March 21, 2010

RELAX: Don't let those weeds bother you

Morning Sentinel Staff

Weeds, even invasive species, are not necessarily evil. The word "weeds" is pejorative, showing a bias on the part of those snooty people who have good soil and control what grows on their property.

A more appropriate -- vegetatively correct, if you will -- term would be cosmopolitan urban vegetation.

Peter Del Tredici, senior research scientist at Harvard University's Arnold Arboretum, said in a talk at New England Grows that these urban landscapes are not the problem but a symptom of the problem, and they provide a glimpse of what the landscape will look like in the future.

Urban environments create a heat island, with buildings retaining and creating heat. With the entire world getting warmer, people will see changes from global warming first in urban environments.

"The hemlock woolly adelgid dies at minus 5 degrees Fahrenheit," Del Tredici said. "With winter temperatures moderating, the adelgid will spread."

He believes that in the future, forests will be filled with black birch instead of hemlock when the adelgid spreads north and kills hemlocks.

Other changes come when people bring in plants to help. When Dutch elm disease killed off most of America's street trees, the elms were replaced with Norway maples. The maples turned out to be prolific and naturalized into the environment, and people are now trying to get rid of them.

"Today's savior always seems to become tomorrow's villain," Del Tredici said.

But people keep trying to put in street trees, although most of them last about 10 years. Even in Portland, a small city, you can walk along the brick sidewalks and see paved squares where trees used to grow.

If the squares don't get paved over, they produce spontaneous vegetation (weeds) that can survive a highly compacted soil and lots of road salt.

Del Tredici said there are three types of urban landscapes.

A few natural landscapes remain, whether woodlands or wetlands, with mostly native plants. These have moderate maintenance requirements.

Managed urban landscapes include parks, estates, lawns, often on imported soils. They have high and expensive maintenance requirements.

"There are no weeds in Central Park," Del Tredici said, "because they grind up dollar bills and use them as mulch." (Having never been there, I assume he was speaking metaphorically.)

Finally, you have "Ruderal" (adaptive) landscapes. These include vacant lots, roadway edges, and degraded wetlands and woodlands.

"They are dominated by spontaneous (weedy) plants on compacted or fill soils with zero maintenance requirements," as Del Tredici wrote in his handout for the talk.

And zero maintenance requirements is the key. With all governments strapped for cash, they don't have to spend any money on these landscapes.

Del Tredici had dozens of examples of thriving urban vegetation. Many plants use chain-link fences to thrive. Mowers and weed-whackers can't reach the plants when they grow close to metal fencing, so they climb the fences and make what amounts to hedges. Others, like green foxtail, do better than anything else sprouting in pavement cracks. Japanese knotweed grows very well in cities because it has a long taproot that will penetrate the soil. Plants grow in abandoned lots, among rocky outcroppings and along railroad tracks.

And what is Del Tredici's reaction to these plants? "If it's green, it's good," although some are better than others and a few -- like poison ivy and imported bittersweet -- can cause major problems.

Woody plants and long-lived perennials contribute most to human welfare, while annuals and biennials contribute less, he wrote in his handout.

For starters, all plants -- not just urban plants -- produce oxygen and take in carbon, which helps reduce global warming. All plants control erosion. On river and stream banks, plants like wild roses and river birch stabilize the rivers. In marshes and along ponds, the vegetation absorbs nutrients, which will cut down on algae.

Urban vegetation makes it cooler in paved areas of the city because plant material doesn't retain heat the way buildings and pavement do.

Phragmites australis, the common reed, grows up to 20 feet tall in dense patches of wetland, crowding out native plants. But it can be used to clean up wastewater, trapping heavy metals in the plant matter so they cannot do other damage.

Del Tredici is not saying that things would not be better if we could have native plants or tended gardens in the urban landscape. But he is saying that if you cannot figure out a way to get native stands or tended gardens, you had better be happy with your cosmopolitan urban vegetation and think positiviely about it.

Tom Atwell can be contacted at 791-6362 or at:

tatwell@pressherald.com

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