Weed control is the key to successful tree planting, and without it even the best planting stock in the most expensive shelters will either fail or be very slow to establish.
The importance of weeding for tree establishment was shown many years ago in trials by the Forestry Commission (Davies, R J, 1987), and the evidence from tree planting schemes over the years since have backed up the findings. Young trees surrounded by long grass or mown grass will struggle to survive. At best, they will put on very slow growth, leaving them vulnerable to trampling, browsing and disease. Trees which are weeded establish fast, and quickly grow out of the reach of animals, grow too big to be easily vandalised, and are not so prone to wind damage and disease.
The use of larger planting stock is not a substitute for weed control. Stock over 120cm high, including whips, feathered stock and standards need similar or better weed control as small transplants, as they have a poor root/shoot ratio. Just because a transplanted tree is tall enough to be visible above long grass doesn’t mean that it is thriving. Bare-root, cell-grown and container grown stock all have the same requirement for regular weeding.
How weeds compete
Moisture and nutrients
Weeds interfere with tree growth in several ways. Weeds use up moisture in the soil through transpiration, and also intercept rainfall before it reaches the soil. This lessens the amount of water available to the tree. The tree responds by growing fewer and smaller leaves, which reduces photosynthesis and in turn reduces root growth. Nutrient uptake is also reduced. The young tree is starved of food and water, and either fails to put on growth or dies back. The weak growth is subject to disease. Heavy rainfall or frequent watering does not help, as there is insufficient root growth to take advantage of it, and the water merely washes away nutrients and encourages the growth of weeds.
Grass is the most damaging of the weeds which affect young trees, partly because grass grows almost year round, starting before trees come into leaf and continuing after leaf fall. Mown grass is even worse than long grass. Mowing stimulates the grass to regrow, using up more soil and nutrients, and produces a thick mat of turf which reduces the amount of water percolating through. A side effect is that the mower or strimmer often damages the bark of the tree. Young trees should never be surrounded by mown grass. Mown grass also similarly interferes with the growth and health of mature trees, which is the reason why commercial fruit trees are always grown in grass-free ground. Established trees which are not flourishing can be helped into growth by killing off the grass surround. Care also needs to be taken when a thicket of newly-established trees is thinned. If the increased light allows grass to grow, the growth of the trees will be checked.
The suckers of trees and shrubs such as elm, cherry, aspen and blackthorn can compete with grass, because their moisture and nutrients are being provided from the roots of the nearby mature tree.
Light and shelter
Tall grass, bracken, nettles and other weeds may reduce the light reaching young trees, but this is much less important than the effect on roots. Most species of tree are adapted to regenerating in the shade of other trees, and reduced light levels are not usually a problem. There may be a benefit in tall weeds providing shelter from the wind, creating a humid microclimate, but again, this benefit is normally outweighed by the damage weeds do to root growth. Tall weeds which shelter young trees can also encourage height growth at the expense of stem diameter and root growth, in a similar way to staking. However, tall growth between rows of trees, at least 500mm away from the stem, and short enough that when it collapses it will not smother the trees, may be beneficial in providing shelter.
Physical damage
Any climbing or scrambling weeds including goose grass, wild hop, bindweed, honeysuckle and bramble can severely damage young trees by smothering and distorting tree growth. These weeds need controlling until the trees are physically large and strong enough to support them, or until shade prevents their growth. Herbaceous weeds such as bracken, nettles, thistles and docks, which die down in winter, can smother young trees as they collapse. Tall weeds also give shelter to voles, slugs and snails which can damage young trees. Voles are reluctant to cross bare ground due to fear of predators. Beneficial side effects of tall weeds may include the habitat provided for ground nesting birds, reptiles and other animals, and the fact that the weeds hide the trees and make the ground uninviting to walk through, so reducing accidental and intentional damage by people.
Mycorrhiza
Trees, shrubs, grasses and other plants have an association with soil fungi, called mycorrhiza, which have a beneficial effect on plant growth by helping the plant absorb water and nutrients. Different types of plants have different mycorrhiza, and it is thought that mycorrhiza associated with grass and some herbaceous plants have adverse effects on those associated with trees.
Minimum standard for weed control
On most large planting schemes the minimum which can be achieved is to keep weed-free a one metre diameter circle around each tree, or a one metre width strip along the line of trees. This is sufficient to allow quick and healthy establishment. For very small planting schemes, the gardening approach of keeping the whole planting area free of unwanted growth will result in very rapid growth of trees and shrubs. A combination of herbicide to kill weeds, and a loose mulch to retain soil moisture, soil warmth and to increase organic matter in the soil will give the best results. If this level of maintenance is kept up for three years, the site will normally have been ‘captured’, with the trees shading out other growth, and producing their own mulch of leaves. Only minimal weeding will be needed in future years, with thinning the next operation to consider.
Trees planted within woodland normally suffer less competition from grasses, but are easily overwhelmed by brambles, nettles, bracken, old man’s beard or other plants which can thrive in woodland clearings. Planted trees should be clearly marked by their tree shelters or guards, or if not individually protected, by tall canes. A clearing in winter is normally overgrown by early summer, hiding the young trees. Check the trees at least twice during the growing season, and either trample or cut back any encroaching growth.
Trees planted on exposed sites, poor soils or in upland areas may appear to suffer less weed competition. However, Forestry Commission trials (Davies, R J, 1987) indicated that because poor soils have fewer reserves of nutrients and water, the presence of even sparse weed growth can be detrimental to the trees.
