Winter annual weed development

Winter annual wake-up time

Winter annual weeds have had a successful winter with very limited mortality and are beginning to grow as heat units accumulate. Even with a wide-regional cooldown expected over the next 5 days here in the Corn Belt, many of these weeds have entered the reproductive phase. Henbit and its near-look-alike Purple dead nettle can bloom in as little as 50 GDD₅₀, a process that is well underway in the Central Corn Belt. Chickweed and shepherd's purse are also early to flower. These winter annuals that were left uncontrolled in the Fall can provide a potential host for insects and disease ahead of planting, also accumulate and sequester soil-available nutrients, and can insulate the soil from solar radiation, thus slowing seed-zone warming. Early reproduction, often before burndown herbicide treatments, ensures an ample seed bank for germination next fall. Controlling winter annuals in the Spring is particularly critical in regions with marginal rainfall, as the growing weeds extract moisture from the seedbed in advance of planting.

📸: BASF Contributing Writer/Featuring Henbit (purple flower L) and Field Pennycress (R), which are among the first to flower in the spring, often between 50 and 70 GDD_50.

The heat units needed to promote bolting and flowering are often in a predictable range. An exception might be species like marestail that can germinate in both fall and spring, thus creating a continuum of flowering and seed dispersal when left uncontrolled.

📸: GDDTracker.edu/Featuring the advance of the yellow and gray (71–150 GDD₅₀), which approximates the zone of early-flowering winter annuals such as henbit and pennycress (Adapted from https://gddtracker.msu.edu/).

Tillage is an option

Many of the infestations of winter annuals occur in fields slated for no-till production, so tillage is not an option. When tillage is the main option, it is still important to get to the field when weeds are still small. Larger weeds have the ability to stay rooted or slide between field cultivator shovels during tillage operations. Smaller weeds are more easily dislodged and easier to control. The chances of killing weeds with tillage are greatly enhanced if the weeds are tossed out into sunny drying conditions to further desiccate them and prevent re-rooting. In some cases, where conventional tillage is planned, an early herbicide burndown treatment can clear the soil surface of interfering weeds and result in a better seedbed.

Post-emergence herbicide use in early Spring

As winter annuals come out of dormancy and begin to enter the reproductive phase, many external factors may influence your ability to both get herbicides applied and to have them be effective. Soil conditions must be able to support ground traffic for herbicide application, and frozen soils are not an option. If soil surfaces are wet, the winter annuals may be rapidly growing as you wait for the opportunity to treat the field. Systemic herbicides need warm temperatures and rapidly growing weeds to be effective. Glyphosate and growth regulator herbicides such as 2,4-D are examples of herbicides that require active plant growth to be most lethal.

📸: BASF Contributing Writer/Featuring Marestail and sow thistle (L), which bolt and flower later and are still in the rosette stage at 100 GDD₅₀. Daisy fleabane and cressleaf (R) groundsel flower a little later than henbit.

PPO herbicides for burndown

PPO herbicides such as Sharpen^® Herbicide (key component safluenacil) have a distinct advantage in the cool Spring because they are primarily contact herbicides that destroy plant cellular tissue under sunny conditions. As with all contact herbicides, coverage and a good adjuvant package are key to maximum herbicide performance. Sharpen^® Herbicide is a key tool in managing broadleaf weeds that are resistant to glyphosate (marestail being the most important).

Read these two excellent articles (authored by Mark Storr and Jared Roskamp) to learn more about BASF's burndown product line.

• Tips & Tricks for Controlling Winter Annual Weeds
• Know how to get the most burndown from BASF burndown herbicides

Use of glyphosate for winter annual control

Glyphosate is still very effective on most grasses and is still effective on a number of broadleaf weeds, provided they are small. In many of our fields, the traditional approach of glyphosate with 2,4-D alone has become risky based on the large proportion of marestail that is glyphosate-resistant. Glyphosate can still bring value to your burndown operation.

Growth regulator herbicides for spring burndown

If field operations can be conducted early enough prior to planting and coincide with active growing conditions the growth regulator herbicides are still very effective on a number of broadleaf winter annuals, provided they are still in the target size range. Pay attention to plantback restrictions for corn, soybean, sorghum, and other crops. There are a number of different time periods based on your selection of traited crops and, in some cases, rainfall amounts after burndown application.

📸: BASF Contributing Writer/Biennials such as some thistles (L), wild carrot (R), and poison hemlock may be treated as winter annuals, with the best herbicide treatment occurring before flowering.

If you are dealing with winter annuals in your no-till fields, try and get the weeds controlled as soon as fields are trafficable, weeds are actively growing, and temperatures optimize herbicide activity. Herbicide applications when daytime temperatures are above 65 are optimal, and those days under 40F should be avoided. Know the weeds you are dealing with and consider a PPO herbicide to give strong contact activity that will be helpful against weeds in marginal temperatures and when broadleaf weeds are glyphosate-resistant.

Coming next: Cover crop scenarios

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