About 20 years ago horticulturist and author Thomas Ogren became interested in allergy-free gardening because his wife suffered from allergies and asthma. He began research and went back to college to get his MS degree, focusing on plant/allergy connections. He discovered that many 'dioecious male' trees triggered severe allergies. Ogren lives in San Luis Obispo, CA. His books have been reviewed in dozens of publications, been the focus of a CBS Evening News special, and the Discovery Channel in Canada filmed a documentary about his research results.
Pollen is the main culprit in allergies (although molds are also important). In 1972 a pollen scientist named Raynor demonstrated the importance of locality in allergy. A simplification of his findings boils down to: a male pepper tree in bloom in your yard will expose you to ten times as much allergenic pollen as a similar pepper tree in a neighbor's yard down the street. As gardeners, we can control what we plant and dramatically limit allergens in our own yards: the fewer allergenic plants in the garden, the less chance of exposure.
Ogren realized that the allergy problem was increasing because male trees, which do not produce fruits, seeds, messy flowers or old seedpods, were being planted by the millions in California cities and landscapes. Perhaps we should digress and briefly discuss the sex of plants, since most people don't realize that some are male, some are female, and many are bisexual.
Perfect flowered. Many flowers, such as apple blossoms or roses, have both male and female parts all inside the same flowers. These flowers may also be called bisexual or complete. The pollen in a perfect flowered plant doesn't have far to travel, and often, but not always, these plants cause few pollen allergies.
Monoecious flowered. In Latin monoecious means "one house". In this system there are separate male and female flowers; however, they are both on the same plant. Corn is a good example of a monoecious plant. The tassels are male flowers and the silks in the undeveloped ears are female flowers. The pollen from the tassels drifts down to fertilize the silks and when one silk is pollinated a kernel of corn develops. Oak and cypress are two examples of monoecious flowered trees. They are pollinated by the wind and each is capable of causing allergy.
Dioecious flowered. In this system, plants have flowers that are all male or all female, and the plants themselves are either all male or all female. In most cases, the pollen from male plants is carried to the female plants by wind. Dioecious plants include ash, willows, poplars, hollies, pepper trees, some maples, mulberry, all ginkgos, many palms, and others. Pollen from dioecious males is typically quite abundant and causes a great deal of allergy.
Ogren's discovery. The relationship of plant sex to allergic reactions bears repeating. Ogren discovered that many 'dioecious male' trees triggered severe allergies. Male trees shed a lot of pollen. All the flowering plants, except 'dioecious females', have pollen. Through his intense research Ogren found several trees to be pollen-free or close to it. He rated more than a thousand species of plants on a scale of 1-10 with 10 being severely allergenic.
A few examples of trees with a rating of 1-3 are firs, cedars, and strawberry tree (Arbutus unedo), as well as the fruit trees like apricots, avocados, peaches, plums, nectarines, persimmon, pineapple guava, but also almonds and flowering red leaf plums, bronze loquat, and flowering pears. Also the female trees of ash, Chinese fringe tree, Chinese pistache, junipers, some named maples, palms, podocarpus, poplars, sour gum or tupelo, and willows.
Some of the worst trees are acacia, alder, beech, birch, buckeye, CA pepper, camphor, catalpa, Chinese evergreen elm, cypress, liquidambar, mimosa, oak, olive, pecan, sycamore, walnut, and zelkova.
Ogren's Best Plants. In general, the best plants for an allergy-free garden are those that produce large, very showy, lightly scented flowers, in which the male flower parts are either few in number or deeply recessed within the blossom (these are usually pollinated by insects and rarely cause pollen problems).
Then there are the separate-sexed species, the 'dioecious females' that do not produce pollen. They are often commercially named or are easy to distinguish from the male tree (because they contain fruit clusters). Be aware that there are drawbacks to female trees. When pollinated they produce fruits/seed capsules that can be troublesome. Research your species before you plant as some trees are considered messy, while the fruit of another species are not a bother at all.
Reference: Allergy-Free Gardening by Thomas Leo Ogren, 2000. Ten Speed Press, Berkeley, CA