Society for Allergy Friendly Environmental Gardening



OPALS® (Ogren Plant-Allergy Scale) is a science-based tool designed for improved, smarter, allergy and asthma-friendly plant selection. OPALS® has been used by many individuals worldwide, by allergy and asthma coalitions, by cities, schools, parks, hospitals, state departments of public health, by lung associations, and by the United States Department of Agriculture.

OPALS® uses a numerical scale to rank plants, ranging from 1 (least allergenic) to 10 (most allergenic). More than 130 different criteria are used to develop allergy rankings for plants. Each factor is either positive or negative. All factors are not weighted the same because some are more important than others. Most plants will have a combination of positive and negative factors that are computed to determine their OPALS® ranking.Some of the factors used to rank a plant are:

  1. The amount of pollen produced, if any
  2. Potency of the individual pollen grains
  3. The amount of time during a year that the plant is in bloom
  4. The size, density, shape, and weight of the pollen grains
  5. The dryness or stickiness of the grains
  6. Is the tree perfect flowered, monoecious, dioecious, or polygamous
  7. Sex of plant, if dioecious
  8. Position on the plant of pollen flowers
  9. Average rankings in actual skin scratch, patch, and sniff tests
  10. Cross-reactivity to food allergies
Chinese pistache tree (female)
Note: Additional information regarding the factors used to build OPALS® may be found in the book - Safe Sex in the Garden (Chapter 4 – Understanding OPALS) by Thomas Leo Ogren.OPALS® ranks each type of plant in relation to other plants of the same type. All perennials are ranked against other perennials, the shrubs are ranked according to other shrubs, and trees are ranked only against other trees. Thus, a tree ranked 8 will have far more potential for allergy than will a perennial also ranked 8. This is simply because the tree is so much larger.A plant with a ranking of 1 or 2 has very little potential for causing allergy. If the plant has a rank of 2 to 3, it still has very low potential to cause pollen allergy unless directly sniffed, but it may have potential to trigger skin rash from its sap.Plants that rank from 1 to 5 should be considered low-risk plants. However, the allergy potential does rise as the number increases, so while there is little wrong with having a few 5s in the garden, you wouldn’t want to plant too many of them.Plants with allergy rankings of 10 are the worst. These can often cause both hay fever and asthma. They may also trigger skin rashes. A plant ranked 10 is known to cause the worst kind of allergy and to have the potential to affect a high number of people.

For more information, go to OPALS in Wikipedia.

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Below are OPALS Labels - now available at Queux Plant Centre; Castel - Guernsey, United Kingdom