Archive for the ‘Plants’ Category

I should have known, of course.  In a recent post on trees and pollen, I wrote:

Bee-pollinated trees don’t bother to release the kind of pollen that makes you sneeze, and wind-pollinated plants don’t bother to attract bees. (There may be belt-and-suspenders plants out there that can be pollinated by wind but would like also to be pollinated by bees, but this non-botanist isn’t aware of them. Readers?)

I like taxonomy and either/or, but nature likes redundancy and both/and.  Nature is right; I stand corrected.

Species with a Plan B tend to stick around longer. So it turns out at least two genera of New England trees (and probably many more elsewhere) are both wind- and insect-pollinated. Maples and Willows have small but clearly visible flowers that are pollinated both by wind and by insects.

Our native Columbine, Aquilegia canadensis, has a back-up plan too. Its long red nectaries attract hummingbirds and its yellow bits attract bugs.

Thank you to Frances Clark, in her excellent Wildflowers of New England class at Garden In the Woods, for cluing me in on this.

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I wrote here that “a big tree does more to keep a city cool and clean than a small one does.” Adrian Benape, New York City’s parks and recreation commissioner, has the numbers.

The first number is 102. New York, he says, plants “102 different unique cultivars and species of street trees in pursuit of a more diverse urban forest.”

The second number is 65. Benape writes:

I wouldn’t trade in the street trees planted by our predecessors for anything. These venerable specimens of the urban forest give us the most benefits right now — more than 65 times those of a smaller tree, according to research by the United States Forest Service. They clean our air, shade our streets, reduce energy costs, increase property values and beautify our neighborhoods.

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ZZZZZZZZZZZZSSSSSSDDDDchew!! Excuse me; Toby’s photo of the pollinating pine in that last post just makes my nose tickle.

His points about pollinating trees make sense to me; wind-pollinated trees are different in nature from insect-pollinated trees, and have quite different effects on those allergic to fine particulates.

I have to say that Mr. Ogren’s original Op-Ed piece stood out as much for its hypo-allergenic focus as for its assertions about what trees do get planted in cities. To set the record straight, a number of the plants he mentioned (both favorably and unfavorably) — box elder, mulberry, silver maple, willow, cottonwood, and even red maple — are considered either outright weed trees or are simply weak-wooded and would never pass muster for planting in dense urban areas. More often than not, most of these trees volunteer in the city, and are able to thrive because they are wind-pollinated, produce a lot of seed, and are able to grow quickly and under conditions that would stymie less tenacious trees.

Landscape architects and arborists working in cities tend to avoid these plants for street tree plantings; in fact, I would think of using some of them — if any — only on larger sites where space was not an issue, where I wanted fast and effective growth, and where there was no chance that they would invade other areas. Municipal lists of acceptable street trees are actually, quite limited, as the piece points out, and mainly include slower-growing or more structurally sound trees than some of these worst culprits — the criteria for street tree selection in a city have to take into account those trees that have relatively reliable structure and habit, can deal with difficult soil, exposure, and moisture conditions, and that don’t create walking hazards with heavy fruit set.

What struck me most, after reading Mr. Ogren’s piece, was the realization of how much vegetative biomass there is in cities, despite our best efforts to pave wherever possible. The ocean of pollen we swim in every spring and summer comes from volunteer as well as planted trees, shrubs, grasses, and forbs, and reducing the pollen count in any area feels a little bit like setting aside a piece of the ocean to make a freshwater pond…

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On Saturday the Times published two letters that responded to its recent op-ed piece by Thomas Lee Ogren on trees, pollen, and allergies.

One reader, Christine Lehrer, wrote:

Honeybees collect pollen from the very trees that are causing all the sneezing and runny noses. By taking a spoonful of honey daily, approaching and during allergy season, you inoculate yourself against the offending pollen and greatly reduce your allergic reaction.

Another, Joan Edwards, wrote:

Plants with showy colorful flowers like magnolia, black cherry, redbud and flowering dogwood … make pollen that is designed to affix to their pollinators. Their pollen is sticky, produced in small quantities, and large so it is less likely to be blown in the wind, where it can be inhaled by an unsuspecting passer-by.

Both of these cannot be true. My money is with the Edwards: Bee-pollinated trees don’t bother to release the kind of pollen that makes you sneeze, and wind-pollinated plants don’t bother to attract bees. (There may be belt-and-suspenders plants out there that can be pollinated by wind but would like also to be pollinated by bees, but this non-botanist isn’t aware of them. Readers?)

So by planting only insect-pollinated trees and the female cultivars of wind-pollinated trees, we can reduce pollen counts. Good information, as far as it goes, but neither Ogren nor Edwards successfully addresses the other qualities that urban trees require. Ogren writes:

In New York City, street trees are selected only for their hardiness in winter; their resistance to disease, insects and drought; their ability to withstand smog; and their size, shape and color.

Even when selecting “only” for those criteria, the trees that pass muster make up a pretty short list.  Adding a new criterion will only make the list shorter. And in fact, very few of the trees that Ogren and Edwards recommend would perform well as street trees.

It turns out New York’s criteria make sense.  Size, for example, matters. A big tree does more to keep a city cool and clean than a small one does.  It casts more shade, transpires more water, absorbs more noise, and (if its leaves are fuzzy) traps more particulates. Plus, a sidewalk tree’s lowest branches have to be higher than a tall pedestrian’s head, so the tree has to be large enough that it can be limbed up without looking like a broken umbrella.

Trouble is, most big trees are wind-pollinated. (Evolutionary biologists, pitch in here: Is it because more insects fly at dogwood-height than at oak-height, or are there some other phenomena at work? What is it that the tall insect-pollinated trees — tulip poplar, cucumber magnolia, caltalpa, horse chestnut — have in common?) Whatever the benefits of planting only the kinds of trees that don’t release a lot of pollen, our urban forests would be poorer for it, and our cities would be hotter, to the detriment of trees and people alike.

Instead of banning wind-pollinated trees altogether, what about mapping prevailing spring winds and vulnerable populations (schools, hospitals?), and designating zones in each city where only female and bee-pollinated trees would be planted? If we can put some allergists and public health specialists in the same room as some planners and urban foresters, we might get somewhere on this.

In the meantime, stir a little local honey in your tea. Probably won’t help. Can’t hurt.

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As I was driving down a Cambridge street last Tuesday this scene caught my eye. My heartfelt good wishes and thanks to whatever forward-thinking kind soul who planted these crocus bulbs and let them naturalize through the lawn; after a long and cold and grey winter they were balm for the eyes.

Apologies for the small image size — the light was about to turn green, so I had to snap before I could zoom in.

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Last week a friend mentioned seeing a yellow-flowering shrub on the VFW Parkway in Boston. It reminded me of the show of Hamamelis that used to appear outside of the Harvard Business School’s Baker Library back in the early 80s; when I first saw it (this was a few years before I became a landscape architect and learned what Hamamelis was) I thought a bank of Forsythia was blooming in February.

Saturday I was in Cambridge, and drove down a street that used to be on my route home from work at CRJA. As I turned the corner, this sight greeted me.

This street runs roughly east-west, and with buildings on both sides gets a slice of sun in the middle of the day. The sycamore maples further along the street add a bit of dappled shade to that slice, too. These Witch Hazels don’t seem to mind. And they have the room they need to spread and fill out their characteristic fountain habits.

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Cattleya in bloom; Ripsalis in background

Another grey and cold day in a long, cold month. Going to my desk and working is a good antidote to the gloominess, especially when the Cattleya next to my drawing board blooms (as it did last fall), or the Ripsalis in the window each January reliably turns from a mop of green string into a mop of green string and yellow confetti. Each plant has its own delicate flower fragrance, which rests quietly in the air until someone walks through the room and stirs up faint currents of deliciousness. It’s a good reminder to use scented plants in the landscape.

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