Both trees and shrubs give height, structure, a sense of scale, as well as a feeling of permanence to our gardens, and no garden is complete without them. All trees and shrubs contain woody tissue which forms the skeleton of the plant and stays unchanged throughout the year. They can be either deciduous, losing their leaves in winter, or evergreen, retaining them through the seasons. Trees have a single trunk supporting a branching head, while shrubs have several stems at ground level.
Tree pollen is a major allergen and causes asthma and hay fever in a great many people. However, only wind-pollinated trees produce enough buoyant pollen to be a problem, and then only certain trees produce pollen which is also allergenic. The following families of trees and shrubs are known to produce large quantities of allergenic pollen: Aceraceae (maples), Betulaceae (alder, birch and hazel), Fagaceae (beech, oak and sweet chestnut), Juglandaceae (hickories and walnuts), Moraceae (mulberries), Oleaceae (ash, olive and privet), Salicaceae (poplars and willows) and Ulmaceae (elms). All wind-pollinated plants from the these families should be considered suspect as far as the low-allergen garden is concerned. Either exclude them to be completely safe, or investigate the reactions of you or members of your family to each such plant before allowing them to remain in, or introducing them into, your garden.
Selecting and using low-allergen shrubs
Shrubs provide a great deal of the living structure for the smaller garden, particularly where there is little room for trees. In larger gardens they provide the middle layer of plants, linking the lower-growing herbaceous plants and the trees. Shrubs are used in all parts of the garden and can form hedges, lend impact to ornamental beds and borders, or act as focal points. Evergreen shrubs are particularly useful for providing a green background to gardens throughout the year.
In the low-allergen garden there are some shrubs to avoid as they are known to provoke allergic reactions. Among the worst culprits is privet, Ligustrum spp., a member of the Oleaceae family. Other problem shrubs are those that produce highly-scented blossom, which acts as a trigger for allergies. These include the mock orange, Philadelphia spp.; flowering currant, Ribes sanguineum; lilac, Syringa spp.; and the butterfly bush Buddleja davidii, any of which may need to be excluded.
Skin allergies may be caused by the various daphnes, many of which are both beautifully scented and toxic, and therefore must be treated with extreme caution. Rue has already been mentioned as a major cause of photodermatitis. The other main offenders in this respect are the various species of Rhus, one of which, Rhus radicans, is the notorious ‘poison ivy’, which causes severe blistering dermatitis. Other, less obnoxious plants which may nevertheless cause problems in susceptible people are the hairy, prickly and grey-leaved plants. The key to choosing shrubs is to avoid the real villains and then exclude only such other plants as you know affect you or any member of your family.