. Low-Allergen Landscapes

by Derek Burch

There may be a new consideration of which landscapers need to be aware. The spread of allergies and asthma induced by plant pollens may make it necessary to be cautious in the use of certain plants close to occupied areas.

Wherever statistics are available, they show clearly that the incidence of allergies is rising sharply. Allergy sufferers know all too well the symptoms of an attack, which can range from the relatively mild "runny nose and blocked sinuses" to the debilitating inability to get sufficient air into the lungs to support the body's need for oxygen.

A number of factors have been identified as possible reasons for the increase in the number of people suffering these attacks. Allergists explain that the symptoms represent the body's over-reaction to the presence of a foreign substance, particularly certain proteins. Eating one of these substances or coming into contact with it by touch or by breathing it in from the air, or even from a sting or bite, triggers the production of antibodies intended to neutralise the effect of the foreign material by binding it up. Unfortunately for us, this reaction also causes the release of histamines that bring about tear production and running noses, and, in the lungs, can cause blockage of the air passages.

The first time that the foreign material is experienced may or may not give a noticeable reaction, but the body "remembers" it, and reacts quickly and severely in later exposures. This is called becoming sensitised. The body actually remembers very specific proteins, and in addition the chemicals which surround us in the modern world appear to affect our immune systems so that sensitisation can happen from scarcely discernible levels of exposure to the proteins.

Some Japanese researchers also believe that our obsession with cleanliness prevents the immune system from experiencing the mild exposures to organisms that can toughen up the body in the same way that vaccination with the cowpox virus used to be used to prepare the body to resist deadly smallpox. My mother, who was right about so many things, would often use the particularly English saying "you have to eat a peck of dirt before you die."

What is the significance of this to the landscape industry?

Pollen is one of the common causes of allergy attacks, although the number of pollen grains in a given volume of air that we breathe is very low compared with the number of microorganism spores that also induce allergies.

The pollen from most plants is carried from one flower to another by specific animals (insects, birds, bats or other small mammals are the most likely carriers). Long periods of interchange between plants and animals have set up these relationships. Other plants, however, set their pollen adrift on the wind usually in great quantities to make sure that at least some arrives at its intended destination. These plants may live naturally in environments where there are few animals to harness as pollinators, and may flower when there are no leaves on the trees to interfere with the passage of slow breezes.

Studies have shown that much of the pollen cast into the wind in this way travels no more than 50 feet from the parent, but other researchers have found pollen that has been carried hundreds of miles from its source. Obviously these long distance travelers are irrelevant to questions of what should and should not be included in the landscape, but when considering the typical wind-pollinated plant whose pollen falls close to home, it makes very good sense for allergy sufferers to exclude them from their immediate surroundings as far as they can.

What plants are the likely offenders?

In theory, since all pollen has a high protein content, any plant can cause mischief if its flowers are sniffed or waved around violently. In practice, it is the wind-pollinated types that should be avoided where practicable. Wind pollinated flowers do not need to attract pollinating animals, so most of them are small and inconspicuous. The element of chance in distributing their pollen successfully, however, often means that very large numbers of the tiny flowers are produced.

A rule of thumb would be to avoid trees and shrubs that flower when the leaves are off or just regrowing, particularly if they have large numbers of flowers in the dangling inflorescences that we call catkins. This includes oaks, birch, alder, willow, poplar, hickory, pecan and hazel. Maples, elms, lindens, sycamore, sweetgum, wax myrtle and hackberry are also known to be strongly allergenic. Conifers may have serious allergens but fortunately the pines which produce large amounts of pollen seem to affect relatively few people.

A few groups of weeds cause serious and widespread problems - the pigweeds, chenopodiums, plantain and the members of the daisy family allied to ragweed. The other group easily recognised is the grasses, and many of these can cause problems when growing in large acreages, in a situation in which they come into bloom.

You will notice that many of our most widely used street trees are on this list. Should the potential for causing allergy affect the choice of tree species for future planting? This is a question that must be discussed, but in most cases the fact that pollen production occurs in only two or three weeks out of the year may mean that overall excellence in other ways will weigh heavily in the decision. People who have had these trees identified as the source of their allergy may be able to minimise their exposure by staying home in the mornings when pollen counts are often highest.

For the home landscape the same question must be faced, with the decision perhaps going towards avoiding the worst offenders since the tree will be so close to living areas. The same is true of plants other than trees, but here the variety of material available is much wider, and it will be easier to select non-allergenic substitutes for the known offenders.

Contact allergies are another matter, and it is harder to generalise here about the groups of plants which may cause problems. Gardeners will very quickly establish their own lists of "untouchables" in their surroundings, but maintenance contractors would be well advised to learn what plants their workforce should be warned to approach with caution.

This topic of landscapes with low allergy potential is starting to produce books which list non-allergenic plants, and the prudent designer will consider allergy potential and look for landscape techniques that avoid making the problem worse. Bear in mind, however, that we are unique individuals, with our own particular set of plants to which we may be sensitive so that there is no guarantee that any plant that we touch or smell may not turn out to be a wrong'un. As if being a landscaper didn't bring enough other things about which to worry!

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