Browning of Leylandii Hedges
A Technical Report from The Bartlett Tree Research Laboratories
Figure 1: Browning of Leylandii hedge
Figure 1: Browning of Leylandii hedge
Leyland and Lawson cypress are commonly planted to make quick-growing hedges. Over the past few years brown patches (conifer die-back) has become more frequent and severe throughout the UK and Ireland (Figure1).
A number of reasons have been identified as to the cause of this problem. In some cases die-back can be linked to one, or more generally a combination of these problems.
Pruning: Due to their excessive growth regular trimming of Leyland/Lawson cypress hedges are required. Overzealous pruning has been identified as a major cause of die-back. Interestingly research by the royal horticultural society has shown a greater chance of die-back following excess pruning in autumn (predominantly October) than at other times of the year Vigorous conifers should be trimmed at least two (spring, summer) or more times a year with a final cut in mid to late August to deter excessive regrowth in autumn. Most conifers will not regrow from old wood, and so hard pruning should be avoided. If pruning exposes brown areas stop pruning, you have reached the overzealous stage! Height reduction of hedges is carried out in spring removing no more than one-third of the overall hedge height.
Insufficient water: Prolonged warm temperatures, i.e. 22° C or higher, can result in leaf death mainly by dehydration. Because leylandii and Lawson cypress are evergreen then water loss occurs continually throughout the year. Drying winds also produce drought damage when water lost from leaves or buds cannot be replaced fast enough from the soil. Death of one side of a hedge is a common response to this form of damage. Use soil moisture probes to determine when to water.
Two insect pests have been associated with browning of conifer hedges:
Conifer shoot miner: (Argyresthia dilectella and/or Argyresthia cupressella) are small moths, with a wingspan of about 8mm. Their prevalence has increased due to the increase in garden juniper (Juniperus) and cypress (Chamaecyparis) plantings. Both moths can cause severe and highly visible damage (shoot tip dieback and foliar scorch) to infested hedges. Moth larvae hollow out entire shoot tips. Infested trees appear scorched and the dead hollowed out twigs are easily broken off.
Conifer aphid (Cinara cupressivora) are small sap sucking insects abut 1-3mm in size. Colonies consist of winged and wingless individuals. Both young and adults feed almost continually and large volumes of sap are ingested. Initially foliage becomes severely yellowed and browned. Subsequently affected needles fall in large numbers. Eventually crowns become thin consisting of a few bare needles. Secondary effects result from fouling of the leaves and stems with honeydew that encourages the growth of a fungus known as sooty mould. Heavy feeding by both these pests will injure leaf cells and reduce leaf tissue area leading to a loss of plant vitality. If complete defoliation occurs the plant may die.
Conifer browning has on occasion been associated with attack by the fungal diseases Cercospora needle blight, or Katatina shoot blight. Symptoms include dieback of twigs and branches and yellowing/browning of foliage. These fungi generally invade stressed plants that have been weakened for example by chemical, drought and/or cold temperatures.
To aid in the recovery of overzealous pruning and drought damage see Bartlett’s Five Point Drought Recovery Program Technical Note or contact the UK Laboratory for this information.
Chemicals which are available for control include; spray oil, soap, Bandu and Dimlin Flo (leaf miner only). Dimlin Flo although expensive is highly recommended due to its long persistence within the plant. Dimlin Flo, however, kills only moths and caterpillars, having no effect against aphids.
Winter washes based on spray oil plus a residual insecticide kill overwintering eggs of aphids as well as controlling other pests such as psyllids, scale insects, mealybugs and some moth eggs.
Soil drenches around the base of the hedge with Admire in February/March or October/November provide the optimal solution for growing season long control of aphids. Results against conifer leaf miner are conflicting. In some cases positive control has been recorded, in other little or no control. Please be aware of this fact if recommending Admire as a control option.
Biological control with adult ladybird beetles is inconsistent. If requested by the client release at 500 (3 tablespoons) per small hedge but aphid control cannot be guaranteed in this instance. Ladybird beetles will have little effect against leaf miners.
Fungicide sprays will effectively control blight if applied at the proper intervals with good coverage. At least three fungicide sprays should be applied during the growing season (spring, summer, autumn). Synthetic or organic (copper) can be recommended.
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