On a hot, sunny afternoon, walk outside and find a parking lot bordered by grass, shaded by a tree. Put one hand on the asphalt and the other on the grass. The surfaces of stone, brick and cement that keep weeds and water out of our way can sizzle in the sunlight and raise local temperatures. To make room for all these buildings and roads, cities squeeze out vegetation that would otherwise cool its surroundings by evaporating water. Added to the mix are car motors, hot air from air conditioners, clothes dryers, airplanes, even smokestacks. All of these heat sources work together to raise temperatures. The annual mean temperatures of urban areas can be up to 12 degrees warmer than surrounding non-urban areas; on extreme occasions the temperature differences between the city and the country side can be as high as 21.6 degrees. Black rooftops in New York can reach a staggering 160 F on the hottest summer days. This 'heat island effect' means that the core areas of many cities have already warmed up to the levels predicted for the surrounding countryside 20-50 years from now.

Today, 54 percent of the world's population lives in urban areas, a proportion that is expected to increase to 66 percent by 2050. How do we live in a city that gets unbearably hot? The answer is pretty straightforward: not just more trees and plants for the city but choosing the right ones; the ones that will survive de-icing salts, compaction, drought, heat, flooding, and warmer, shorter winters.

Japanese knotweed (Fallopia japonica), is a large, shrubby perennial. Native to Japan's volcanic mountains, it was born to be inundated by ash and poisonous gasses for years at a time. Japanese knotweed was originally introduced into the United States as an ornamental plant during the 1870s where it quickly escaped cultivation. Plant nurseries advertised the hardy newcomer as fodder for cattle. They said that its canes were good material for matchsticks, and its underground stems or rhizomes, an effective agent for stabilizing sand dunes. Knotweed was, they promised, 'impossible to destroy'.

It was another century before biologists began to realize a systemic collision was taking place. Humanity's project to connect the world was overriding the earth's natural barriers of water, mountain and desert. Biologists sometimes use the term "enemy release theory" to describe the ability of foreign creatures to overwhelm a new habitat. If there is a food supply and nothing trying to eat you, then even humble beings can become monsters. The two hundred or so insects that plagued knotweed in Japan and the stronger rivals that stole her light were nowhere to be seen in the US.

But here, all it does is grow. Although knotweed produces seeds, they are rarely viable and reproduction occurs primarily through extensive rhizomes. Rhizomes can stretch for sixty feet and are adapted to lie dormant in the ground for years at a time, in case the land above is destroyed by an eruption. Its stems grow up to four inches a day in the summer, cracking through roads, undermining buildings, and filling in riverbeds. Up close, knotweed leaves are the size of young children's faces. They hang in the thousands from tubular, smooth stems that stand twelve feet tall and emanate in groups from heaped, brown mounds known as crowns, which serve as the weed's carbohydrate store in winter.

The plant is present in at least forty-three states and is considered 'noxious' in seven of them. Trying to dig it up is a fool's errand. Just 0.7 grams of knotweed rhizomes, a fingernail, can spawn a new infestation. Unfortunately, the only reliable method for killing knotweed is poison. You can use ordinary old glyphosate, also known as Roundup, the systemic herbicide developed by Monsanto in the 1960s. Because Roundup is a dangerous chemical, it is best to hire a professional applicator. There are two ways to go about it: spray the leaves and canes, or inject the poison into the stems. But what you really need is patience. It take 5 years of repeated applications, in the spring and in the late summer, for the chemicals to go through the plant and kill the rhizomes. The characteristics that make invasive plants and animals such a fundamental threat to the planet's biodiversity- their desert monocultures, their boring, uncanny vitality- are both impressive and frightening.

A typical, healthy mass of Japanese knotweed in flower in late summer

Its sturdy red-brown asparagus-like shoots appear in early spring and grow rapidly, as much as four-inches a day.

Japanese knotweed has hollow, bamboo-like stems that can reach 10 feet in height.

The leaves, pale to bright green and strongly veined, are broadly egg shaped, two to five inches wide and two to six inches long, with sharply pointed tips. 

For more information on the Japanese Knotweed