Plants for health
Jackie Herald discovers how to select plants with allergies in mind
We believe green space is great for people’s health and wellbeing – the perceived antidote to grey hardscapes and urban pollution – and the benefits of plants, in private gardens, public parks and indoors, are beginning to be documented.
But what if the green prescription inadvertently triggers discomfort and even chronic illness? Usually, as garden designers, when selecting plants our emphasis is primarily on ‘right plant, right place’, for the sustainable good of the plant. Having established the aspect of a site, more often than not the choice is an aesthetic one. However, when we factor human health into the equation, there are many more features to consider. The process of selection is nuanced, and requires more information than the standard encyclopaedias of plants and nursery catalogues provide.
One of the main things we have discovered is that in weighing up which plant to use, it is well worth considering not just the physical and chemical features of the individual plants and their impact on people, but also where they are to be placed.
Safe trees & healthy hedges
As a rule of thumb, what’s good for bees and biodiversity is also good for people. Thus prioritising insect-pollinated (entomophilous) trees over wind-pollinated (amenophilous) species is a win-win approach.
The UK has the highest rate of asthma in the world, and there is a proven link between pollen allergy and asthma. We mass plant alder and its relative birch to filter the roadside air with their foliage, only then in springtime to shower the unsuspecting public with particularly fine, allergenic pollen. Alder and birch are amenophilous; they are major triggers of allergic rhinitis (hayfever) and asthma, and they also trigger allergies to some fresh fruit and vegetables.
Reassuringly, of the 2,951 vascular plants listed in The New Atlas of British Flora, only a handful trigger respiratory allergy in the UK. That leaves plenty of choice for trees to function as our “green lungs”. Many non-allergenic species can trap particulates efficiently with their canopies of textured foliage – tulip tree (Liriodendron tulipifera) and rowan (Sorbus) are good examples.
Studies show that roadside hedges can be more efficient than the street trees at protecting pedestrians from traffic pollution. However, here it is important to avoid the especially harmful mix of noxious emissions with pollen. As pollen is the male DNA of the plant world, when choosing dioecious species, select female plants for lower pollen impact.
American horticulturalist Tom Ogren has devised the OPALS™ 1-10 measurement of allergenicity – whereby a female plant may score a low 1, and the male of the species gets 8 or 9.
The trouble is that many nurseries and suppliers don’t know the sex of the clones on their stock lists. One exception is Nigel Clarke, of Queux Plant Centre in Guernsey, who is growing a range of guaranteed female plants. These include varieties of Coprosma, Griselinia, Pittosporum and Taxus, each tagged “allergy friendly” with an OPALS™ label.
Approximately 85% of people with hayfever are allergic to grass pollen. The symptoms are often dismissed as a “summer cold”. In fact, the impact on lifestyle and opportunities may last more than a season; one study showed that children may achieve one grade less than their potential in summer exams, due to hayfever symptoms leading to loss of sleep, disrupted concentration and, in the worst cases, absence from school.
Avoidance of the trigger is the first principle, and in the plant world there are always alternatives. One striking ornamental grass is sterile, and therefore produces no pollen: Calamagrostis x acutiflora ‘Karl Foerster’ – a staple in the Oudolf-inspired palette. Another option is to use non-toxic grass-like plants such as Libertia and Ophiopogon.
In principle, for a healthy planet, the more herbaceous flowers and flowering trees and shrubs, the better. The key is in the shape of the flower. Aim for forms in which the pollen is hidden inside, and to which beneficial insects have adapted to probe. This includes bell, funnel, labiate and tubular shapes, and double rather than single blooms.
The snapdragon (Antirrhinum majus) has often been cited as the perfect low allergy plant. The antithesis, for example, is Callistemon, on whose bottle-brush tips the pollen is exposed. It’s a plant that is spectacular to get up close to, and so it tends to be sited next to paths where the pollen is easily brushed off onto clothing and skin – the allergic reaction for some kicks in hours later, by which time the garden source of the problem has been forgotten and a different association is often suggested.
Also avoid using highly scented plants. Lovers of perfume might wonder if there is any point to a garden without it, but if you know someone with asthma triggered by heady scent, you will understand the issue.
This approach requires being very specific about your choice of bloom, down to the cultivar. For example, flowers in the rose family tend to emit relatively low amounts of pollen, but the degree of perfume varies enormously. The effect is intensified in enclosed spaces, such as a small courtyard. The intensity can be gently dispersed by planting diverse varieties with different flowering times, which has the advantage of extending the seasonal interest of the garden.
Ground covering plants spilling over pathways and cascading down walls are tempting to touch – or, indeed, sometimes difficult to avoid. The wonderfully soft hairy lamb’s ear leaves of Stachys are relatively safe, and a good choice for family gardens. However common ivy, so often recommended for green walls and cleaning the air, is one of the most common triggers of contact dermatitis in the UK. This causes extreme discomfort for up to six weeks, and is likely to recur unless precautionary measures are taken.
Taking due care over planting for health does not necessarily make a sterile environment – in fact, quite the opposite. Remember, diversity is the spice of life.