Dry Weather and Weed Management: Good News or Bad News?

By Jared Goplen and Liz Stahl 

Planting has progressed rapidly this spring given the favorable planting conditions across much of Minnesota. The week of May 10th is still forecasted to be cool and dry, but there will be a gradual warm up this week, with some chances for rain going into next week. Now that planting is wrapping up, what do dry conditions and a dry forecast mean for weed management?

Was my preemergence herbicide “wasted?”

No. Even though preemergence (PRE) herbicides need timely rainfall after application to allow “activation,” or movement of the herbicide into the weed seed germination zone, most PRE herbicides used today stick around on the soil surface until rainfall is received. Most commonly-used PRE herbicides are resilient from photodegradation and volatilization. It is true, however, that in order for herbicides to be active they need to be in the soil solution so that they can be absorbed by germinating seeds. Residual herbicides are not as active in dry soils since less herbicide is in the soil solution. One-half inch of rain is typically adequate for activation of most herbicides. Some herbicides require less or more rainfall to activate, but from a practical standpoint, there are not large differences. When the soil surface is very dry like it is now in some areas, three-quarters to an inch of rainfall may be needed to sufficiently “activate” herbicides.

If rainfall does not happen in a timely fashion, incorporation with a rotary hoe or harrow may help move some of the herbicide off the soil surface, putting it in a better location for when precipitation does occur. Rotary hoes should be operated at a high speed and are most effective at controlling weeds in the white root stage (germinated but not yet emerged).

Some PRE herbicides do have some “reactivation”, “recharge”, or “reach back”, where the residual herbicide can kill emerged seedlings if rainfall is received after weed emergence. This can only occur with some herbicides, including group 27 herbicides (i.e. HPPD inhibitors in products such as Balance Flexx and Acuron). However, these products still perform best when they are absorbed during germination.

Is it dry (or cool) enough weeds won’t emerge?

Maybe. Weed emergence is influenced by both soil moisture and soil temperature. Just like crops, weeds need to imbibe water from the soil solution to germinate. In situations where there is insufficient moisture in the top several inches of soil for imbibition to occur, weeds will not germinate. In some areas this spring, the upper several inches of soil is likely dry enough to prevent weed germination. This may help minimize emergence for a little while, but large-seeded weeds like giant ragweed are able to emerge from deeper depths in the soil where there is usually sufficient moisture for germination (Table 1). Small-seeded weeds that emerge from shallower depths, like common lambsquarters and waterhemp may be more affected by the dry conditions.

Table 1. Typical depth of emergence of several common weeds in Minnesota.

Figure 1 shows a prediction of emergence for various weed species in Minnesota based on heat unit accumulation. The prediction model used in this graph assumes adequate moisture for germination to occur. Given the accumulation of growing degree units so far this year, there are fewer reports of emerged weeds than expected in some areas. Dry conditions are likely driving the reduction in expected weed germination and emergence. Many of our weeds are expected to emerge quickly once adequate rainfall is received, meaning a residual herbicide applied ahead of precipitation will be especially helpful.

Figure 1. Cumulative weed emergence predictions from growing degree day emergence models (Archer et al. 2006) averaged across southern Minnesota as of May 7th, 2021.

Herbicide strategies

If you have already applied PRE herbicides and have yet to get sufficient rainfall for activation, it is just a waiting (and scouting) game. A rotary hoe or harrow could be used to help incorporate the herbicide while providing weed control benefits if weeds are in the white thread stage. If weeds emerge before herbicide activation, an additional postemergence (POST) herbicide application may be needed to clean fields up before weeds exceed 2-4” in height. Be prepared to apply a POST herbicide a little earlier than expected if dry conditions reduce PRE herbicide effectiveness.

If you have not applied a PRE, perhaps you were planning an early POST program with residual herbicides instead. An early POST system may work well this year given the dry weather. The concept here is to apply a tank mixture of POST and residual herbicides. This may minimize trips through the field by allowing the POST herbicide to clean up more weeds, but it will be essential to watch the weather forecast and make applications ahead of any precipitation to provide timely activation of the residual products. It is also necessary to check herbicide labels to ensure delayed application is acceptable. Many PRE herbicides require application within 3 days of planting.

Layered herbicide programs

Layering a PRE herbicide, where a PRE residual herbicide is applied at planting and then approximately 30 days later (i.e. Group 15 herbicides such as Dual II Magnum, Outlook, or Warrant), can enhance control of weeds like waterhemp that have an extended emergence period. An earlier planting season means that pre-plant tillage removed few, if any, weeds prior to planting, as very few weeds were emerged by late April. Thus, a significant percentage of the weed seedbank has yet to emerge. This places greater emphasis on the ability of PRE and POST herbicide applications to provide season-long weed control.

With an early planting and PRE application date, a lower concentration of the residual herbicide applied at planting will remain by early to mid-July. This unfortunately is peak emergence time for waterhemp. Layering with a group 15 herbicide about 30 days after planting will provide an effective concentration of residual herbicide later in the season to help in controlling late-emerging weeds like waterhemp. If you trust your POST herbicide (e.g. Liberty, Enlist, Xtend) to control emerged waterhemp, you may be able to stretch the application window beyond the 30 day recommendation (we have waited up to 42 days in UMN research), but rainfall will still be essential for activation. Earlier application is preferable in situations where the extended-range forecast may be drier than normal, so that the herbicide is able to activate whenever you do receive rainfall.

Questions? Want to learn more?

We will go into more depth on weed management topics on this week’s “Field Notes”

Debalin Sarangi, University of Minnesota Extension Corn & Soybean Weed Specialist and Tom Peters, University of Minnesota Sugarbeet Weed Specialist will be on along with moderators Jared Goplen and Dave Nicolai, Extension Educators in Crops, to talk more about weed management considerations this year.