Allergy Plants: Worst Plants for Schoolyards/The Most Allergenic Plants Commonly Found in Schoolyards
The plants below are the most allergenic and most commonly used in schoolyards across Canada and the United States. For new schoolyards, and for replacement shrubs or trees, we recommend using low-allergy and pollen-free selections, and to only plant with an OPALS ranking of 1 – 5. Based on OPALS findings, male species produce the most pollen, although male trees don’t create pollen year round.
Our recommendations for already existing allergenic shrubs or trees in schoolyards are as follows:
- Ideally, fruitless mulberry trees should be removed and replaced with allergy-free selections. If the trees can’t be removed, they should be pollarded on a yearly basis. (Pollarding is a pruning system in which the upper branches of a tree are removed, promoting a dense head of foliage and branches.) This will not remove all the pollen but will lessen the amount produced. *Please see further notes below on other ways to deal with these large “fruitless” male mulberry trees.
- Male juniper shrubs should be replaced with female juniper plants.
- Male yews should be replaced with less allergenic shrubs.
- Male red maple trees, if not too large, should be kept pruned as much as possible. Annual pruning in late winter will remove much of the pollen exposure. Any red maple tree that produces no seed is probably a male.
- Cypress shrubs and trees should be removed and replaced with female juniper selections.
- Birch trees can be tip-pruned each fall to remove pollen exposure the next spring. The best long-run recommendation is to replace them with less allergenic trees.
- If not too large, male: poplar and willow trees can also be tip-pruned each fall to limit springtime pollen.
- Oaks, if not too large, can also be tip-pruned early each spring to limit pollen exposure. They can also be pollarded with success.
- Sycamore and London plane trees should be hard pruned and then kept pollarded. Yearly pollarding during the dormant season works well with these trees and will eliminate their pollen.
- Male Podocarpus shrubs and trees should be replaced with low-allergy selections.
- Norway maple trees normally get too large to prune annually, but they can be pollarded. They should be considered for replacement.
- Acacia shrubs and trees should be replaced with allergy-free species.
*Fruitless mulberry trees are almost always male clones of Morus alba, the White mulberry, a non-native species. They are extremely common and highly allergenic. The leaves of these trees are used to feed silkworms, a school project at many schools. If the school wants White mulberry leaves for these projects, it’s recommended that they plant Weeping mulberry variety ‘Pendula.’ This is a female selection, a small tree, and it will produce fruit, but only if it receives pollen from a nearby male (fruitless) mulberry tree. ‘Pendula’ will not produce pollen.
It’s also possible to graft a male “fruitless” mulberry tree with scion wood from a ‘Pendula’ tree. This will quickly result in a full-sized weeping tree that will not produce pollen. However, to do this, you must have grafting skills. Dormant mulberry trees can be cut back hard to 7 – 10 centimetres (3 – 4 inches) stubs and the stubs can be cleft-grafted with pencil-thick scion wood from ‘Pendula.’ Any sprouts that arise from below the grafts should be removed promptly, as they will be male. Mulberry trees are considered among the easiest trees to graft.