Breathe easy

Published 08/09/16

Low-allergy gardening with indigenous plants. Dr Philip Taylor has researched and published several papers on allergenic pollens. He offers us this thought-provoking argument for greater use of indigenous plants in our garden

HAYFEVER symptoms include a runny nose and itchy eyes, and this is one of the most common chronic respiratory conditions, affecting 18% of the population in Melbourne. Allergic asthma affects 10% of the population, and is experienced as wheezing, coughing and shortness of breath because it involves the lower airways. Melbourne is a global hotspot for the prevalence of both hayfever and asthma. 

Wind blown exotic grass pollen, especially from ryegrass (Lolium species) is a major trigger of hayfever in Melbourne, and has also been implicated in outbreaks of asthma. There are extensive pastures north and west of Melbourne. Hayfever symptoms peak in the spring and early summer when strong winds associated with hot, dry weather entrains pollen from these pastures and distributes them across Melbourne’s suburbs. Local exotic vegetation can also be a major contributor of pollen to the atmosphere. So, personal exposure to pollen allergens will be a result of both local and long distance transport, dependent on the weather. 

Pollen from introduced trees, such as Birch (Betula), Alder (Alnus), Ash (Fraxinus), Oak, (Quercus), Olive (Olea), Privet (Ligustrum) and Cypress (Cupressus), are also triggers of allergy. However, pollen from native trees, such as Eucalypts, Wattles (Acacia) and Bottle brushes (Melaleuca and Callistemon), are mainly dispersed by insect and birds and their shape is not conducive to transport by wind. Allergenic pollens are also emitted from introduced weeds, such as plantain (Plantago lanceolata), Fat hen (Chenopodium album) and Wall pellitory or ‘Asthma weed’ (Parietaria judaica). However, native herbs, such as Blue bells (Wahlenbergia), Native Pelargonium (Pelargonium austral), salt bushes (Einadia, Atriplexa and Enchylaena), Flax lilies (Dianella) and everlastings (Chrysocephalum, Coronidium and Xerochrysum), release very little pollen into the atmosphere. 

Notably, most allergenic pollens are emitted from plants that are native to Europe and North America. Australian native plants are mostly insect or bird pollinated, have low pollen yield, low allergen content, and/or are self-pollinated. Among the few locally indigenous plants around Melbourne that emit pollen that can trigger hayfever are the native ‘Pine’ (Callitris) and Sheokes (Allocasuarina). Planting only female Allocasuarina will eliminate any pollen production from this species. Only one male Allocasuarina is required for full seed set of 40 females. Callitris is best avoided or used in less populated areas. Kangaroo grass (Themeda triandra) has also been implicated as a hayfever trigger in a few patients, but to a much lesser extent than introduced grasses. 

Melbourne is well known for its highly variable weather compared to cities in the Northern hemisphere. Cycles of wet and dry weather in combination with variable wind speeds can result in the rupture of highly allergenic pollen, especially Ryegrass, and the wind-borne dispersal of particles small enough to penetrate deep into the human airways. Asthma epidemics in Melbourne often occur soon after extreme weather events, such as a thunderstorm. 

Replacement of common exotic plants with indigenous plants in Melbourne will transform high allergy gardens to low allergy gardens. This can have a significant impact on reduction in the amount of allergens in the outdoor air, and thus lowering the burden of allergic diseases on susceptible people. 

USEFUL REFERENCES 

The Asthma Foundation Victoria, The low allergy garden: how to avoid garden allergens, (2013). 

Ragg, M, The low allergy garden, Hodder and Stoughton 1998.