Yellow irises (called flags) are common along our road in Ireland. They are European natives, and we often snip a few for the vase by the hearth.
And now here they are showing up in the water meadow along my New England path, garden escapees, imported from Europe during the last century, and by all appearances a lovely addition to the native flora.
Unfortunately, like many alien species, yellow irises have a way of crowding out natives plants, and many North American environmentalists consider them an invasive pest. The few lovely plants in my photo may make of themselves a nuisance a few years down the road. Which means, by dictionary definition, we must consider them weeds.
Many handbooks define a weed as "a plant out of place." This assumes that we know what is the proper place for a plant. Dandelions are considered out of place in a lawn, but don't tell that to the dandelions. From a Darwinian point of view, a dandelion in the lawn or in a crack in the driveway is very much in place, a flawless adaptation of plant to habitat.
The concept of "weed" can be usefully extended beyond plants. Most of us would categorize starlings as animal weeds. Some of us would apply the term to the white-tailed deer that invade our backyards. Definition: A weed is any species of life adapted for prolific colonization of disturbed habitats, often displacing indigenous species.
Now you know where I'm heading. What is the species that is best adapted for intruding its burgeoning progeny into every nook and corner of the planet, displacing other species of plants and animals, driving many to extinction?