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Pollens and Moulds in the Garden

Of the four main inhaled allergens, house dust mite debris, animal allergens, plant pollens and mould spores, the last is the most difficult to understand and cope with. All of them float in the air, to be taken in with every breath, to settle on the eyes and in the nose as well as the lungs to produce all the well known symptoms, runny nose, itchy puffy eyes and difficult breathing.

Moulds are part of the enormous genus of fungi found throughout the world, not one of which is truly green.  There are, peculiar yellows and blues, but not the rich colour of chlorophyll which is only found in plants and algae. Both fungi and algae spread about by spores, the equivalent of palm seeds but algae spores are much heavier and so do not float about much in the air. Consequently, they are not a problem.

All algae need a lot of water, sea-weed being an extreme example, whereas moulds broadly prefer dryish conditions. Essentially most moulds live upon dead vegetable matter, converting the vegetable cellulose to energy. That is their niche in the great scheme of things, to break down plant material to release the built up carbon and oxygen. They need a little water to survive. Damp wood rots faster than dry.

All fungi consist of a mass of fine threads which spread in all directions spreading the debris in some cases for hundreds of yards. The threads, called hyphae, release enzymes to digest the debris. These can be seen by lifting any rotting piece of wood and teasing it apart. Dead branches and wood scraps provide a site for a succession of different moulds, which follow one after the other in sequence decaying the residue a little bit more each time, until finally there is nothing.

Under appropriate circumstances, the moulds decide to reproduce and it is then that "fruiting bodies" appear classically as a mushroom. When mature, the bodies release enormous quantities of spores into the air in their billions. This can be seen as a blue haze on old leather, cheese and bread scraps. A close look with a magnifying glass will show hundreds of little knobs ready to burst. Obviously, the mould only does this when it has reached the surface, otherwise the spores would never spread. It builds up a sufficient supply of its own food because spores are made of protein that take time to acquire and which cause allergy. This eruption only happens when it is warm and dry enabling them to spread.

The greatest source of spores in the garden is from turning over a compost heap and spreading its contents. If it has to be done then, either do it during rain which washes the spores into the ground, wear a face mask or better still, ask someone else to do it!  Leaf mould and compost is produced better if it is covered, as it keeps the heat in and stops the leaves getting too sodden which slows down rotting. Covering also discourages the moulds from fruiting as there is no air to release the spores into.

Tree and Shrub Pollen

Allergy to pollen is extremely common. All plants produce pollen, which is in effect male sperm, very high in protein and hence allergenic. Female flowers of plants are fertilised either by wind borne or insect borne pollen. It is windborne pollen that is the major problem, as the air is filled with minute particles when the male flowers are open.

Hence all trees and shrubs which have catkins are hazardous. Virtually all our native trees have wind spread pollen. After the last Ice Age, there were no insects, so many varieties, such as Alder, Ash, Beech, Hazel, Juniper, Oak, Pine, Poplar, Sweet Chestnut, Walnut, and Yew cause allergy symptoms. Similarly grasses, including a lawn, no matter how closely mown, will still have flowers low down, and are wind pollinated. An exception might be bamboo which, depending upon the species, flowers as rarely as once every thirty years.

Trees and shrubs which have open flowers, particularly those which are scented, are designed to be attractive to bees and insects so are relatively safe for allergic individuals. Obviously, if a sufferer puts their nose into a rose they might knock off some pollen and could inhale it.  This is unlikely as insect borne pollen is very heavy so stays on the fur.

Below is a list of trees and shrubs which are insect pollinated and hence safe to grow. To save confusion most of the names are in Latin: Azalea, Berberis, Buddleya, Ceanothus, Cistus, Cornus, Cotoneaster, Cytisus, Escallonia, Fushia, Hebe, Holly, Lavateria, Lavender, Lime,Mahonia, Potentilla, Prunus, Pyracantha, Quince, Ribes, Rhododendron, Rubus, Rose, Skimmia, Spirea, Syringa, Ulex,Viburnum,Weigelia, and Wisteria.

As well as producing hay fever symptoms, a number of plants can cause skin reactions. These are not usually true allergies but a real problem in sensitive people. There are too many to list, but as a guide anything with hairy stems or leaves could be a hazard. Insect pollinated plants include: Geranium, Iris, Clematis.

Try replacing lawns with gravel or paving, ornaments and water features!


Last updated: March 2012