Few sights are more disturbing to golfers than weeds poking through the soil where turf should be.
Although flowering weeds are indeed among the most troublesome challenges for golf course superintendents, what lurks beneath the surface likewise should not be dismissed.Weed seed can lie dormant for years, waiting for just the right time to show itself.
"In demonstrations when you cut 2 inches of sod off well-maintained turf, people who come to a field day and look at that are wowed by how much weed seed is actually there," said Jim Brosnan, Ph.D. weed scientist at the University of Tennessee.
That window of opportunity might come in the way of an unrepaired divot or ball mark, anything that leaves even the narrowest window of bare soil. All it takes is an oblivious golfer wielding a 7-iron to set into motion millions of years of evolution.
"They are opportunistic, just waiting for the right time to complete their lifecycle," said Zane Raudenbush, Ph.D., at Ohio State Agricultural Technical Institute in Wooster. "That's all they are about, growing up and producing more seed."
All weeds, however, are not created equally.
Although most of the seeds that can be a nuisance to superintendents reside in the top few inches of soil, there is more to weeds than meets the eye. Scores of seeds can subsist in their dormant state several inches below the surface just waiting for something to bring them closer to the surface.
Although turning the soil and bringing weed seeds closer to the surface where they awake from their dirt nap is a bigger problem in production agriculture than it is in turf, but it is not completely foreign to golf.
"The top inch or two is what people focus on, but any time they renovate, they should be concerned with what they are bringing to the surface," said Dave Gardner, Ph.D., weed scientist at Ohio State University's main campus in Columbus.
Weed seed moves in a multitude of ways, including wind and through bird and animal droppings.
According to a series of fact sheets published by the Weed Science Society of America, seeds have been known to travel hundreds of miles on the wind, and seed from horseweed has been found thousands of feet into the atmosphere. Some invasive aquatic weeds even survived a transoceanic journey when they arrived on American shores after the Japanese tsunami in 2011.
Earthworms, according to the WSSA, have been recorded moving seed to different depths throughout their subterranean ecosystem.
Many factors can trigger seed germination in weeds, including changes in temperature and available sunlight, but there is still much work to be done to fully understand weed ecology, Gardner said.
"It's probably a little of both," Gardner said. "You won't have crabgrass germinating in winter in bare spots, and it won't germinate in summer until it has bare soil."
How long seeds can remain viable in their dormant state still is not fully understood, but some can germinate after lying dormant in the soil for decades, while others die off in as little as a few days. Stan Zontek, the late USGA Green Section agronomist, once told TurfNet that the seeds of some common golf course weeds could survive in dormancy for 40 years or longer.
"Weeds have evolved these mechanisms to continue to propagate their species, and one of these mechanisms is a long dormancy period," Gardner said.
"You can have soil that you think is devoid of weeds, but there is a surprising number in there waiting for the right opportunity."
The rule of thumb, Gardner said, is the larger the seed, the longer it can survive extended periods of dormancy.
"That's one of the things in our favor: many of the weeds that are common in golf are small-seed plants," he said.
However, Gardner said you can't take much for granted when discussing weeds.
"It's wise not to generalize," he said. There are all kinds of weeds."
The best way to keep them in their dormant slumber is to provide a healthy stand of turf, against which they typically are unable to compete. Even then, some can subsist for long periods.
In 1879, professor William Beal began a research project on weed seed dormancy at Michigan Agricultural College, according to the WSSA. Beal buried seed from 20 common weed varieties to learn how long each could remain viable in dormancy. He attempted to promote germination at 5- and 10-year intervals.
Beal eventually retired, leaving his research to others at Michigan Agricultural College, which has since been renamed Michigan State. Beal died in 1924, but some of the seeds he buried 138 years ago still germinate today.
Most of the seed that is of immediate concern to superintendents, those that reside in the top few inches of the soil profile, can be kept in check with a healthy cover of turf, and cultural practices.
What about projects that require tilling up massive amounts of soil, unearthing weed seed that has been buried for generations, or those acres of native areas that have been planted in place of managed turf in the past decade?
"Where I see people getting caught, they have a good herbicide program, and they think they are weed-free," Raudenbush said. "Then there is a renovation project, and because of the soil disturbance associated with that they have weed problems they never had before."
Much more research on the subject, particularly in turfgrass, still is needed, Gardner said. But research requires money.
"A lot of our understanding of weed ecology in turf is borrowed from production agriculture," Gardner said. "Their behavior and they way they adjust should be somewhat similar.
"Most of the focus has not been on understanding how weeds work, but how to kill them. If we had a better understanding of the ecology of weeds, that might be a benefit in killing them."