Weed of the Month for March 2013 is Large Crabgrass

Large Crabgrass 

Biology: Large crabgrass ( Digitaria sanguinalis ), smooth crabgrass (Digitaria ischaemum), and to a lesser extent southern crabgrass (Digitaria ciliaris) are all species found in the Midwest that are collectively referred to as crabgrass. Large crabgrass is probably the most common of these in Indiana lawns. Combined, these crabgrass species are often considered to be the most problematic weed in lawns. Large crabgrass is summer annual grass that typically germinates in April in the Midwest (late March in southern areas and late April in northern areas). A germination tracker at www.gddtracker.net can be used to track the germination of crabgrass.

Identification: Large crabgrass is one of the first summer annual grasses to germinate in Indiana. Look for it first in bare soil areas like a garden or very thin turf areas and in areas with south facing slopes. First germination has occurred between March 22 and April 20 in the Lafayette area the past several years. Initial seedlings have an almost oval shaped, cotyledon. 

After the first leaf emerges, plants may remain small for several weeks if the temperatures are cool. Additionally leaves will develop and eventual plants will begin tillering in June. If you look close at large crabgrass (also sometimes called hairy crabgrass) you will see hairs on the leaves (blade and sheath) and a membranous ligule. If the leaf sheath and upper leaf blade are mostly smooth (hairless) then you likely have smooth crabgrass.  

As the plants mature they tiller and form small clumps. Large crabgrass also can produce stolons and root although it doesn’t spread far and remains mostly bunch type in growth habit.

Cultural control: Proper mowing (higher mowing heights), proper fertilization (some rather than none to improve turf density), irrigation to prevent summer dormancy during drought, and aerification of compacted areas to improve turf health are all cultural practices that can be used to reduce crabgrass.

Biological control: Some organic herbicides are available. The predominant organic herbicide in turfgrass systems is corn gluten meal for preemergence crabgrass control. This product has shown to be effective in northern states in lawns, although tests in Indiana and states south of Indiana show it has limited efficacy on crabgrass and that encouraging a dense turf through proper mowing, irrigation, and fertilization is equally effective.

Chemical control: Crabgrass can be controlled using preemergence or postemergence products.

Dithiopyr (Dimension), pendimethalin (Pendulum), and prodiamine (Barricade) are common ingredients in preemergence crabgrass products. These herbicides inhibit cell division and prevent crabgrass seeds from properly emerging. Since these herbicides work on germinating seeds, you must apply them prior to germination — with the exception of dithiopyr, which controls crabgrass after germination until it reaches one tiller.

Another approach to controlling crabgrass is to use postemergence herbicides after crabgrass has already emerged. Dithiopyr, quinclorac (Drive and others), MSMA (sod and golf), mesotrione (Tenacity), Fenoxaprop (Acclaim Extra) and other herbicides containing these ingredients can be used for postemergence crabgrass control. Each of these ingredients offers a slightly different spectrum of activity and works on specific sizes of crabgrass. Pick one based on the size of crabgrass you're treating and the other weeds you have present in the area.

For more information on weed control, search this blog (search box in upper left corner of page) and archived turf tip postings and check out our Turfgrass Weed Control for Professionals Publication.

Aaron Patton, Turfgrass Extension Specialist

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Turf Notes: Disease and Fungicide Update

Early Season Dollar Spot Sprays

The pathogen must be active in order for the mycelium to absorb a fungicide.  We know that Sclerotinia homoeocarpa grows when temperatures remain above 55F in the presence of ample moisture.  If such conditions prevail in March (as they did in 2012), then early season sprays may help reduce dollar spot severity later in the season.  If temperature and moisture conditions do not favor pathogen growth (as they did in 2010 and 2011), then the March application will have no effect.

Snow Molds

Winter conditions in the lower Midwest have been quite favorable for pink snow mold (aka Microdochium blight).  Springtime outbreaks on putting greens may require application of a remedial spray to limit disease spread, especially on sites with a history of the disease.  Often, a single application of a tank mix that includes chlorothalonil, iprodione, and a DMI fungicide will be effective in suppressing existing infections and limiting spread.
Our research with springtime fungicides on bentgrass fairways has had mixed results.  In most cases, fungicides applied to fairway height bentgrass after the outbreak is evident, did not hasten turf recovery.  If you are inclined to try a “fairway clean-up” spray, do a simple experiment by setting out a check plot—where an area of the turf remains untreated.  By comparing the check plot with the fungicide-treated areas, you will be able to evaluate the benefit of your fungicide spray.

Fungicide Notes:  Secure (fluazinam)

Throughout the season, I will try to make a point of discussing new and/or interesting fungicides for turf disease control.  The first is Secure (Syngenta) -- its active ingredient (fluazinam) was developed for crop disease control in the mid-1990s –but it is brand new to turf markets.  Secure is a contact fungicide with a reportedly multi-site mode of action.  Therefore, the risk of resistance is very low (although there is a report of resistance among populations of the Botrytis pathogen on rice).  The major target turf disease is dollar spot, although there may be some efficacy against other diseases.  Fluazinam’s multi-site inhibitor makes it very attractive as a component in fungicide programs that must deal with season-long dollar spot threats—especially where chlorothalonil limits are at issue.

Rick Latin, Department of Botany and Plant Pathology, Purdue University


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A Much Different Start to the Year

A year ago we had record high temperatures and we were scrambling to apply preemergence herbicides for crabgrass control. On March 22 last year, I had crabgrass germinating in my garden! However, this year I can tell you that crabgrass is a long ways off from germinating today. The chart below shows the dramatic difference between 2012 and 2013, especially starting on March 7. I’ll continue to keep you updated as we progress this year on how the weather impacts our turf management.

Dr. Aaron Patton, Turfgrass Extension Specialist

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Use Growing Degree Days to Better Time Your Applications

Monitoring growing degree days can be an effective way to better time applications and increase your weed control. A helpful website (www.gddtracker.net) is available to help you monitor growing degree days for timing herbicide and plant growth regulator applications. What can this website help me with?
  1. Need help determining when to apply your preemergence herbicide? Visit www.gddtracker.net
  2. Need help estimating when crabgrass is going to germinate in your area? Visit www.gddtracker.net
  3. Should I use an amine or an ester this spring to control broadleaf weeds? Visit www.gddtracker.net (I’ll also cover this more on tips in the upcoming weeks).
  4. When should I apply Embark or Proxy/Primo to my putting greens to regulate annual bluegrass seedheads? Visit www.gddtracker.net
What is a Growing Degree Day (GDD)? A growing degree day (GDD) is a method to track the heat units that have accumulated and are needed for plant growth and development. The formula for calculating GDD is:
GDD = ((max temperature °F + min temperature °F) ÷ 2) - base temperature °F, where the base temperature is normally either 22, 32 or 50 °F but varies based on the model.

For example, if the high today was 74°F and the low was 52°F and we used a base temperature of 50°F, our calculation would be, GDD= ((74°F + 52°F) ÷ 2) - 50°F = 13

In another example on a cooler day you might have a high of 58°F and a low of 40°F and with a base temperature of 50°F, our calculation would be, GDD= ((58°F + 40°F) ÷ 2) - 50°F = -1. When a GDD calculates to <0, we simply make it a zero and determine that no growth (or plant development) occurred on that day.

Models that help us predict plant development use accumulated GDD which is simply adding the GDD calculated each day and determining how many GDD units have accumulated thus far.
In some cases accumulated GDD can be used to monitor when weeds might germinate or flower or when grasses might produce a seedhead while In other cases accumulated GDD can be used to help optimize application timing such as with preemergence crabgrass applications or the selection of amines or esters for spring broadleaf applications. Research into plant development and optimal herbicide application timing has determined a window of accumulated GDD needed to best predict when to time these applications or when these events might occur.

While it is possible to track your own GDD using a local weather station, an easier way to do this is to use an online tool. The Purdue University Turf Science Program and the Midwest Regional Turf Foundation help provide financial support of Michigan State University Extension’s GDD Tracker, www.gddtracker.net. This website tracks GDD for the Midwest and provides updates for golf courses on timing annual bluegrass seedhead suppression applications, updates for all in the turf industry on crabgrass germination and preemergence herbicide application timing, and updates on broadleaf flowering and the timing of amines and esters for spring broadleaf applications.

Watch a video from Dr. Ron Calhoun (creator of www.gddtracker.net) to learn how to use this website.

Dr. Aaron Patton, Turfgrass Extension Specialist

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New Weed of the Month Series: February is Prostrate Knotweed

This post kicks off a new series that we will be posting via turf tips called, “Weed of the Month”. In each post we’ll discuss a timely weed (one that you’re seeing at that particular time of year) and we will highlight its biology, identification, and control.


February's weed of the month is Prostrate Knotweed 

Biology: Prostrate knotweed (Polygonum aviculare) is an early germinating summer annual broadleaf that is often found in low-oxygen soils, including compacted areas next to sidewalks and previously flooded areas. Germination starts in late February and early March in many Midwest states.

Identification: Prostrate knot weed is the earlier summer annual weed to germinate in Indiana. Look for it first next to driveways and sidewalks. First germination has been in late-February or the first week of March the past three years in Indiana. Initial germinating weeds have a red hypocotyl. 

The seed leaves (cotyledons) are very narrow and it almost appears to be a germinating grass upon first inspection. 

As the plants mature they become tough, wiry, and prostrate in growth. This plant develops a thin taproot. It can be distinguished from prostrate spurge which exudes a milky white sap when it's stem is broken.

Cultural control: Reduce soil compaction through aerification and improve drainage in areas prone to flooding.

Biological control: None known.

Chemical control: 2,4-D by itself will provide only fair control of prostrate knotweed, but when combined with triclopyr (Turflon Ester Ultra or Triclopyr 4) or dicamba (Banvel, Vanquish), it should provide excellent control in cool-season turf. There are many combination products that contain 2,4-D and dicamba, including Trimec 992 and SpeedZone. Combination products that contain 2,4-D and triclopyr include 4-Speed XT, Chaser, Chaser 2 Amine, Momentum FX2, Turflon II amine, and TZONE.

In warm-season turf, metsulfuron (Manor, Mansion, MSM) or the herbicides listed above will provide effective postemergence control of prostrate knotweed.

Preemergence control of prostrate knotweed can be achieved with late fall (November or December) applications of isoxaben (Gallery, Isoxaben 75WG). Other preemergence herbicides will also work, but are less effective than isoxaben. You can apply isoxaben in late winter, but spraying conditions are not typically favorable at that time of year and it is difficult to predict exactly when prostrate knotweed might germinate although it is usually in early March.

Aaron Patton, Turfgrass Extension Specialist

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