Warmer temperatures and rainfall occurring across much of the state are causing weeds to flourish. Many of these weeds started their growth last fall and overwintered as a rosette. However, there will be some that germinate from seed this spring. These weeds will cause problems with planting in no-till settings if they are not adequately controlled. Before burndown herbicide programs are considered, it is best to know what weeds you are targeting to best determine what products to use and when to apply them for optimum performance. Therefore, scouting fields now can be worthwhile. Below are several common weed species that are present in fields now. Depending upon your location in the state and accumulated heat units, some of these species may have flower structures and have more advanced growth. Once you correctly identify the weeds, use resources such as the Penn State Agronomy Guide for herbicide suggestions.
(The following images were taken by D. Lingenfelter, Penn State Weed Science)
Aster family: Common spring species in this family include marestail/horseweed, fleabane, common groundsel, corn chamomile, dandelion, common burdock, thistles, and prickly lettuce.
Marestail/horseweed (Conyza canadensis; annual)
The majority of marestail populations in the region are resistant to both glyphosate (Group 9) and ALS herbicides (Group 2) such as FirstRate and Classic. Since they can germinate during the fall and spring the populations can be different sizes at burndown making them more difficult to get complete control. Many of the larger ones germinated last fall, survive the winter as a rosette, and then can sometimes start to bolt before the burndown herbicides are applied. Marestail that are taller than 6 inches may not be completely controlled by the burndown herbicides. Sharpen tends to be the primary product of choice but other herbicides such as 2,4-D, dicamba, Elevore, and Gramoxone (paraquat) can provide control if used when marestail are smaller. Make sure to optimize the application by using the correct adjuvants and spray volume to get good coverage on the leaves.
Fleabane (Erigeron spp.; annual)
Annual fleabane species can be confused with each other and with marestail. Sometimes they can be difficult to burndown especially if they are larger. Make sure to include glyphosate and 2,4-D or dicamba in the spray mixture for best results.
Common groundsel (Senecio vulgaris; annual)
In most cases, common groundsel can be easy to control with various burndown herbicides. However, there are populations in the region that are resistant to triazine herbicides making its control more difficult to manage.
Common burdock (Arctium minus; biennial)
Since burdock is a biennial it forms a rosette in the fall. Once it starts to bolt in the spring it can be harder to control. Make sure to spray the burndown herbicides before it gets too large. Glyphosate alone in the spring tends to be weaker so make sure to include 2,4-D and/or dicamba in the spray mixture. Mesotrione can help improve control as well in corn burndown programs.
Dandelion (Taraxacum officinale; perennial)
Dandelion is a common weed in early spring no-till fields. Typically spring applied glyphosate is weak on it so the addition of 2,4-D +/- Harmony Extra (or similar product) usually helps to control it.
Mustard family: Typical early-season species in this family include field pennycress, shepherd's purse, hairy bittercress, pepperweed, yellow rocket, birds-rape mustard, and Whitlow grass.
Field pennycress (Thlaspi arvense; annual)
Field pennycress and other mustards are very common in no-till fields in the springtime. In most cases, they are easily controlled with many of the commonly used burndown herbicides. However, since they can grow in cooler temperatures they may start to bolt and form seedheads much sooner than other species. Once they are starting to set seeds, in most cases it is too late for effective burndown control. Pennycress forms larger seed pods about the size of dime.
Shepherd's-purse (Capsella bursa-pastoris; annual)
Shepherdspurse and other mustards are very common in no-till fields in the springtime. In most cases, they are easily controlled with many of the commonly used burndown herbicides. However, since they can grow in cooler temperatures they may start to bolt and form seedheads much sooner than other species. Once they are starting to set seeds, in most cases it is too late for effective burndown control. Shepherdspurse forms seed pods that are heart-shaped.
Yellow rocket (Barbarea vulgaris; annual)
Yellow rocket and other mustards are very common in no-till fields in the springtime. In most cases, they are easily controlled with many of the commonly used burndown herbicides. However, since they can grow in cooler temperatures they may start to bolt and form seedheads much sooner than other species. Once they are starting to set seeds, in most cases it is too late for effective burndown control. Yellow rocket is one of the earliest weeds to start flowering in the spring. It has bright golden yellow flowers and forms long and cylindrical seed pods that are about an inch or so in length.
Hairy bittercress (Cardamine hirusta; annual)
Hairy bittercress is one of the earliest maturing weeds in no-till fields. It usually starts to set seed by late March/early April, so it is usually not a major problem around corn and soybean planting time. However, if is still present during burndown applications it is easily controlled by commonly used herbicides.
Whitlow grass (Draba verna; annual, two samples on left); vs. marestail/horseweed on right
Pink family: Common chickweed and mouseear chickweed are the most common species in this group but others include white and bladder campion and knawel.
Common chickweed (Stellaria media; annual)
Common chickweed is an annual species and can have very aggressive growth in the late winter and early spring. Glyphosate and paraquat generally provide effective burndown. Sharpen, 2,4-D, and dicamba are usually not as effective. Sharpen tends to provide initial control but after a couple weeks, chickweeds usually starts to regrow.
Mouseear chickweed (Cerastium vulgatum; perennial)
Mouseear chickweed is a perennial and can sometimes be difficult to control with certain burndown herbicides. As with common chickweed, glyphosate and paraquat are required to burndown mouseear chickweed.
Mint family: Henbit and purple deadnettle are very common in early spring. Ground ivy is another species in this family but is most prevalent in turf and landscape settings.
Henbit (Lamium amplexicaule; annual)
Henbit usually starts its growth early in the spring so it can sometime flower earlier than when burndown programs are sprayed. Once it is flowering and sets seeds, burndown herbicides are not effective. Otherwise, glyphosate is usually the foundation herbicide but it is best to include other products such as atrazine or metribuzin. 2,4-D or Sharpen alone generally are not effective but can be included with glyphosate. Paraquat alone only provides suppression so it will need to be tankmixed with a triazine to control henbit.
Purple deadnettle (Lamium purpureum; annual)
Since purple deadnettle is related to henbit, they are often confused with each other. Purple deadnettle has leaves and flowers that grow on the upper portion of the plant as compared to henbit which the leaves are more spaced out along the stem. The same principle apply to controlling deadnettle in a no-till burndown setting. It tends to be very aggressive early so it can be difficult to control sometimes if it is too mature.
Henbit (left) and purple deadnettle inflorescence
Figwort family: Speedwell populations are common this time of year. Corn speedwell is frequently found in no-till settings, however, there are several different speedwell species, and they can be difficult to distinguish – some are annuals and other perennials.
Corn speedwell (Veronica arvensis.; annual)
Corn speedwell is frequently found in no-till settings. However, there are several different speedwell species and they can be difficult to distinguish. Some are annuals and others perennials. Speedwell species tend to be difficult to control in general. Usually a mixture of glyphosate and 2,4-D plus other products like atrazine and mesotrione in corn programs and metribuzin and flumioxazin containing products in soybean can have activity. Effectiveness of the herbicides can be impacted by the kind of speedwell being controlled.
There are miscellaneous other species that can be found in no-till settings prior to burndown such as dock, poison hemlock, bluegrass. downy brome, and quackgrass, among others.
Curly dock (Rumex crispus; perennial)
Curly and broadleaf dock species can be found in no-till fields and can be challenging to control. 2,4-D, glyphosate, and many of the other typical burndown herbicides are usually weak on dock. Harmony Extra and similar herbicides tend to provide adequate control of it when included in the spray mixture.
Poison hemlock (Conium maculatum; biennial)
Since poison hemlock is a biennial, it is most easily killed in the fall or early spring before it starts to bolt. Once it starts to bolt and flower, it can be more difficult to manage. Glyphosate, 2,4-D, and dicamba usually are effective on it when its in the rosette stage.
Annual bluegrass (Poa annua; annual or short-lived perennial)
Annual bluegrass can sometimes be a difficult weed to control in the spring. Glyphosate generally provides the best control. Basis Blend (1.5 oz/acre) can provide some suppression. Also, high rates (>5 oz DF) of metribuzin can provide fair control of bluegrass; especially if mixed with paraquat. ACCase herbicides (clethodim, Assure II) can work if 60-degree day and not tank mixed.