no run-off.



The recommended retail price of all the non-motorised applicators is less than 100, the pepper-pot and fish tail considerably less. Taking an average usage of 50 days per annum pre and post planting and an output per day of between 2 and 4ha, the cost of a 100 applicator will be from 1 to 50p. per ha. a marginal cost when compared with the cost of labour for one day or herbicides for one ha.

Comparative costings are shown in bar chart form, with the three physical/mechanical methods for comparison only. Due to the danger of contact damage, the cutters cannot remove the weeds closest to the trees, thereby reducing the initial effectiveness. Additionally, if the grass or weed is cut before it seeds it will regrow, effectively making the duration of competition, even longer each year than if left uncut. If physical/mechanical methods are adopted, they may be required for up to six years. If herbicides are adopted for the first two years, no more treatment should be necessary, as the plants rise above and finally overcome competition.

With all applicators, allowance should be made for wear and tear, breakage and loss of parts on site. A figure of 20% of the purchase price should be a standard allowance per annum, if the equipment is maintained to original specification, the savings in accurate chemical application will more than cover this small outlay. After the initial outlay, the cost of using the equipment will obviously reduce in succeeding years. This would lead to an annual cost of 20% reducing to 10% of purchase price.



By controlling competitive vegetation before planting, and for two years after planting, the transplants will thrive rather than survive. Losses and consequent beating up costs will be lower. More plants will be available from which to select final crop trees. If rotation is determined by size/price ratio, the rotation period will be reduced by virtue of transplants not having an extended period of check and also having a greater percentage of dominants in the plantation. The risk of fire spread will also be reduced and vermin damage will be more visible, leading to earlier corrective action.

All these benefits have a marginal additional cost in terms of net discount revenue but, with a shorter rotation, these costs will accrue for a shorter period and the final crop will be more valuable. As previously stated, the final choice of applicator is subject to many variables, as is the choice of herbicide. With forestry planting in Ireland now approaching 30,000ha. per annum, the competition in forty years time with other produce of yield class 24 will be intense. Whoever gets to the market first will obtain a premium over the competitors. Finally, 25% of forestry grant is payable upon satisfactory establishment. Is it worth foregoing a minimum of 400/ha. at year four for an outlay of 30/ha. before planting and for each of the two following years?

Irish Timber & Forestry, April 1996