Maintaining History, Shade, the Birds and the Bees: Trees in Potrero Hill and Dogpatch

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Potrero Hill and Dogpatch are home to a number of “Landmark Trees,” so designated for their size, historical value, or beauty by the City. These include the Acer rubrum, “red maple,” at 862 Kansas Street; the Callistemon citrinus, “lemon bottlebrush,” on the east side of the 400 block of Arkansas Street; the Carpinus betulus ‘Fastigiata,’ “Erect European Hornbeam” at 227 Connecticut, at Mariposa; and the Syagrus romanzoffianum, “Queen palm,” at 1601 Mariposa, at Connecticut. According to Mei Ling Hui, urban forest and agriculture coordinator for the San Francisco Department of the Environment (SFDoE), Landmark Trees are chosen by the Board of Supervisors and protected from physical damage and removal.

Many trees find the Hill and Dogpatch challenging. The communities have steep slopes, full sun, high winds, serpentine soil, and dry spots. According to Geoff Coffey, general partner at Bay Natives, located in Bayview, California natives, particularly species local to San Francisco’s hills and flat areas, are best suited to Southside neighborhoods. “They don’t mind the fog or a sunny day. They’re also adapted to serpentine soil, which has a very low nitrogen content and a very high magnesium content,” said Coffey.

Coffey said the best trees for Dogpatch and the Hill are Ceanothus, known as “Mountain lilac” or “California lilac,” particularly C. arboreus,

C. thyrsiflorus, and the “Ray Hartman” cultivar, a hybrid of C. arboreus and C. griseus. Ceanothus have small, tightly packed blossoms that can be purple, blue, or white.

Coffey also suggested Sambucus cerulean, known as “blue elderberry” or “blue elder,” particularly Sambucus nigra ssp. cerulean. These plants have bunches of small blossoms, with flowers that range from white to light yellow to pink. Another native tree is Myrica californica, known as “California bayberry,” “California wax myrtle,” or “Pacific wax myrtle,” which has thick magenta blossoms and small waxy purple fruit.

Coffey said native trees with fruit or blossoms attract a variety of pollinators. “The native specimens are the ones those birds and bees and butterflies absolutely recognize. Wax myrtle is especially good for wildlife. I’ve never seen one that’s not absolutely alive with twittering birds,” he said.

For those interested in non-native species, Michele Palmer, education coordinator of San Francisco nonprofit Friends of the Urban Forest (FUF), recommended Arbutus marina, known as “strawberry tree,” which has small, round, red fleshy fruits; Gingko biloba, the “maidenhair tree,” with half-moon-shaped leaves that turn yellow in the fall; and Liquidambar styraciflua, known as “liquid amber,” “sweet gum,” and “alligator wood.” Liquid ambar has five-pointed, star-shaped leaves that turn red, orange, and yellow in the fall, and hard, spiky brown fruits with a thick hull.

Peter Brastow, senior biodiversity coordinator at the San Francisco Department of Environment, said the City encourages planting more native trees “as appropriate throughout the built environment for habitat and sustainability.” Brastow said residents can consult SF Plant Finder, http:// sfplantfinder.org, to determine plants suitable for their location.

Palmer and Coffey agreed that pruning is important. “You want to prevent plants that grow out, like wax myrtle, from becoming a shrub,” said Coffey.

“After the first 18 months, we usually do some structural pruning on trees,” said Palmer. “You want to leave enough foliage for the tree and space for birds to build nests.”

Coffey said fall is the best time to plant. “You want to get trees in the ground by mid-November, before the rain starts. We usually get the most rain in December,” he said.

This year, it’s important to get trees planted early so they’ll be well-rooted and strong enough to withstand the heavy rains expected because of El Niño.

Coffey said that if a tree is being planted on a steep slope, a “French drain,” or perforated pipe, should be installed on its high side. The pipe should be inserted vertically into a hole near the tree’s root ball. The tree should be staked when it’s first planted. It’s helpful to put up a sign to let passersby know to protect it. “A tree spends its first year just putting roots down,” said Coffey. “Suddenly, in its second year, it’s going gangbusters on top. But it needed to get that root system established first.”

According to Coffey, it’s hard to judge a tree’s progress in its first year. After the second year, a tree that’s not growing may benefit from fertilizer. Almost all fertilizers contain a good amount of nitrogen. Many native species have root nodules with nitrogen-fixing microorganisms that free up nitrogen to be used by the plant. As a result, fertilizer gives native plants a double dose of the mineral.

Unfortunately, there’s no way to remove excess magnesium from soil. When soil contains a high amount of magnesium, it usually lacks a normal amount of other nutrients, such as calcium and potassium. Plants that cannot draw enough calcium from the soil produce less fruit and can’t grow normal storage roots. Plants that don’t get enough potassium cannot photosynthesize well.

Amber Hasselbring, executive director of the San Francisco non-profit Nature in the City, said trees in low-lying areas have access to the water in aquifers and streams. They tend to grow differently than trees on hilltops. Hasselbring recommended Prunus ilicifolia, known as “hollyleaf cherry,” a species that used to grow by Islais Creek, for those who live by a stream.

She said reintroducing native trees to a neighborhood can add shade and change interactions in the soil, because trees collect water. “San Francisco has over 44 hilltops, each with their own microclimates,” said Hasselbring. “When you remove concrete and put in trees, you allow wildlife…to take advantage of what resources are there.”

The trees appropriate for Potrero Hill and Dogpatch today aren’t the ones that may have grown there before the City was well settled. Some native species grow to great sizes. They rain down leaves and seeds, and send out roots that break up sidewalks. “(California) buckeyes, along with oaks, grow along north and east-facing shorelines around the Bay, such as at China Camp State Park in Marin,” Brastow said. (They) would have characterized the rocky shoreline of San Francisco.”

According to Hill resident Mary Purpura, a non-native but also notable plant is a mature, fruit-bearing fig tree on De Haro Street, a few houses up from 26th Street. “The house is in the Parkview Heights housing development, where most of the front yard trees are landscaping trees, not fruit trees. It means to me that the homeowner there had a different idea, took the initiative, and now has a beautiful fruit tree,” said Purpura.

Palmer said Potrero Hill and Dogpatch property owners have been receptive to having FUF plant trees as part of a cooperative effort to green streets. “We’re looking forward to putting more trees in these neighborhoods,” said Palmer.

Coffey said for those who have or want trees, fall is a good time to consider installing a rainwater catchment system. “The urban environment is very challenging for plants, because many areas lack water. Capturing storm water and using it allows you to save on water bills later on,” said Coffey.

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