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 and in the UK there are about 12 million people who suffer from hayfever. 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. They also have to produce much more pollen than insect-pollinated plants as it is a more random process. 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.
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
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 as 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 (often named as 'Flore pleno'). 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. The pollen count is at its highest between 9am and mid-day, and lowest around 8pm.
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:-
Trees - Ash, Alder, Willow, Birch, Cedar, Pine, Elm, Maple.
Shrubs - Bottlebrush
Herbaceous - Most Daisies, Goldenrod, Lilies
Strongly scented plants and flowers exacerbate the problem.
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.
The symptoms can also be reduced by avoiding certain foods that can exacerbate hayfever. Dairy products and grains stimulate mucus production and sugary foods are thought to make the immune system more sensitive to pollen.
There may also be plants in the garden that can be used to help hayfever sufferers. The bioflavinoid antioxidants found in purple or red fruits and vegetables are said to boost the immune system and have anti-inflammatory effects that reduce the response to allergens. (Tomatoes are an exception as they contain histamine which causes the allergic symptoms). A tea made from the dried leaves of Stinging Nettles have been used to treat respiratory problems and are thought to block the allergic reaction to pollen.
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 Common Rue (Ruta graveolens), Giant Hogweed, Poison Oak and Poison Ivy. The sap from Ruta graveolens makes the skin more sensitive to the action of the sun and this reaction can recur for many days after first contact with further exposure to sunlight.
One way to encounter plant sap in the garden is when using a line trimmer and some doctors use the term 'strimmer rash' to describe the skin reaction which occurs. The problem can be exacerbated by sunlight. Other means of exposure to such irritants are when pruning or weeding.
Below are some examples of plants which can cause an allergic reaction.
PLANT ALLERGEN REACTION Alstromeria sp. chemical allergens usually affects people who handle plants frequently, eg. florists Dieffenbachia sap burning sensation, sometimes blistering Echium sp. irritant hairs itching Euphorbia sp. milky sap sensitises skin to sunlight Ficus benjamina, Weeping Fig chemical allergens skin rash Fremontodendron irritant hairs itching, respiratory problems if breathed in Heracleum, Hogweed sap sensitises to sunlight, may cause blistering and long-lasting discoloration Monstera deliciosa, Swiss cheese plant sap burning sensation, sometimes blistering Pulmonaria sp. irritant hairs itching Ruta graveolens Rue sap sensitises to sunlight, may cause blistering Symphytum sp. eg. Comfrey irritant hairs itching
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