Weed Management: Trade Tips

By Matthew Doyle, Assistant Editor

Brigg Abercrombie of Forestry Suppliers, Inc. noted the importance of growth patterns, like height, in identifying weeds. Determining the correct species allows you to take advantage of the targeted nature of many herbicides to kill the threat without damaging desirables. To determine the species, use an identification key.

Today a wide range of pre-emergent and post-emergent herbicides are available to the landscape contractor managing weed threats. So when should each category of products be applied? What should guide product selection? How should products be applied? While these questions could fill a book (or ten), LCN spoke with industry experts to get some basic answers.

When to Use Pre-emergents

A major key in successful pre-emergent applications is timing, as you’re attempting to prevent germination and sprouting. So you’ll need to time your application for early spring, when this is happening. Midge Dreelin, with St. Gabriel Organics, claimed that daphadils and crocuses are a good guide for when soil is just warm enough for germination.

An important thing to remember when applying pre-emergents is that you don’t want to prevent germination in desirable plants via residual product that’s still in the soil. Tracking application times and dates on paper and communicating this information to those spreading seeds will prevent wasted plant material and time.

According to Midge Dreelin of St. Gabriel Organics, daphadills and crocuses are great indicators for when the soil is becoming warm enough for germination. It’s at this point or slightly earlier that you’ll want to apply pre-emergents. However, remember to maintain a paper trail of applications while planning enough time for your chosen pre-emergent to decay in soil before desirables germinate.

Where to Use Pre-emergents

Obviously, you don’t want to use pre-emergents where planting new materials. So places like established vegetable gardens (assuming no human toxicity concerns exist) or mature lawns are very good candidates for pre-emergent applications.

How Much Pre-Emergent to Use

Dreelin claimed that most corn gluten based products require putting down roughly 20 pounds per 1000 square feet. So simply determine your square footage and multiply that number by the pounds recommend per 1000 square feet for your chosen product. Dividing the result by 1000 should determine how many pounds are required for your square footage. Note that the safest approach is always to follow any instructions on the herbicide label.

Post Emergent

So what about once a weed threat has sprouted? The first step is to get a reliable identification, as this will guide many of your decisions to follow.

Identification Process

When making identifications, simply examine the unknown plants characteristics as prompted by the identification key. With each additional yes or no answer regarding the presence of x characteristic, more possibilities will be eliminated until only one species remains.

According to Brigg Abercrombie of Forestry Suppliers, Inc., the growth pattern is a common indicator utilized by identification keys: Is the new sprout growing in a heavily lit area, in plenty of shade or somewhere in between? Another is the height. Is it sticking close to the ground, or stemming up? The same source claimed many noxious weeds exhibit the latter. Is the plant in question woody or grassy? Another major characteristic to pay attention to will be the venation.

ID Elusive? Check the Veins, Check the Web

Venation is the pattern formed by the veins in an unknown leaf. Abercrombie claims venation often determines the family, a big help in moving through identification keys. Depicted is pinnately venation, in which veins pair off from either side of a large, central vein.

Palmately venation; several equally sized main veins diverge from the leaf stock where it meets the blade. Identification keys can be found in several forms in several locations, but according to Abercombie those found on university and government websites are among the most useful due to constant updates and plentiful pictures.

In netted venation the large veins branch repeatedly as they enter the leaf blade.

An example of palmately compound venation, in which the central leaf stem (rachis) is not elongated and leaflets can be found at one location closely connected. According to Abercrombie, a great print source for identifications is the “Color Atlas of Turfgrass Weeds: A Guide to Weed Identification and Control Strategies” by L. B. McCarty, David W. Hall, John W. Everest, Tim R. Murphy and Fred Yelverton.

Depicted is an example of pinnately compound venation. This is characterized by an elongated central rachis (the structure or stem to which leaves are attached, called a petiole in the context of non-compound venations) with leaflets exhibiting pinnately venation attached opposite one another along its length. A final leaf is often running in a parallel direction at the end of the rachis.

Several sites featuring identification keys can be found at:

www.weeds.cropsci.illinois.edu/weedid.htm.

Weed Identification Pitfalls

An important thing to note while categorizing new growth as a weed is how different your client’s tastes in landscape aesthetics may be. As Abercrombie put it, “…one persons weed is another’s desirable.” So communicating with your client on what they consider desirable and undesirable in the landscape can prove very beneficial. That being said, a big part of that conversation is relating your knowledge regarding any human, pet or overall landscape health threats a newly sprouting plant may represent. That way, clients can’t (at least rightfully) be outraged when you kill the pretty but noxious wildflower that you explained and they agreed needed to die.

The only solution to this conundrum is balancing these two conflicting aspects of weed management as best as you can.

As aforementioned, it’s a wise decision to communicate with your client on control programs and what they consider desirable in their landscape as much as possible. However, remember that part of that communication is informing them of any threats to human, pet or landscape health an identified weed may present. Depicted are burns from giant hogweed sap.

Application Tips

You’ll obviously have to ask yourself what to use. Explore the options for your identification and find the most targeted and effective product you can. For instance, Abercrombie pointed out that several products are available that will kill all noxious turf threats while leaving St. Augustine grass unharmed. When to apply is simply a matter of proceeding through the process of identification, determining a control strategy/product and executing as quickly as possible after an unusual sprout is sighted. As far as amounts, simply follow the label.

Finally, make sure your chosen application method (foliar spray, granular, broadcasting, ext.) won’t result in any non-targeted organisms (humans, pets and beneficial/desirable plants) being harmed.

Safety

A final point that Abercrombie felt isn’t said often enough is to keep local regulations and restrictions in mind. In particular, make sure all your certifications are up to date for restricted use products. Also make sure all staff are adequately trained to safely apply all chemicals.