When you hear weed specialists toss around the terms “large-seeded weeds” and “small-seeded weeds,” you might wonder which are which and what difference it makes? Good questions.

“Weed seed size comes into play in particular with no-till and strip-till,” points out University of Wisconsin weed scientist Chris Boerboom. “With those systems, large-size weed seeds don't get incorporated and therefore don't germinate well. For example, velvetleaf, a medium- to larger-seeded weed, becomes much less of a problem in no-till farming.”

In contrast, Boerboom notes, small-seeded weeds and annual grasses, if not managed, can become more prevalent in no-till and strip-till. That's because they're able to germinate near the surface.

“When the seeds of large-seeded weeds lie on the surface, as they do with no-till, they make only one flush and are easier to control with a single herbicide application,” explains Jeff Stachler, weed scientist at Ohio State University. “Also, since they are on the surface, they are more likely to be eaten by rodents, insects and birds. Over time, because of predation, good weed control, and lack of incorporation, the number of large-seeded weeds declines in no-till fields.”

But small-size weed seeds in no-till fields usually get incorporated by rainfall, tire tracks and other natural soil disturbances just enough to germinate, Stachler explains. Thus, they become dominant weeds.

When fields are tilled, large-seeded weed seed gets incorporated. That allows them to germinate better and over a longer period of time, says Stachler. But small-seeded weeds, when incorporated by tillage, become less competitive because they're pushed below their optimum germination level, he points out.

Germination and growth patterns of medium-seeded types, says Stachler, fall between the large and small.

In general, he adds, large-seeded weeds are more competitive on a per-plant basis than small- or medium-seeded weeds. “The larger seeds tend to germinate faster because of more energy, and they tend to be larger weeds,” he notes. “It may take four to eight times as many small-seeded weeds to be as competitive as a single large-seeded weed.

“On the other hand, small-seeded weeds like pigweeds and lambsquarters tend to produce more seeds per plant and can spread faster,” Stachler says. “That's the reason they can take over so quickly in no-till if not properly controlled.”

Herbicide-wise, soil-applied products such as dinitroanilines (Prowl, trifluralins), acetamides (Harness, Dual II Magnum, Lasso, etc.) and pigment inhibitors (Command, Balance, Callisto) are the most effective for small-seeded weeds, says Stachler. Except for the few products that need incorporation, all can be used with no-till.

Pre-emergence PPO inhibitors (Authority, Valor, etc.) control mostly small- and medium-seeded weeds. “To control large-seeded weeds with pre-emergence products, go with triazines or ALS-inhibitors, assuming there is not a resistance problem,” Stachler advises. “They also get medium- and small-seeded weeds.”

As a rule, large-seeded weeds are more difficult to control with pre-emergence herbicides than are small-seeded weeds, notes Wisconsin's Boerboom. “That's partly because they emerge from a greater depth and don't take up as much herbicide as do those weeds that germinate near the surface. We can usually control large-seeded weeds more effectively with post products.”

Boerboom points out that most pre-emergence grass killers, especially for corn, also are touted to help control small-seeded broadleaf weeds. But, he says it's important to keep in mind that their control can vary depending on the broad-leaf. When those grass herbicides are premixed with atrazine, control is good and consistent for most small-seeded broadleaves.

Weeds And Their Seed Sizes

The following list of broadleaf and grass weeds, found in the Midwest, comes from Ohio State University:

Large-seeded broadleaf weeds: common cocklebur, giant ragweed, the morningglories, common sunflower and burcucumber.

Medium-seeded broadleaf weeds: common ragweed, velvetleaf, jimsonweed, smartweed, Canada thistle, kochia and common dandelion.

Small-seeded broadleaf weeds: pigweeds (including waterhemp), lambsquarters, eastern black nightshade, marestail, field pennycress, chickweed, purple deadnettle, wild mustard and shepherd's-purse.

Large-seeded grass weeds: shattercane, johnsongrass, field sandbur, woolly cupgrass, downy brome and wild oats.

Medium-seeded grass weeds: foxtails, barnyardgrass and wild proso millet.

Small-seeded grass weeds: crabgrasses and fall panicum.