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Garden Allergies
Pollen Guide
Month
Plants
releasing
pollen
March April May June July Alder Birch Oak Grass Grass Poplar Plane Horse Chestnut Plantain Nettle Elm Ash Oil-seed Rape Pine Sweet Chestnut Yew Willow Hornbeam Lime Dock There are many things in our environment which can trigger an allergic reaction. These can range from house dust to chemicals introduced as improvements to modern living. To gardeners it is probably plant pollen which is most likely to be an important allergen. As plants are static and need to share their genetic material to reproduce, it has to travel between them. The male gametes in the form of pollen are transported to the female ovaries by a number of mechanisms, but the plants which use the wind to distribute their pollen are the main culprits when it comes to causing allergic problems - grasses and most trees are wind-pollinated. Usually the flowers of wind-pollinated plants are not very showy and consist mainly of the male and female parts without much adornment or scent as they do not need to attract insects.
The table above gives examples of plants which are known to cause problems for those affected by pollen. Birch trees are one of the main producers during spring in the British Isles followed later by grasses. There is usually a small respite between the two releases, but in some years they can overlap and this causes greater problems for allergy sufferers. The release can vary depending on location and temperature, so may be later in the north - recently the warmer weather has meant earlier onset.
Plants such as oil-seed rape are insect-pollinated, but as it is planted in concentrated blocks, a lot of pollen is produced at the same time so it can be a problem at close quarters and it can be carried by strong winds. The weather conditions can have an effect on pollen dispersal. During warm, dry conditions, more is released and it can remain in the air for longer. Rainfall captures the pollen so reduces the problem.
Since it is the male half of the reproductive equation that is the problem it might be a conclusion that excluding male flowers of plants would remove the allergens. Unfortunately it is not that simple, most plants are monoecious, ie. bear male and female flowers on the same plant or are hermaphrodite having flowers with male and female parts. They can self-pollinate or to improve the genetic mix the pollen is transferred to another plant.
Dioecious plants carry either male or female flowers on a single plant so require a mate to be growing nearby, eg. Holly. With the dioecious plants it is possible to use specimens which are all female, but they may not look very exciting. Some varieties of Holly can have a misleading name - the variegated 'Golden King' is actually a female plant as it develops berries, and 'Silver Queen' is male so produces lots of pollen, both were named before the male-female differentiation was known .
Choosing showy, insect-pollinated plants helps as their pollen is sticky and does not travel far in the air. Variants or Cultivars which have compound petals tend to have fewer anthers which produce the pollen or they can be absent. Plants with attractive foliage are a good choice and if they produce flowers they can be trimmed before they are due to bloom - there are plenty of verigated plants or ones with sculptural shapes.
If grass is a problem a non-sensitised person can cut it to remove any flowering stems a few days before an affected person visits.
Some plants which could be used in a low allergy garden are:-
Trees - Apple, Dogwood, Magnolia, Holly (female), Plumb.
Shrubs - Hydrangea, Rhododendron, Box (trimmed), Skimmia (female).
Herbaceous - Begonia, Crocus, Hosta, Rose, Daffodil, Geranium, Bergenia, Tulip.
Flowers of some cultivars and hybrids have double or more petals which have developed instead of the male anthers which produce the pollen, so these make ideal plants.
Plants to avoid:-
Most grasses.
Trees - Ash, Alder, Willow, Birch, Cedar, Pine, Elm, Maple.
Shrubs - Bottlebrush
Herbaceous - Most Daisies, Goldenrod, Lilies
Strongly scented plants and flowers exacerbate the problem.
Many weeds are wind-pollinated so their removal will help: eg.Plantain, Stinging Nettle
Since gardens are surrounded by others or the countryside, there are plenty of pollen-producing plants around, so it is impossible to be pollen free. Unless it very breezy the pollen may not travel too far and it has been shown that for trees, most of it falls within about 10 metres of the edge of the canopy. Female trees are able to attract and trap pollen, so planting one in the garden should reduce the amount floating freely. Choose cool, cloudy days after rain to be in the garden when the pollen count will be low.
There are other allergens in the garden apart from pollen. Some plants have sap which can induce an allergic reaction. This is part of their defence against insect attack, but some people are very sensitive to it as well. For example, Euphorbias have a white sap which exudes in copious amounts if the stems or leaves are damaged and it is very irritant to the skin or eyes, especially in sunlight. Other plants which can cause an allegic reaction include Giant Hogweed, Poison Oak and Poison Ivy.
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