Mistletoe: parasite and priority species at once

Mistletoe germinates on a branch from a seed deposited in bird droppings or wiped from a bird’s beak, and produces a structure that penetrates the bark and connects to the host’s water-conducting tissue. From there it draws water and dissolved minerals continuously, including through winter when it remains in leaf and the deciduous host does not.

The consequences are proportional to the load. One or two clumps on a mature tree are of no practical concern. A tree carrying a heavy burden loses a substantial share of its water supply to a competitor that gives nothing back, and the effect is most severe in dry summers when the host is already under stress. Branches carrying large clumps become swollen at the point of attachment, fruit less, and are prone to breaking.

Apple is a particularly favoured host in Britain, along with lime, hawthorn, poplar and false acacia. Pear is used less readily. The old orchard tree with a spreading, open crown is close to an ideal habitat: plenty of accessible branches, plenty of light, and plenty of bird perches.

Mistletoe in Britain is heavily concentrated in the south-west Midlands and the Welsh Marches — essentially the Severn and Wye catchments — with the Three Counties at its core. That distribution overlaps almost exactly with traditional orchard country, and traditional orchards hold a substantial share of the national population. Orchard loss and mistletoe decline are therefore the same story told twice.

It also supports a specialist fauna that exists nowhere else. Several invertebrates are dependent on mistletoe, including a weevil, a bug and a moth associated with no other plant, and the mistle thrush and blackcap feed on the berries in winter and are the principal agents of its dispersal. A mistletoe-bearing orchard is supporting a small food web rather than merely hosting a curiosity.

Mistletoe has a cultural and economic role as well. Tenbury Wells in Worcestershire has held mistletoe auctions supplying the Christmas trade for generations, and harvesting for that market is a genuine minor income from orchards that otherwise produce little. Cutting for sale is also, incidentally, a form of management: it removes growth before it becomes a burden.

The practical guidance that has emerged from orchard conservation work is to keep mistletoe at a level the trees can carry rather than to remove it or to let it take over. That means cutting back heavy clumps, prioritising trees that are carrying a disproportionate load, and leaving a distribution across the orchard rather than concentrating everything on a few trees.

Cutting has to be repeated. Removing the visible growth leaves the embedded tissue in the branch, and the plant regrows from it; complete removal means taking the branch. In an old tree that is often not a trade worth making, and periodic reduction is the sustainable approach.

The situation to avoid is the one that arises when an orchard is abandoned. Unmanaged, mistletoe can build to the point where it dominates the crowns of already declining trees and accelerates their collapse — which loses both the orchard and, eventually, the mistletoe. As with most things in a traditional orchard, the failure mode is neglect rather than either extreme of intervention.