Crabgrass, and oh so many weeds

To Adam he said, “Because you listened to your wife and ate from the tree about which I commanded you, ‘You must not eat it,’

Cursed is the ground because of you; through painful toil you will eat of it all the days of your life.

It will produce thorns and thistles for you, and you will eat the plants of the field…” –Genesis 3:17-19, KJV

Thorns and thistles.

And common lambsquarters, Palmer amaranth, redroot pigweed, horseweed, waterhemp, nutsedge, field bindweed, velvetleaf, foxtail, common and giant ragweed, barnyardgrass, kochia and nightshades. These last named in a recent national survey of weed scientists as among the most common and troublesome weeds plaguing American farmers, but there are many others.

Then there’s my poor backyard lawn, riddled with common and English plantain, white clover, cheeses, chickweed, cinquefoil, henbit, morning glories, dandelions and crabgrass.

Ah, but what a marvelous summer it’s been for crabgrass!

OK, that was sarcasm, but when we jumped in mid-June from an abnormally cool and wet spring into three months of blisteringly hot summer weather, it did seem the cards had been heavily stacked in favor of the hard scrabble weed.

In contrast to our cool-season turfgrasses like Kentucky bluegrass, fescues and perennial ryegrass whose optimal growth occurs around 65-75 degrees, crabgrass is a warm-season plant that flourishes at 80-100 degrees.

As an annual (it dies in the autumn), crabgrass relies on its prolific seed production–150,000 per plant in an unmown lawn–to carry on from one year to the next. And it doesn’t help that its miniature seeds are reported to remain viable in the soil for 30 years or more.

In the spring when melting snow reveals a tiny patch of bare soil, say in a crack in the sidewalk or the empty spot occupied by last year’s weed, light falling on an exposed seed triggers germination and it’s off to the races again. (This is why experts say one of the best mechanisms for controlling crabgrass is to keep the lawn mown to about 2-4 inches–enough to shear off its seed-producing inflorescences while minimizing its seeds’ access to open soil.)

Worldwide, something under 8,000 species of plants have been identified as agricultural weeds although only 250 of these are purported to have significant economic impact. But that impact is enormous. The U.S, Cooperative Extension System reports that weeds cause more yield loss and add more to farmers’ production costs than insect pests, crop diseases, root-feeding nematodes or warm-blooded pests like rodents, deer and birds. A lot more

Why do we have such problems with weeds? Where do they come from?

“Because you listened to your wife and ate from the tree…Cursed is the ground because of you…”

A 2017 Gallup Poll found that 38% of American adults interviewed held the view that “God created humans in their present form at one time within the last 10,000 years.” While recognizing that a sizeable number of serious-minded people would find a metaphorical interpretation of the Holy Word unacceptable, perhaps it would be of interest to consider how this seminal passage might be viewed from the perspective of an evolutionary biologist.

Archeological data indicates that domestication of plants for agricultural purposes began in numerous locations around the world 10,000-12,000 years ago. In his Evolutionary Ecology of Weeds, Iowa State University’s Jack Dekker argues that weeds are the direct and inevitable result of that human behavior.

By learning how to clear a plot of land in order to plant a seed in hopes of a future bounty — having tasted of the fruit of the Tree of Knowledge, in one sense — our ancestors had set the table for their enemies.

Dekker argues that it is the competitive relationship between humans and plants that defines a weed. Long before the advent of agriculture, numerous species of plants already had specialized in taking advantage of natural disturbances such as fire, erosion, landslides, storm events and drought that temporarily opened up patches of nutrient-rich soils.

Agriculture was just another disturbance, albeit an increasingly large one.

To take advantage of such transitory opportunities, a plant would first need to be able to find the disturbed opening and then to grow and reproduce at lightening speed before being crowded out by new growth from the previous owners.

These are the traits typically found in many of our most troublesome weeds. Prolific production of small seeds that readily disperse far and wide, rapid seed germination and growth, the ability to grow from a fragment of root or stem and high tolerance to various stressors (drought, repeated mowing/grazing/tillage, herbicides, etc.)

Ecologists commonly refer to such opportunists as pioneer species because of their ability to take advantage of newly available habitats.

I do admire your pioneering spirit, crabgrass, but I’m still mowing the backyard this afternoon.

Ken Baker is a scientist and a retired biology professor. If you have a natural history topic you’d like the author to consider for an upcoming column, email your idea to newsroom@advertiser-tribune.com.

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