The not so incredible, undesirable and flexible weed

by Doug Toburen

Wouldn’t it be nice to come up with a plant that would grow virtually anywhere, on any soil type, taking very little work and very little fertilization?

Too late, those plants have already been introduced and every producer is growing them.

And they’re commonly referred to as weeds.

By definition, according to Curtis Thompson, Kansas State University Extension weed scientist, a weed is a plant that is considered undesirable, or troublesome, especially one growing where it is not wanted.

No doubt. That about sums up what everyone out there considers a weed.

And in order to control weeds, Thompson told producers at the recent Weed Management School in Columbus, Kan., an Integrated Weed Management system is essential.

“Weeds have growth cycles just like other plants,” he explained. “There are winter annuals, summer annuals, biennials and perennials.”

So not only do weeds grow nearly all year, they also produce thousands of seeds which can be spread around in a number of ways.

“Weed seeds can be dispersed by wind and rain as well as by mechanical means, such as on tires or through combines,” Thompson said.

The tough part about controlling weeds, according to him, is that even though the seeds are dispersed all over they don’t all grow every year.

“Weed seeds can be alive in the soil for years and they won’t grow until there is tillage or something that triggers them to grow,” he explained.

There are a number of factors that affect weed seed germination including moisture, oxygen/carbon dioxide, temperature, light and the presence of inhibitors.

“Weed species vary in their requirements,” he said. “Pigweed species require light and won’t grow until they are tilled up out of the ground. Some weed seeds can remain dormant in the soil until certain inhibitors take place and then they grow.”

And then, just to add a little insult to injury, there are not only grass-type weeds, but broadleaf weeds as well.

The type, however, may not be nearly as important as the size, according to Thompson.

“The larger weeds get the harder they are to control since they have more growing points,” he explained.

Both grasses and broadleaves have growing points that include a terminal growing point, nodes, base of leaves and root tips.

“At the base of every leaf there is potential for a new emerging tiller,” Thompson said. “In order to get adequate control we need to get herbicide to the active growing areas to kill the plant.”

This is where an Integrated Weed Management system comes into play.

IWM is simply putting together a system of practices and techniques that are detrimental to weed germination, growth and development, according to Thompson.

The key to success is having a weed control toolbox full of options, or tools you can use to work on controlling weeds.

“The weed control toolbox includes cultural control, mechanical control, chemical control and other miscellaneous tools,” he explained.

From a cultural standpoint, Thompson stressed the importance of using cropping systems that don’t let weeds adapt.

“With crop selection and/or rotation we add diversity and competition,” he explained. “If you are planting continuous crops year after year and using one herbicide you will have problems.”

This, according to him, is how weeds become more adaptive and competitive.

One of the best ways can combat weeds from a competitive standpoint is to give the crop a head start.

“We need to plant into weed-free fields with the proper soil conditions, temperature, pH and fertility. Establishing good seed-to-soil contact will allow the crop to emerge quicker even without rainfall,” he explained. “The quicker we get the crop up and growing in weed-free fields the healthier the crop is and the more competitive it is with weeds.”

Another aspect of cultural practices includes tillage. Whether using no-till, minimum-till or conventional methods, each has its own set of advantages and disadvantages.

According to Thompson, no-till and minimum-till tend to favor the development of small-seeded weeds like crabgrass, foxtail and pigweeds. However, it helps control more large-seeded broadleaf weeds including velvetleaf, cocklebur and morningglory.

“Systems with minimal tillage provide a disadvantage for weeds since residue cover has a negative effect on weed seedling germination, growth and development,” he explained.

Conventional tillage methods, on the other hand, can bury and preserve seed providing an environment suitable for several weed species.

Conventional tillage systems can also kill weeds, according to Thompson.

“Tillage tools kill weeds by burial, chopping and dessication,” he explained.

However, tillage can also stimulate germination of dormant seeds and buds.

When looking at mechanical control Thompson touched on hand-pulling and hand-hoeing which are applicable in small-scale operations and gardening.

“We see this type of control in high-value crops. The problem is the labor expense,” he said. “To make this feasible labor should be cheap or free.”

Another form of mechanical control, usually associated with grass or hay production, is mowing.

“Mowing reduces competitiveness, prevents seed production and may reduce spread however it usually does not kill the weed,” he said.

The final tool in the weed management toolbox is chemical control.

According to him, chemical control that will burn-down the weeds is a substitute for tillage.

“The key to chemical control is knowing your rotation plan for the next year,” Thompson said.

The thing to remember when looking at controlling weeds, according to Thompson, is that weeds are opportunists.

“Weeds are adapted to capture a number of resources and they aren’t particular about the size of the resource,” he concluded.

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