City Trees

26 CityTREES What do urban forestry professionals like about palms, and in what ways can palms be problematic in the urban environment? Palm trees have been used in many municipal- ities and are considered historic in some cities, complete with a Palm Avenue. Palm use is, of course, limited by climate. In my work in California and in discussions with friends in Nevada and Florida, we've observed that palms often are signature trees at city entry points, and they are often used in sites with limited crown canopy space. My personal observation on palms from an urban cano- py and design perspective is that palms have a limited crown effect. The fronds and cano- py of fan palms are small and the appear- ance is generally effective when palms are between 15 to 40 feet (5 to 12 m) in height—as they exceed that, all one sees is a trunk. While the crown may be visible via craning the neck, there is no shade component, and the appearance is that of a decorative utility pole. Palms with long feathery fronds provide wider canopy; still, canopy shade decreases as the trees grow taller. Even adjacent to tall buildings, the taller palms lose their effect as the crown gets too high. In the positive column, palms are easy to transplant. This is good news because we can transplant large trees to facili- tate in-kind replacements; as the trees grow too tall or die, they can be replaced promptly with a similar size speci- men. Another positive is that palms can be used in tight spaces such as narrow islands, sites with narrow crown space, and narrow planting strips, where they can still provide some benefits, including a vertical element where many other trees with spreading crowns just can't fit. Their root systems adapt better to narrow planting spaces than do the root systems of many angiosperms or conifers. I find that a cycle for replacement needs to be con- sidered for the long-term effectiveness of the palm plantings, changing out the trees when they get too tall. Many designers don't agree with my perspective, but I consider long-term maintenance in addition to canopy benefits and appearance. Most designs are static in nature and only consider the trees at planting time. I bear in mind, for instance, that some palms if allowed to mature get over 80 feet (24 m) tall, and at that height, their tops are very challenging to prune. Palm trees are often either under-pruned—in which case, fronds can fail and create hazards—or they are over-pruned—which shrinks the crown and stresses the tree. Palms may need to be pruned on a shorter cycle than other tree species. Correct pruning provides the largest crown but removes dead fronds that may fail. Palm diseases also create a challenge. Diseases like the fatal Fusarium wilt (Fusarium oxysporum) on Canary Island date palm (Phoenix canariensis) are highly contagious and must be carefully managed. When removing a Fusarium-infected tree, the aerial sawdust from cuts and the chainsaw itself can spread the disease. Also, the root system usually carries the disease, and the planting hole may be challenging to disinfect when attempting to protect a replacement tree from being infected. We have tried many planting-hole care strategies including soaking with bleach and other disinfectant solutions, chemical treatment, surrounding the new root ball with a root barri- er to avoid contact with the original soil, and replacing with a different species not known so far to be suscep- tible to the disease. Fortunately, there are other diseases that often can be treated, and pruning can improve the appearance of diseased trees that don't have the dread- ed Fusarium wilt. The challenge with palms is similar to the chal- lenge arborists face with other tree design elements. Designers tend to think about the appearance of trees more than the long-term growth, maintenance needs, and site limitations of trees. As arborists, we should have the opportunity to comment on the design based on site conditions; above- and below-ground growing space; long-term maintenance, growth cycle, spacing, and pruning needs; and the Useful Life Expectancy. — Gordon Mann, President of California Tree and Landscape Consulting, Inc. (CalTLC) and Owner, Mann Made Resources S M A R O U N D T A B L E Palms in the Urban Forest Editor's Note: Not all "palm trees" are trees in the botanical sense, and not all plants called "palms" are true palms. We use the blanket term "palm trees" for the purposes of this Roundtable.

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