Herbicides
Herbicides are the cheapest and most effective method of weed control. The combination of a winter application of pre-emergent/residual herbicide, and a spring and summer application of a translocated/foliar acting herbicide in spring and summer is widely used in commercial tree planting schemes. The foliar acting herbicide can be used as the residual product wears off, and also on any weeds which are resistant to the residual product. This leaves bare ground, to which the residual herbicide can be applied again to delay reinvasion. Using a translocated herbicide does have the disadvantage that it can only be applied to growing weeds, so by the time it takes effect, there has already been some interference with tree growth. The dead growth, particularly of grass, can form a useful mulch around the tree which reduces evaporation from the soil, and helps delay reinvasion, but may provide unwanted cover for voles.
The Control of Pesticides Regulations 1986 covers all aspects of the development, sale, supply, storage and use of herbicides. The regulations cover proprietary products rather than active ingredients, and classify these products according to approved use. ‘Professional’ products are formulated for use in agriculture, horticulture and forestry, and ‘amateur’ products are formulated for use in the garden. Amateur products may contain the same active ingredients as those in the professional category, but at lower strength, so there is less dilution required before application.
From the range permitted for amateur use, there are two products which are widely available from garden centres and tree growers, and which are suitable for use on small-scale tree planting schemes. For large schemes, or for groups undertaking regular tree planting, training in the use of professional products is advisable, as these are much more cost-effective than the equivalent amateur products.
When using herbicides, whether professional or amateur, follow the instructions on the label. Note the recommended dosage and the required weather conditions for effective use. Clothing and equipment should be correct and appropriate, as identified by the assessment of the chemical’s use.
Propyzamide (Kerb Granules) is a residual and foliar acting herbicide, although it is more effective as the former. It can be applied to bare ground to prevent weeds germinating, or it can be applied to weeds in growth and is translocated to kill the roots. It remains active for at least 12 weeks. As a residual herbicide, results are best if it is applied to soil with a firm, fine tilth. If applied to very lumpy soil, as the lumps crumble untreated soil will be exposed and weed growth will result. Rain is needed after application to move residual herbicide into the soil. Kerb Granules are available in a 120 tree shaker pack (£10.50 at 2002 prices), or in larger quantities.
Glyphosate (eg Roundup Biactive) is a widely used garden herbicide, which is applied by spray to the leaves. It is translocated through the plant, killing all of it above and below ground, and is effective on all grasses and herbaceous growth. It is inactivated and biodegrades on contact with the ground. Contact with tree foliage, shoots and bark must be avoided, or tree growth will be damaged. Glyphosate can be used at any time when vegetation is actively growing. The addition of a dye makes it easier to apply the spray evenly and effectively. Depending on weed growth, glyphosate should be applied in March/April, and again in August to benefit late summer tree growth. It costs about 5p per tree per application (2002 prices), and can be applied using a garden sprayer of one litre capacity or greater.
The sale and use of professional herbicides is strictly regulated. Products can only be sold to and used by holders of the National Proficiency Training Council (NPTC) Certificate of Competence. Professional products are only available from specialist suppliers, whose staff must also be certificate holders in order to advise on herbicide use.
Training leading to the NPTC Certificate of Competence is provided at agricultural colleges and other centres throughout the United Kingdom. Enquire through the NPTC or Lantra. For further information on training see here.
Information on herbicides for professional use in forestry and farm woodlands is given in The Use of Herbicides in the Forest (Forestry Commission, 1995) and Herbicides for Farm Woodlands and Short Rotation Coppice (Forestry Commission, 1996).
Mulches
Mulches are materials which are put down on the ground to suppress weeds and retain soil moisture by reducing evaporation. Mulches can be either sheet material, such as polythene, bitumen or various textiles, or loose materials such as bark, chipped wood or gravel. Most sheet mulch materials also increase soil temperature, which encourages early root growth in spring. Mulches, either sheet or loose, are not recommended for use on damp ground, as by preventing evaporation they increase the tendency of the ground to become waterlogged and anaerobic, which kills tree roots.
Proprietary sheet mulches
Sheet mulches are available either in a roll, for hedge or shelter-belt planting, or in pre-cut squares or mats to fit around individual trees. Sheet mulches need to be substantial enough to last at least two years, thick enough to resist wind or animal damage, and large enough to be effective. The sheet mulch tends to invigorate the growth of grasses around its edge, so a mat smaller than 500mm square will be ineffective. The larger size mats, usually 840mm, 900mm or 1m square, are recommended.
The cheapest mats are made of black, single-sheet polythene, but are not sufficiently durable. The ‘Thermat’ is a multi-layered anti-tear polyethylene mat which is durable for at least two years, and is effective in raising soil temperature. It degrades after about three years, so no clearing up is necessary. Other mats are available made of mixtures of wool, jute and hair, or flax and hemp. Some mats are supplied with a cross-shaped slit in the centre, and others have a slit from the edge to the centre, making them easier to fit to multi-stemmed shrubs. Depending on type and quantity ordered, expect to pay between £1 and £2 per mat.
Mulch mats can be fitted directly over short mown grass, into which trees have been notch planted. Otherwise, fit the mat onto bare soil. Do not try and fit the mat over long, tufty grass. The mats are normally fitted after planting, by carefully threading the growing tip of the plant through the slit, and then anchoring the mat. The mats should be fastened at the corners by pushing down into a slit in the soil, or by pegs available from the supplier. Suppliers will recommend the best method for particular products. Non-biodegradable mats can be further anchored by stones, gravel, upturned turves or a small amount of soil, to weight down the mulch and discourage animals from disturbing it. Don’t use too thick a layer of soil, or weeds will establish in it. Biodegradable mats should be anchored by pegs or stones, and not by covering with soil, as this makes the material degrade more quickly.
Mulch mats need checking at least once during the growing season. Weeds can grow up through the gap around the stem. Mats may be disturbed by vandals, or torn by the wind, and may need replacing during the initial three year establishment period. Voles may use the shelter provided by the mulch and then damage the tree stem, and foxes may tear the mulch to get at the voles! Some fibrous mats may be damaged by large birds seeking nesting material.
Loose mulch materials
Loose mulch materials such as composted woodchips or bark can be used for mulching, although they are not as effective as herbicides or sheet mulches in controlling weeds. As the material breaks down it is incorporated into the soil, which has the benefit of adding to the organic matter content and improving soil structure, and may introduce beneficial mycorrhiza. However, a layer at least 100mm (4”) thick is needed, and the mulch will need replacing every year, as it becomes incorporated into the soil. Addditional hand weeding will be needed at least once during the growing season. Mulches can be used in combination with herbicides, for example in early spring after a winter application of residual herbicide. This reduces herbicide use during the summer, and gives the soil benefits of mulching. The mulch also looks more attractive than bare, ‘weedkilled’ ground. If the whole tree planting area can be mulched, this will speed tree establishment and the formation of a woodland-type ecosystem.
The mulch should cover an area at least 1m diameter. Keep it away from the stem, or rot may result. Mulch can be laid at the time of planting, or in very early spring, before growth starts again. Replace annually in winter. Continue until the canopy starts to close over and the trees produce their own mulch of fallen leaves.
Loose mulch materials are only worth using for tree planting schemes if they can be acquired cheaply in bulk, or home-produced from woodchipping. Woodchips, bark, leaves and other organic material should be composted for about six months before use, or the young trees may be deprived of available nitrogen as the material decomposes. See page 88 for information on woodchippings. Leaf mould is a useful and beneficial material. Leaves swept from roads and parks may be available for free from local councils.
Hand weeding
Hand weeding is labour intensive, and only suitable for small planting schemes. As in a garden situation, weeding will be needed regularly throughout the growing season. Avoid using any tools near the stem of a tree, but hand-pull weeds instead. Where grasses have taken hold, it’s generally best to use a contact herbicide to destroy the sward, and then try to keep the area weed free by mulching or hoeing.
Weak herbaceous growth such as nettles, creeping thistles and many tall annuals and biennials can be hand pulled, but make sure this doesn’t then allow the growth of grasses, which will be much more damaging. Pulled material that hasn’t gone to seed can be spread around the trees to act as a mulch. Larger areas of nettles, thistles or bracken can be trampled, which suppresses rather than stimulates growth, but may need to be repeated a few times through the season. Cutting with a grass hook must only be done with extreme care, because it is so easy to damage the young trees. Bracken can be ‘whipped’ with a stick as the fronds open, to weaken the growth, but beware of damaging the trees.
Bracken can be a problem in old pasture woodlands and parks. One of the best control methods is to trample or roll the growth in July, which flattens and bruises the stems, but does not sever them. This encourages water loss from the plant, and prevents re-supply of nutrients to next year’s dormant buds. Various commercial rollers are available which can be towed by small tractors and four-wheel drive vehicles (Enact Vol 5 No 3). A deep accumulation of bracken litter can be a serious fire risk on tree planting sites, and should be raked away before planting is carried out.
Inter-row vegetation
The 1m radius circle around each tree must be kept free of weeds. At 2m tree spacing, this weed-free area amounts to about one sixth of the total area. The remaining area between the trees can be left to grow up, or can be mown.
Leaving the inter-row vegetation to grow up unmanaged is useful for hiding the trees from vandals, and for discouraging unofficial access across the site. On exposed sites, it may help shelter the young trees from wind. However, you need to check that growth does not get so tall that when it collapses in late summer, it crushes the young trees. This can be a problem if there is a wet spell in late summer. Voles are more likely to be a problem in long grass, so vole guards may be needed. Tall vegetation can provide useful habitat for ground nesting birds, and may include plants which add to the wildlife value of the site.
Where neatness is important and vandalism unlikely, mowing is an option. This prevents brambles, thistles or other troublesome weeds establishing, and discourages voles. Take great care when mowing, brush cutting or strimming, and never be tempted to cut near the tree stems, even if there is weed growth there, as damage to the stem in inevitable.
Long-handled scythes are not recommended for use near trees, and short-handled grass hooks should only be used with extreme care. Shelters and guards make it easier to see the trees, but are not robust enough to protect trees from a sharp tool.
Techniques to encourage the development of a woodland flora are discussed in Chapter 7 – The young woodland.
