Weeding

Newly planted trees require a weed-free area of at least one metre diameter around the stem, for at least three years after planting. This area can be kept weed-free by herbicides or mulches. Hoeing or other hand weeding can also be used, but is much more labour-intensive.

The frequency of weeding and the type of control used will depend on the situation, soil fertility and other factors. The weed problem will be worse where soils are fertile, or where there is a large bank of weed seeds in the ground or spreading from nearby. Weeds, and especially grass, will grow more prolifically in unshaded sites. young trees planted in woodland clearings will need to be kept free of brambles, nettles and herbaceous growth, but in general weeding will be less of a problem than in open situations.

Fertile, open sites, and particularly pastures, arable fields and other farmland will have high fertility and may have a large seed bank, and are likely to require the most frequent weeding and for the longest number of years.

How weeds compete

Weeds, especially grasses and clovers, compete strongly with young trees for water, light and nutrients.

At the early stage of growth, young trees rely on moisture in the soil’s surface layer. If weeds are allowed to grow up around the base of a tree, they will compete strongly for this water by drawing it up and transpiring it through their leaves. During dry weather, weeds continue to draw up moisture from deeper in the soil. An unweeded tree avoids the moisture stress caused by this competition by reducing its area of foliage, for example by producing smaller leaves. This reduces the tree’s capacity to photosynthesise, which in turns slows root growth. As root growth is needed for the uptake of water, the unweeded tree will be caught in a vicious circle, and will die back or fail.

On an area of bare soil, the rate of water lost through evaporation will be much lower, and the tree will be much less stressed. Evaporation will slow further during dry weather, when a skin of dry soil forms at the surface. An exception are clay soils, which crack and dry out during drought. A thick covering of mulch will reduce cracking and help retain moisture.

Nutrients are only available to the tree through the mechanism of water uptake. Thus, a tree which appears to be suffering from a nutrient deficiency in the soil, may actually be suffering from competing weeds taking up most of the available water and nutrients. Where fertiliser is applied to young trees, it’s even more important to keep a weed-free zone, otherwise the extra nutrients will merely stimulate the growth of weeds, at the expense of the planted trees.

Regular mowing or strimming around the base of trees is not a method of weed control, as repeated cutting simply stimulates grass growth, and makes moisture and nutrient deficiencies even worse. Mown grass is therefore more damaging than long grass, and causes further problems, as damage to the bark from mowers or strimmers is almost inevitable. As well as the physical damage to the stem, disease often enters through damaged bark. Mown grass is the worst way of managing the ground around newly planted trees.

Mown grass can also compete with older trees. Trees that have been growing for many years in grassland and which are not thriving can sometimes be encouraged into growth by killing off the surrounding grass.

In some situations weed control does not have to be total. Too heavy a weeding of woody and herbaceous growth should be avoided, as these weeds do not compete so strongly for water and nutrients, and their removal is likely to encourage grasses to establish instead. In some cases, tall, sparse weeds such as bramble and nettles can be beneficial, as they protect trees from drying winds and damage from deer, hares or vandals. Periodic checks are needed to make sure they are not interfering with tree growth. Some plants collapse as they die back in autumn, and need clearing before they smother the young trees.

Bracken should be controlled, as it competes strongly with young trees for light in the early part of the growing season, and then collapses, often smothering young trees. Goose grass, an annual, can also smother young trees in a season. Old man’s beard must also be controlled.

Bluebells and other low herbaceous woodland flora can be left, as they do not harm trees. Mosses, which have no roots, do not appreciably lower the soil’s water content and may help to stabilise and protect the soil around the tree by forming a natural ‘mulch’.

Timing and duration

Competition from weeds is usually at its strongest in April, May and June. April is also the month in which soil moisture deficits begin to develop over much of england, and the transpiration from weeds from this time will result in even greater deficits for the entire growing season. Thus, even one week’s weed growth in april can significantly reduce water availability right through to October.

Timing will vary with the method of control, the locality and the season. Sheet mulches are fitted at the time of planting and left in position until they deteriorate. Herbicide application must be carefully timed according to the type of herbicide, the amount of growth, and the weather. Translocated herbicides can only be applied to weeds in growth, but the aim should be to have bare ground around the tree for the maximum length of time throughout the year. Grass, the greatest enemy to tree growth, can grow at any time of year, provided the weather is mild. Tree roots also grow in mild spells in winter, even when they are not in leaf. Bare ground is therefore important even in the winter. A young tree that puts on root growth through the winter will show significantly better leaf and stem growth in the following season than one which has not.

If the early season weeding is thorough, the trees may be able to withstand some weed competition later on in the season, but they will grow best if kept weed-free throughout the year.

The number of years that weed control is needed depends on the site, the species planted, the spacing of the trees and other factors. Three years from the time of planting is usually the minimum. Single trees in open ground and trees at the edges of planted areas need weeding for a longer period than trees within woodlands, where the relative lack of light does not encourage the growth of grasses. Trees in grassland should have a weed-free surround for at least five years. The ground under trees cropped for fruit in orchards and gardens should be kept permanently bare.

Herbicides

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 strengths, 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 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 2000 prices), or in larger quantities.

Glyphosate (eg Roundup Biactive 120g/l) 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. 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 (2000 prices), and can be applied using a garden sprayer of one litre capacity or greater.

The combination of using a winter pre-emergent/residual herbicide and a translocated/foliar acting herbicide in spring and summer is widely used for 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 creates 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 is applied, there has already been some interference with tree growth. However, herbicides are the most cost-effective method of weed control for tree planting schemes, and are preferred over mulches by most authorities responsible for tree planting.

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, the Arboriculture and Forestry Advisory Group (AFAG), or Lantra. For contact addresses 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 keep moisture in the ground by reducing evaporation. They can either be sheet materials, such as polythene, bitumen or various textiles, or loose materials such as bark, chipped wood or gravel.

Proprietary sheet mulches

Sheet mulch materials or mulch mats have the dual advantage of suppressing weeds, and reducing or preventing evaporation from the soil surface. They also increase soil temperature, which encourages root growth in early spring. Sheet mulches which prevent evaporation are not recommended for badly drained sites, as this will exacerbate waterlogging and encourage anaerobic conditions.

Proprietary sheet mulches are quite expensive, but it is not usually worth economising by using cheaper products or recycled waste materials. Sheet mulches need to be substantial enough to last at least three years, thick enough to resist 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 50cm square will be ineffective. The larger size mats, 1m square, are recommended.

Table 7a: Comparison of mulch mats and herbicides

Mulch mats can be anchored into the soil at the edges or corners, or fastened by proprietary pegs. Suppliers will recommend the best method for particular products. Pegs add to the cost of the product, with the 1m square mats requiring nine pegs per mat. Non-biodegradable mats should be further weighted down by stones, gravel, upturned turves or a small amount of soil, to weight 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.

Weeds can grow up through the gap around the stem, or along the slit which allows the mat to be fitted around the tree. Some products have an overlap to prevent this weed growth occurring.

Mulch mats are not foolproof. Mats can be disturbed by vandals, or torn by the wind, and may need replacing during the initial three year weeding period. Thinner materials can be scratched and torn by animals, especially if voles take up residence beneath the mat, as foxes will then tear the mats to reach the voles. Some fibrous mats may be damaged by large birds in search of nesting material.

A wide range of sheet mulches are available from Acorn Planting Products and Greentech. Woven polypropylene is probably the best compromise for price versus quality for most projects, but situations differ, and new products may become available. Some products use recycled materials, including wool.

DIY recycled materials

Probably the best recycled product is disused carpet, provided it is made of natural, biodegradable material, rather than nylon or acrylic. It’s long-lasting, heavy enough to stay in place without weighting, is relatively tidy and unobtrusive, and is air and water permeable. Old carpet should be readily obtainable from carpet fitters. Disused books of samples are ready cut in squares, and only need slitting to fit. lay with the underside up.

Old lino or other flooring materials are also suitable, as is roofing felt. Thick black plastic, as used for covering silage clamps on farms or for other wrapping purposes may be salvageable. large sheets of black plastic can be laid down and the trees slit planted through them. Sideways water movement through the soil is sufficient to keep the trees watered, although voles may be a problem. The plastic looks unsightly, but can result in very rapid tree growth and leaves the ground clear for establishing woodland flora.

With all recycled materials, make sure that they are safe to handle, and do not contain residues that may either harm the people laying the material, or contaminate the ground.

Loose mulch materials

Loose mulch materials such as composted bark are commonly used in parks and gardens, both for new plantings, and as general soil improvers. As the material breaks down it is incorporated into the soil, adding to the organic matter content, improving soil structure and helping to maintain nutrient levels. However, a layer at least 10cm (4”) thick is needed to be effective, and this will only remain on the surface for about a year, before it becomes incorporated into the soil.

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.

For use in tree planting schemes, loose mulch materials are only worth considering when there is a supply of suitable material readily available. One of the best methods is to combine the need for mulching materials with disposal of prunings, thinnings and other woody material. Instead of being burnt, material can be put through a wood chipper, preferably composted for a few months, and then used for mulch. Many woodland management schemes use this method of recycling within their own area of woodland. Woody material is best composted before use, or it may deprive the trees of available nitrogen as it decomposes. In practice, it is often applied fresh, without apparent problems. If possible, compost it within existing woodland, or close to but not under the canopies of existing mature trees. This may encourage mycorrhizal activity which will enhance its use as a mulch.

Decaying leaves, either gathered from existing woodland, or swept up from parks and gardens are excellent mulch material, and are beneficial in encouraging mycorrhizal activity.

Any woody material which can be brought into the young woodland will help the development of a woodland ecosystem. logs and larger branches can be left in small piles, or scattered through the wood where they will not hinder access or weeding operations. They can be placed next to young trees, where they will suppress weeds and break down to form a mulch, or can be used to weigh down other mulching materials. Brashings or prunings can be used to deter deer, access and vandalism, although this needs to be weighed against the chance of attracting arson, as well as giving cover for rabbits. Brambles also tend to grow up through piles of brashings, which may or may not be helpful!

Spoiled hay, straw and other bulky materials are suitable as long as they are well rotted. Don’t use silage, slurry or other farm wastes as these can be toxic to young trees and other plants.

loose mulch is not totally effective in suppressing weeds, whether or not it is laid on bare ground. Some weeds will come up through the mulch, and others will seed into it. Goose grass for example can spread very quickly after germinating in loose mulch. Hand weeding will normally be needed twice or more during the growing season. If grasses get a hold, a herbicide is the best way of getting rid of them. loose mulches should be replaced each winter for three or more years, until the young trees are established. Once the canopy starts to close, the young woodland produces its own mulch of fallen leaves.

Hand weeding

Hand weeding is labour intensive, and only suitable for small planting schemes. Hand-pull any weeds that come out easily, and avoid using tools. you should never use a fork or spade near a young tree, as it is easy to damage the roots or stem. A hoe can be used on weed seedlings, but take great care not to damage the stem of the tree. If hand weeding is to be used, it should be combined with using a mulch of leaf-mould or compost. This will keep down the growth of grasses, so you should only need to pull out those plants which seed into the mulch. Where grasses have taken a 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 or 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 can be piled up 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. Bracken can also be ‘whipped’ with a stick as the fronds open, to weaken the growth, but beware of damaging the young 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). Bracken can be a serious fire risk where there is a deep accumulation of litter, which should be raked away from tree planting sites.

Inter-row weeding

The 1m diameter circle around each tree must be kept weed free. The remaining vegetation is called ‘inter-row’ growth, whether or not trees are planted in rows. Even with trees at a regular, close spacing of 2m, this leaves a greater part of the ground unweeded. Some management of this inter- row growth is normally needed for the first few years after planting, particularly on former agricultural land or other fertile ground.

leaving the inter-row vegetation to grow up unmanaged is useful for hiding trees from vandals, and for discouraging people from crossing the site and damaging them. On windy, exposed sites inter-row growth also helps shelter young trees from the 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 particular problem is there is a wet spell of weather in late summer. Voles can be a problem in tall grass, so vole guards may be needed. Depending on the site, inter-row growth can include plants which add to the wildlife value, and which are not damaging to tree growth. These may include arable weeds or common woodland edge plants. If there is anything particularly special, you are probably planting on the wrong site!

Where neatness is important, and vandalism unlikely, mowing is an option. It prevents bramble, thistles or other troublesome weeds establishing, and also removes the cover for 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 is inevitable.

long handled scythes are not recommended for weeding amongst young trees, as it is far too easy to damage or sever the trees. Short-handled sickles or grass hooks may be useful in some circumstances, but again great care is needed. Treeshelters or other protection make it easier to see and avoid the young trees, but shelters and the trees within them are not proof against damage from an edged tool.