Allergy Scale for Tree Pollen

The Ogren Plant Allergy Scale or OPALS Scale is the creation of US horticulturalist and allergy researcher, Thomas Leo Ogren. I have recently read his book The Allergy-Fighting Garden, subtitled Stop Asthma and Allergies with Smart Landscaping. He draws attention to city landscapers’ preference for male trees over messy female trees. There is widespread ignorance that male trees litter the air with pollen. Male trees cause many allergies and asthma, especially when in close proximity to an allergy sufferer’s home without the barrier of non-allergenic hedges. While it occurs to me that the allergenicity of plants may vary due to differences in the US and the UK climates, Ogren’s detailed classification of plants seems useful to improve air quality. I have picked out some trees of interest to the UK from the book. A tree scoring a 10 is high on the OPALS scale, while a score of 1 is low.

MAIN BRITISH TREES ON OPALS SCALE

OPALS SCALE	TREE	UK POLLEN SEASON	UK PEAK SEASON	COMMENTS
10 (Male)	Yew (Taxus)	Jan to mid-April	Late Feb to late March	Don’t plant near bedroom windows. Pollen also irritates eyes, nose, throat, causes headaches, itching and asthma.
10(-6) (Male)	Willow (Salix)	early Feb to early May	Early March to early April	Allergenicity varies according to Salix male species. White Willow (Salix alba), Weeping Willow (Salix babylonica) & Crack Willow (Salix fragilis) are the worst offenders.
9	Alder (Alnus)	Jan to late April	Mid-Feb to early April	Sheds lots of bright yellow, highly allergenic pollen
9	Birch (Betula)	March to mid-June	Late March to mid-May	Allergenic tree too commonly used in landscaping
9(-5)	Elm (Ulmus)	Early Feb to late April	Early March to early April	Bisexual trees. Allergenicity varies according to species.
8	Oak (Quercus)	Late March to mid-June	Late April to early June	Deciduous Oak scores slightly lower than Evergreen Oak. Pollen is abundant with potential to cause asthma. Tree disease risk. Further research required into degree of allergy between different Oak species. Generally, an allergy to one Oak will mean an allergy to other species too.
8	Plane (Platanus)	Early March to late May	Mid-April to mid-May	Annual pollarding can remove the issue, as male pollen flowers are formed on old wood.
8 (Male)	Poplar (Populus)	Mid-March to early May	Mid-March to early April	Recommendation: White Popular ‘Nivea’ a female cultivar that avoids the pollen issue.
7	Hazel (Corylus)	Jan to mid-April	Mid-Feb to mid-March	Known to cause allergies if allergy-sufferers live close to these trees
7	Ash (Fraxinus)	March to late May	early April to early May	Separate male and female trees. Fraxinus excelsior scores 7, but female just 1. Other non-native species may score lower too. Ash Dieback disease killing huge percentage of trees in Britain at current time.
6?	Beech (Fagus)	March to May	?	Not considered as potent as Oak pollen. Does not feature on Met Office UK pollen season chart at all. Pollen.com website rates both US and UK Beech species as mildly allergenic, so the 6 rating is possibly too high or there are factors other than pollen.
6	Lime (Tilia)	June to early August	mid-June to mid-July	Considered imperfectly insect-pollinated resulting in airborne pollen. However, the heavy pollen does not travel far. Allergy-sufferers are therefore likely to live or work in close proximity.
4?	Pine (Pinus)	Early April to late July	Early May to late June	Author states that although Pines shed large amounts of pollen, the pollen grains are waxy and therefore are not so irritating to mucous membranes. He gives the exception of Pinus contorta at 8 which is found in the UK. Given that Pinus features on the Met Office chart, a general score of 4 may be too low for the UK?

There is currently huge desire and motivation to plant trees due to climate change. The above chart rather leaves would-be tree-planters scratching their heads as which trees are the best to plant. By the time you rule out poisonous trees, disease-threatened trees and trees that will grow too big for the allotted space, what is left? Exotic trees are not necessarily any less allergenic than native ones. The main recommendation is to plant more female trees with OPALS ratings of 1, especially as street trees in cities and near schools. They make less mess with fewer males about.

What tree options are there with allergy ratings under 5? The Rosaceae or Rose family features prominently in the list.

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TREES WITH OPALS SCALE UNDER 5

OPALS SCALE	TREE	COMMENTS
5	Dove Tree (Davidia involucrata)	Deciduous, shade tree.
4	Japanese Snowbell (Styrax japonica)	Ogren considers the pollen innocuous, but the sap of some Styrax species is known to cause rashes in sensitive people. Lightly-scented, attractive flowers. H5 cold winter hardiness according to RHS (i.e. -15 to -10ºC).
4(-6)	Magnolia	Evergreen (Magnolia grandiflora) and small deciduous score 4 to 5. Any allergies caused by Magnolia soulangeana are usually moderate and rarely severe. Also good choice for late frosts. Large deciduous trees may reach a 6 see chart below.
4	Persian Ironwood (Parrotia)	Prized for its autumn colour.
4	Sorbus	Ogren specifically refers to the Mountain Ash/Rowan, Sorbus aucuparia. Brightly coloured berries popular with birds. There are also many rare, endemics in the Cheddar and Avon Gorges.
4	Wollemi Pine (Wollemia nobilis)	Critically endangered Australian tree found in Botanic Gardens.
4-3	Apple/Crab Apple (Malus)	Variation according to cultivar. Pollen is heavy and does not travel far. Living next to an orchard may cause hypersensitivity to pollen. Some Crab Apples are pollen-free.
4-3	Pear (Pyrus)	Ornamentals score a 4, fruiting varieties a 3. Ogren mentions diseases.
3	Hawthorn (Crataegus)	Pollen allergy is not common and is rarely severe if it occurs.
3	Medlar (Mespilus)	A largely forgotten fruit
3	Plum (Prunus domestica)	Scores for Prunus species vary (Almond, Prunus communis is a 10; Cherry, Prunus avian/cerasus is 5-7).
3	Spruce (Picea)	Pollen has a waxy coating that prevents irritation of mucous membranes, even though it is abundant. Tree does not thrive in a hot, dry climate. Better in cool summer and cold winter regions. Notable species: Norway Spruce, Picea Abies.
3	Strawberry Tree (Arbutus)	Drought-tolerant.
3	Tulip Tree (Liriodendron)	Tolerant of a wide range of soil and weather conditions. Downside is that it does not transplant easily. Ogren feels that it it a good, low-allergy tree that deserves to be used more in cities.
2	Abies	All true firs have a waxy coating on pollen that rarely causes allergy. Abies nordmanniana is increasingly popular as a Christmas tree.
2	Cedar (Cedrus)	Large pollen with a waxy coating. Although it sheds abundant pollen, it is not considered responsible for many allergies. Most Cedars score 2 with exception of Deodar Cedar, Cedrus deodara, which scores 4 for monoecious, 6 for males and 1 for female. These trees need space.
2	Larch (Larix)	Does not tolerate warm climates. Slow-growing, but long-living. Usually thrives in damp, poorly drained regions. Unfortunately, the tree is affected by Ramorum disease, Phytophthora ramorum.
1	Female trees!	Many female species of high OPALS male trees are 1 on the same scale. Look for female trees of Yew (Taxus), Willow (Salix), Poplar (Populus) and Ash (Fraxinus).

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OPALS SCALE FOR OTHER TREES

OPALS SCALE	TREE	COMMENTS
10	Cypress (Cupressus)	Profuse amounts of allergenic pollen shed for up to 7 months of the year in warm climates. Used in landscaping in warm Mediterranean region.
10 (Male); 9 (Monoecious)	Juniper (Juniperus)	Primary allergenic in US. Recommendation: use female Juniperus communis ‘Hornibrookii’ for allergy-free landscaping. Female is 1 on OPALS scale
10 (Male)	White Mulberry (Morus alba)	Responsible for the highest pollen count ever recorded in US. Mulberrry species vary in their allergenicity. Black Mulberry is a 2, others much higher.
10	Olive (Olea europaea)	Can trigger severe allergies and asthma. Ogren is even suspicious of so-called low allergy or allergy-free cultivars. Also considered Xylella fastidiosa disease risk to UK.
9 (Male)	Bay (Laurus nobilis)	Common allergenic. Cross-reactions in eating leaves also not uncommon.
9-8	Walnut (Juglans)	English Walnut is considered less of a problem than Californian Black Walnut. Rotting husks can cause allergies as well as pollen. Walnut is toxic or stunts the growth of nearby plants due to juglone compound, especially Juglans nigra.
9-6	Maple (Acer)	Acers vary in their allergenicity. Red Maple (Acer rubrum) male 9: grown in Britain for autumn leaves; Norway Maple (Acer platanoides) 8: all cultivars cause allergies; Sycamore (Acer pseudoplatanus) 8: often regarded as unwelcome tree in UK; Field Maple (Acer campestre) 6.
7 (Male)	Maidenhair (Gingko biloba)	Motile male pollen. Male trees used in landscaping. The female with score of 2 is avoided as the fruit stinks. Considered good tree for very large landscapes where it can be planted far from house. Oily seeds can cause skin rashes.
7 (Male)	Holly (Ilex)	Males are often used by landscapers as hedge plants. You only need a few males to many females to have berries on female plants with score of 1. An allergy that is not well-documented according to Ogren.
7	Hornbeam (Carpinus)	All species can cause limited allergies.
7-6	Horse Chestnut (Aesculus)	Also known as Buckeye. No species recommendations. Ogren mentions pollen is occasionally poisonous to honeybees.
7	Laburnum	Poisoning issue. Pollen can trigger legume allergy.
7 (Male)	Tree-of-Heaven (Ailanthus)	Bad-smelling flowers with lots of allergenic pollen. Fast-growing invasive.
6 (Monecious)	Chestnut (Castanea)	Mostly insect-pollinated. Pollen is sticky and does not travel far. Direct contact is the allergy issue. There are female only and sterile male species with OPALS of 1. Risk of Ramorum disease, Phytophthora ramorum.
6	Catalpa	Well-known in allergy studies.
6	Ostrya	May cause an early spring allergy, but it has a short period of blooming.
6	Sweet Gum (Liquidambar)	Popular street tree in US. Both sexes on same tree. Good autumn colour.
6	Wingnut (Pterocarya)	Related to Walnut. Grown for attractive leaves and flowers.
6-4	Magnolia	Large deciduous scores 6; evergreen (Magnolia grandiflora) and small deciduous score 4 to 5. Any allergies caused by Magnolia soulangeana are usually moderate and rarely severe. Also good choice for late frosts.

© Karen Andrews

References and Further Reading

Books:

• Ogren, Thomas Leo (2015): The Allergy-fighting Garden. Stop Asthma and Allergies with Smart Landscaping. Ten Speed Press. New York. USA.
• Johnson, Owen & More, David (2004): Collins Tree Guide. The Most Complete Field Guide to the Trees of Britain and Europe. HarperCollins. London.
• Sterry, Paul (2007): Collins Complete Guide to British Trees. HarperCollins. London

Online:

• Allergy-Free Gardening (2020): OPALS^TM: The World’s First Plant-Allergy Scale.
• AllergyUK (2019): Allergy UK launches its top 10 recommendations for tree planting in National Tree Week. 22 November 2019.
• Clarke, Nigel & Ross (2017): Allergy Friendly Introduction. Queux Plant Centre, Guernsey.
• Forest Research (2020): Ramorum Disease, Phytophthora ramorum.
• FTD Fresh (2016): The 24 Best and Worst Plants for Allergies.
• Nex, Sally (2018): Not to be sniffed at. RHS The Garden. March 2018.
• Pollen.com (2020): Beech (Fagus).
• University of Exeter (2017): Asthma attacks reduced in tree-lined urban neighbourhoods. 17 November 2017.

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Karen does not seek or receive any commercial interest or advantage from this blog. She is not promoting any business venture. She simply loves to share fascinating facts about plants. These pages illustrate her love of plants, botany, biodiversity, gardens and creative expression. There is always so much to learn about plant diversity. This blog is designed as a showcase for photography, commentary on plants and wildlife, gardens and other places visited, horticulture and related topics. Viewpoints are her own, not those of her employer.

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