Herbicide Resistance Pigweed 1: Identification

Key points to ID Palmer Amaranth and waterhemp, and information about their biology that is important for effective management.
Herbicide Resistance Pigweed 1: Identification - Videos

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- [Narrator] Herbicide resistant pigweeds are already a major economic issue in many states and are a serious growing problem in Pennsylvania.

Their ability to grow rapidly and produce lots of seed, allows them to out-compete crops and cause significant yield loss.

Since these problem weeds have now been reported in at least 17 Pennsylvania counties and counting, proper identification and management are necessary to maintain yield and prevent their spread between fields.

This Learn Now video will introduce the problem of herbicide resistant pigweeds and how to identify them.

Pigweeds are a family of summer annuals that continue to emerge in crop fields from spring to late summer.

Each plant can produce anywhere from 100,000 to one million seeds depending on the size of the plant, and can grow up to one to two inches per day, totaling up to eight feet tall.

Because of this, pigweed can quickly develop large infestations in a single field and out-compete the crop in a short time if gone uncontrolled.

Palmer amaranth and waterhemp are two species of pigweeds that are herbicide resistant.

Both occur in Pennsylvania, as well as in many other U.S. states.

While some populations are resistant to Glyphosate only, there are also populations that are resistant to multiple groups of herbicide simultaneously.

It's common for Palmer and waterhemp populations to be resistant to both Glyphosate and ALS inhibitors.

Populations throughout the U.S. has been identified with resistance to Group three, five, and 27.

For that reason, herbicide options for resistant pigweeds are increasingly limited.

Effective control will require accurate and timely identification and the integration of diverse management practices to prevent the spread of this problem weed.

This map shows the number of farms in counties where both Palmer and waterhemp have been identified.

Currently there are at least 30 known sites around the state, but we expect that there are many more locations that have not been documented yet.

Palmer and waterhemp seed spread is a serious problem.

When we visit an operation that has Palmer and waterhemp, one of the first things we do, is try to determine how it got there, because that can help us in determining the best management recommendations for that particular farm.

It can be accidentally transported into the state by way of agricultural products and equipment, and then be spread from field to field by various means.

Some common modes of transport from one state to another include spread through machinery, hay, cottonseed for dairy rations, and cover crop seeds or rock commodities.

When livestock is fed forage containing pigweed seeds, the seeds can survive digestion and end up in manure that is then distributed on to the field, dispersing the seed.

Combines can easily carry pigweed seeds from field to field, and across long distances because the small seeds get stuck in crevices within the machine.

Machinery purchased from the Midwest or South, and transported to Pennsylvania, may be carrying Palmer amaranth and pigweed seed into the state.

In several cases within Pennsylvania, custom combines have been identified as the major reason why Palmer amaranth and waterhemp seeds have been spread form one field to another.

The small size of the seed makes them difficult to clean out of machinery, so additional complete cleanings are necessary if purchasing equipment from out of state, or if using a combine that's recently been in infested field.

Proper management of Palmer and waterhemp requires understanding their biology.

Palmer amaranth and waterhemp germinate throughout the growing season, mostly from May to August.

Sometimes you'll see Palmer plants that already have a seed head in July, but if you look closer at the ground, there will also be seedlings just emerging as well.

This is why it's important to have good residual herbicides to control it during the entire growing season.

Redroot pigweed typically does not germinate this late into the season.

It's critical to scout and identify Palmer amaranth and waterhemp plants in a field very early on, before the plants get larger, and to continue scouting often throughout the season.

This is especially important because once the plants reach three to four inches tall, herbicides fail to control the plants.

Identifying them when they're young means they're much more likely to be controlled effectively.

After initial germination, Palmer and waterhemp plants grow really quickly.

Even under slightly dry conditions, they can grow up to one to two inches per day, reaching up to eight feet in height by the end of the season.

Floral structures can start developing as early as July and continue developing through August, September and October.

Even the plants that germinated very late in the season can flower and form seed heads in August, in order to drop seed during the fall.

Seeds become viable even when the plants are still green, so as early as mid-August, the plants are producing seed that's able to fall and germinate the next spring.

Palmer and waterhemp can be distinguished from one another fairly easily, even in the very early seedling stage.

Palmer has broad egg-shaped leaves, with notched tips.

Sometimes each leaf blade will have a single small hair coming out of the tip.

Waterhemp seedlings, on the other hand, have leaves that are more narrow, elongated, and oval shape.

As the plant gets larger, leaves remain a valuable way to distinguish species.

Palmer leaves are waxy, while smooth and redroot leaves may have a slightly fuzzy feeling.

In Palmer amaranth, as the plants gets larger, the leaves are arranged in a poinsettia shape, like in the image at the top left.

While it's important to be able to tell Palmer and waterhemp from each other, it's also very important to be able to distinguish these two herbicide resistant pigweeds from other pigweeds that are not typically herbicide resistant.

Smooth and redroot pigweed are typically herbicide susceptible.

While the elongated shape of waterhemp leaves makes them easy to tell apart from smooth pigweed and redroot pigweed, it's usually more of a challenge to tell Palmer apart from smooth and redroot pigweed.

This is because their leaves are a similar shape, and they can also have the V-shaped watermark on the leaves.

Luckily, there's another way to distinguish Palmer seedlings from smooth and redroot, you just have to look carefully.

Palmer has very smooth stems, and will never have hairs on the stem or petioles, while redroot and smooth pigweed do have at least a few fuzzy hairs on the stems and petioles.

The best place to find these hairs is on new stem growth.

Check in areas where there are new leaves.

This is a better place to look, than the old woody parts of the stem.

Even a few very fine fuzzy hairs is a good indicator that the plant is redroot or smooth pigweed, and not Palmer.

Here's another closeup photo of the tiny fuzzy hairs on a redroot pigweed plant.

Another way to distinguish Palmer from waterhemp is the length of the petiole relative to the leaf.

If you fold the petiole over the leaf blade on a Palmer amaranth plant, the petiole is often longer than the blade.

In waterhemp, the petiole will be shorter, or the same length as the leaf blade.

Pigweed seeds can be distinguished from many other weed species from their size and color.

Pigweed seeds, including Palmer and waterhemp, are very small, about one millimeter wide and are dark brown or black, and smooth and shiny.

In the mid to late season, the size and arrangement of the floral structures can be used to distinguish Palmer amaranth and waterhemp from each other and from other types of pigweeds.

Palmer and waterhemp both have male and female plants.

Female plants contain thick, tall seed heads, and male plants contain softer pollen heads.

On Palmer amaranth, the floral structures are tall, erect, and can get to be very large.

In the left most image shown here, Palmer amaranth male and fluorescence is on the left, with the female on the right.

While the floral structures on males are softer to the touch, the female seed heads are very prickly.

Waterhemp, on the other hand, has much smaller flowering structures in seed heads in comparison to Palmer.

These seed heads are not prickly.

The floral structures of smooth and redroot pigweed are arranged in large type bunches.

Palmer amaranth and waterhemp can both grow to be quite tall.

Plants between five to eight feet tall are common from the mid to late season.

In soy and alfalfa, they typically rise well above the canopy as seen in these photos.

In corn, Palmer and waterhemp don't typically grow taller than corn.

For that reason, it's especially necessary to walk through the field in order to spot pigweed in corn.

A plant this size has the potential to produce up to a million seeds, many of which have the potential to germinate the following year.

However, mature seed producing plants can also be quite short, if conditions are not ideal for rapid growth.

The shorter plants will soon develop seed heads with substantial amount of viable seed.

The photo on the left was taken in August, and shows a small Palmer amaranth seedling with a newly developed floral structure.

The image on the right shows a plant that had been mowed off of the base, and then re-sprouted from a bud.

Even though just a few inches tall, this plant is beginning to flower, and will release viable seed to germinate the following spring.

This may happen particularly in the later part of the season when stressed plants are trying their best to produce seed before frost.

Therefore, when doing late season weed scouting, it's important to look underneath the crop canopy as well as above in order to evaluate Palmer amaranth and waterhemp populations.

In summary, Palmer amaranth and waterhemp are herbicide resistant weeds that pose a serious concern to Pennsylvania producers because of their prolific nature ability to spread rapidly and difficulty to control.

They can be distinguished from each other and from other pigweeds by several traits, including leaf shape, stem, and leaf texture, arrangement, and floral structures.

Because control options are minimal and expensive after plants are foraged and tall or taller, early identification is critical for effective management.

In young plants, leaf and stem are the best identifying features.

In mature plants, distinguish them using the floral structures, as well as the leaves and the stems.

Watch these other Learn Now videos to learn the best management practices for herbicide resistant Palmer and waterhemp in conventional field crops, horticultural and organic systems.

Thanks for watching.


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