Whittier, California From a sunbaked peak in the Puente Hills Preserve, the panorama stretches from the skyscrapers of downtown Los Angeles to the edge of the fog-blanketed Pacific Ocean. In between, rolling green and brown hills are dappled yellow with blooming black mustard, an invasive plant that poses a mounting threat in California.

Native to Eurasia, the black mustard plant outcompetes native vegetation because it grows profusely and its roots generate biochemicals that stop the seeds of other plants from germinating. Its growing season makes it a particular menace: It thrives in the spring and can grow to eight feet high, only to die and turn to dangerous tinder by early summer.

The buildup of dead