Mistletoe management – the need for control
(Note that this advice relates to Viscum album, the native mistletoe of Britain & Northern Europe – management issues will be different for other mistletoe species)
The need for, and understanding of the need for, mistletoe management varies considerably across Britain. In areas where mistletoe is rare it is often regarded as sacrosanct, and hardly ever cut, whereas in areas where it is common, or isolated sites where there are large populations, the need for management is often clear. Information on where and why it grows in particular areas is given on the Distribution pages.
Despite this variable attitude, nearly all mistletoe will need management at some time and failure to manage it can result in loss of both the host tree and mistletoe. The outline advice given here applies equally to gardeners, ecologists, arborists, conservation managers, farmers and orchard owners.
The need for management
All vegetation needs management from time to time, and mistletoe is no exception. Indeed, as a parasitic plant, it has particular management needs as it directly affects its host and needs to be kept in check.
Some mistletoe species around the world are major forestry and tree pests, whilst others are relatively harmless. In Britain Viscum album is fairly benign but it will always significantly affect the branch it is on, and may need remedial management if allowed to colonise too many branches.
New conservation or garden-related mistletoe plantings, especially in new locations, will not need such significant management – unless left untouched for a decade or two. Regular management for all mistletoe, new or established, eliminates the need for any drastic intervention. This should be included in vegetation management plans.
Effects on the host tree
Mistletoe affects the host tree in a number of different ways. The most obvious is the way it takes over the branch it is growing on, with the host’s growth (cambial) cells fooled into developing wood and bark enclosing the developing mistletoe plant from germination onwards. The resulting distorted growth (the haustorium) can become quite large and often leads to any distal parts of the original branch becoming stunted or dying. This means that this particular branch is no longer serving the tree.
The evergreen nature of mistletoe is another obvious impact – the mistletoe’s retention of leaves throughout the winter increases windage of the tree – which can be a problem in winter storms.
The biggest impact is rather less apparent and relates to water and gas exchange. Viscum album, like most mistletoes, tends to preferentially absorb and transpire more water than the host. This can be a particular problem in summer and dry periods when the mistletoe’s excessive (and somewhat uncontrolled) transpiration can lead to the host reacting by closing its own stomata, further reducing water for the host. There are then knock-on effects for carbon assimilation, as the closed host stomata mean limit gas exchange and will reduce photosynthesis in host tissues.
These effects are usually only significant where there is excessive mistletoe.
Traditional management in orchards
Impacts of mistletoe on relatively small trees can be significant, especially on crop trees. So mistletoe has, traditionally, been managed in the English SW Midlands, where mistletoe and apple orchards are common.
Work usually takes place in winter when mistletoe is better assessed and is often combined with a ‘harvest’ for Christmas, using the mistletoe as a extra crop. This aspect was, until recent times, a secondary factor, as traditional orchards were primarily for fruit and mistletoe, which would reduce fruit yield, was kept to a minimum. Management simply meant cutting out as much mistletoe as possible.
Mistletoe has also been an issue in continental orchards, particularly in northern France, where management was, at one time, a legal requirement. This obligation, for French farmers to cut mistletoe out from orchards, was met with a programme of winter cutting, happily combined with export of the ‘waste’ to Britain for use at Christmas.
The decline in traditional style orchards over the last few decades, in both Britain and France, has changed the dynamics of mistletoe management in apple orchards. Many surviving orchards in mistletoe areas are not managed at all, with excessive mistletoe often the norm. When there is ‘management’ it is often, in reality, a Christmas ‘harvest’ rather than genuine management. This can mean that only berried female mistletoe is cut, leaving male plants to grow unchecked.
Such neglect also often applies to mistletoe in garden situations, where a few decades of mistletoe growth can smother a small apple tree, looking delightfully romantic for a few years, until premature death of both tree and mistletoe. Far better to manage the mistletoe and keep both tree and mistletoe indefinitely!
Management techniques – regular, proportional, and both sexes!
There is no standard technique for mistletoe management – and much depends on whether the task is regular annual work or irregular remedial work following decades of neglect. For regular management the first need is to decide your objective – decorative mistletoe in a garden, a larger quantity for heritage and/or nature conservation objectives, primarily female plants for a Christmas cash crop, or simply maximising host tree productivity (i.e. attempting total control where mistletoe is common)?
Required quantities, and proportion of male/female plants, will vary with each but similar management principles can apply to all. These include;
- management will not be needed for new colonies for several years after establishment
- male and female should be equally cut, even when harvesting for Christmas and the unberried male is valueless
- the winter shoot tips are spring flower buds, so if berries are wanted some of these must be retained
- a pre-planned proportion of growths can be cut back each season – a rotation system can be helpful
- mistletoe is difficult to kill – it will re-sprout if cut back to host bark. This can be a useful management tool
- total control may only be possible by cutting out whole branches
If the need is for remedial management of neglected trees with excess mistletoe there may be a need for removal of whole branches
Mistletoe Management Survey (Mistletoe League)
The Mistletoe League is a relatively new project, set up in 2011 to promote the need for mistletoe management in traditional apple orchards (as well as on garden fruit trees) and in particular to gather information on attitudes to management and on whether mistletoe affects different fruit tree varieties in different ways.
The project is ongoing – details are available at www.british.mistletoe.org.uk
New text and pictures following soon…