The earth needs trees. They provide food for untold numbers of avian, mammalian, and insect species. They also make the oxygen that most of these species need to exist. They make shade, they collect fog and other moisture, and they aerate the soil that their roots thread through. And those are just the obvious benefits. Even more fungal, bacterial, and archaea also live in sync with them so that scientists in many fields are still finding astonishing reasons to revere trees.

This time of year, some among us also find them an obvious source of discomfort because of irritating pollen. With all the windy weather this spring, many have found themselves sneezing and suffering from watery eyes and dripping noses. Some of the worst offenders are acacias, maples, alders, citrus, eucalyptus, and pines. Industrial and commercial enterprises must consider the impact of their emissions on the general pollution levels, and gardeners should be aware of what our landscape choices are producing.

Besides pollen, trees also give off many different kinds of volatile compounds and fine particulate matter (some species emit up to 10,000 times as much as others) that can contribute to higher ozone concentrations, one of the major components of urban smog. In a perfect world, these trees would be found growing in a natural ecosystem free of other sources of pollution. In our metropolitan world, where the sources of pollution are many and varied, certain species may do more harm than good if planted in high numbers.

High emitters include London plane and California sycamore (Platanus acerifolia and P. racemosa), liquidambar and Chinese sweet gum (Liquidambar styraciflua and L. orientalis), goldenrain tree (Koelreuteria paniculata), and many oaks (Quercus species), eucalyptus, and bottlebrush trees (Callistemon species). Some good choices for low-emission status are Chinese hackberry (Celtis sinensis), avocado, peach, ash (Fraxinus species), sawleaf zelkova (Zelkova serrata), eastern redbud (Cercis canadensis), and many conifers, such as pines and cedars.

Most gardening resources, of course, include details about the size, shape, and requirements of available trees. A few also list the pollen offenders, but one good source of information on this issue of the negative effects on air pollution by certain trees is readily available online. The results of a study that rated 1,400 tree species for their pollen production and other impacts on pollution have been included in an excellent website maintained by Cal Poly San Luis Obispo. Visit for excellent descriptions, images, and recommendations of trees suitable for many planting situations.

Plant trees in the backyard or by supporting reforestation projects worldwide. Everyone will benefit, but you and your neighbors may breathe easier if they aren’t among the most irritating pollen producers.

May Tips

• Subtropical fruits – citrus, avocado, guava – can be planted and pruned now. Also, cut back bougainvillea, tibouchina, and abutilon.

• Plant a block of corn (i.e., four rows of four plants) for best pollination and full ears. Also plant peppers, tomatoes (watch out for hornworms), and eggplant.

• Wash off aphids, but leave a few so that beneficial insects will have a reason to come to your aid.

• Take advantage of National Public Gardens Day, May 10, with free admission to some area favorites and a special tour to celebrate these community treasures. Info at


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