Feb. 1 2008 12:00 AM

Chemical, Biological & Cultural Options

Spring is here, at least in some parts of the country. Soon little green things will be sprouting up all over. As usual, the sprouts will include not only the plants we want but some of the others, too . . . the good, the bad, and the ugly. For landscape contractors this means finding ways to protect the good while getting rid of the bad and the ugly. In other words, weeds.

Spring weed control can mean different things, depending on the contractor and the client. For some, it means applying pre-emergence herbicides. For others, it means using biological applications or cultural practices. For many, it means a combination of several methods targeted to specific needs.

No matter what methods you use, tackling annual weeds on a pre-emergent basis makes sense. While nothing completely eliminates all weeds, preventing most from emerging in the first place will keep them from establishing large patches and will eliminate thousands of seeds that would otherwise develop and add to next year’s stand.

While pre-emergence controls can dramatically reduce weeds, it’s important to remember that effective weed control actually begins before plant beds or lawns are ever installed. It continues on an ongoing basis through all seasons. It’s as much about promoting the optimal health of desirable plants as it is about getting rid of the undesirables.


What’s a weed?

Usually aesthetic tastes determine what people consider a weed, and how many they can tolerate. Weeds detract from the planned color and texture of a planting bed and mar a smooth emerald carpet of lawn. But weed control is more than an aesthetic issue. Weeds compete with desirable plants for water, nutrients, and real estate. When not managed, they can keep the “good” plants from thriving.

Annual weeds like crabgrass typically sprout from seed deposited in the soil during a previous season. They grow, flower, and produce thousands of new seeds from which the next generation will typically sprout in much greater numbers. Stopping these seeds before they can emerge is key to keeping infestations to a minimum.

What you do to prevent infestation will depend on your clients, your own philosophical approach, and most importantly, your experience with what works.

Chemical controls

Pre-emergence herbicides are one option. These chemical applications bond to the first couple of inches of soil, where they form a barrier that kills weed shoots as they emerge. Think of them as a shield that does not allow weeds to pass.

To create this shield, pre-emergence herbicides must be watered in after applying. Watering bonds the product to the soil and puts the chemical barrier right at the ground. Not watering according to instructions is a common reason for product failure. As with a shield, complete protection requires that the barrier remains intact. Therefore, it’s important not to aerate or otherwise puncture the soil after application. Doing so will let weeds pass through at the point of the break, defeating the purpose of the product

There are a wide variety of pre-emergence herbicides on the market, and they are not all the same. Each targets a different set of grassy and/or broadleaf weeds. It’s important to identify the weeds at the site and review the list of species the product targets to make sure it’s designed for the weeds you’re fighting. Failure to use the right product in the first place can lead to over-application of chemicals when you have to go back with the correct product later.

Many products target young roots. Because most pre-emergence herbicides do not distinguish between the roots of desirable plants and those of weeds, it’s important not to use them on newly seeded lawns or those that will be seeded soon. Some pre-emergence herbicides target young shoots instead of roots and are not harmful to turf roots.

In order for pre-emergence herbicides to work they must be applied before weeds germinate and are not effective on weeds that are already actively growing and thriving. Because of this, timing of application is critical. Products are typically applied in early spring, but exact timing varies according to climate and even recent weather patterns. Typically, a rule of thumb is to apply two to four weeks before germination. Products vary in how long they remain active in the soil, and some may offer a longer window of opportunity for application. The benefit of pre-emergence herbicides is primarily one of cost and labor. “Treating with a pre-emergent is going to save the customer a lot of money over having to come back later with a post-emergent product,” says Beth Whitehouse turf and ornamental sales representative with Dow Agro-Sciences, Indianapolis, Indiana.

Chemical pre-emergence herbicides are not to be used without caution, however. Impact on water quality is a concern. Many labels clearly state that they have been shown to be toxic to fish. To reduce environmental impact, it is critical to use them according to the guidelines listed on the label.

“As with all chemicals, it’s very important for people to read and follow the label,” says Don Myers, of Bayer Environmental Science, Research Triangle Park, North Carolina. “I can’t tell you how many times people don’t read the entire label. Remember, the EPA has approved the use of these products when they are used as specified on the label.”

Not following label instructions is environmentally irresponsible. For example, with any herbicide it’s important to apply the correct amount. Application equipment must be properly calibrated to deliver the correct amount. Improper calibration can lead to over-application. This can damage some plant species and lead to excessive loss in runoff. “Make sure to do proper calculations,” says Myer. “Only use the amount of product that is required. All of these label directions are supported by research. These are the amounts the EPA has approved.” Myer points out that you don’t need to blanket an area with chemical applications to provide effective weed control. “Select the areas where the problem is most severe. You can do some or most of the work by hand weeding and then use chemical treatments selectively on those areas where a problem is severe, or if the area is inaccessible.”

It’s important to select and apply chemicals in ways that minimize leaching and runoff. Chemicals that leach through the soil can contaminate groundwater. Those in surface water runoff can enter lakes or streams. Leaching and runoff can depend both on properties of the chemicals chosen, such as water solubility and persistence in the environment, and also on the qualities of the soil being treated. Several organizations, such as university extension agencies, provide information to help professionals make informed choices about product selection and use for minimal environmental impact.


Integrated pest management

Because of their possible environmental impact, some contractors prefer not to use chemical preemergence herbicides at all. Others take an Integrated Pest Management (IPM) approach to weed control, using chemical applications with care on a limited basis.

IPM takes a big picture approach to weed management. It uses a combination of cultural, mechanical, biological, and chemical methods to reduce weeds. The source of weeds is identified and eliminated if possible. Cultural and biological weed reduction practices are explored. If chemical applications are still deemed necessary, they are applied selectively.

Early planning plays a major role in IPM. Site preparation, plant selection, plant health and quality are all important. Weeds are completely eliminated on a new site before planting to reduce problems in the first year and thereafter. When possible, designers select plants that thrive in an area with little intervention, so they will have the best chance to compete with weeds. New plants are carefully inspected before installation to make sure they are not bringing weeds along with them.

Focus on promoting optimal health of desirable plants so they can out-compete undesirables. In lawns, the goal is to create a healthy, dense canopy that shades out weeds. Lawns are cut high to increase shade. Fertilizer or a top dressing of compost (or both) are used to encourage lush growth.

Corn gluten meal is a biological alternative that has been shown to be an effective fertilizer and preemergence treatment. Though some debate whether it is the fertilizer or the pre-emergence effect that has the greatest impact, research has shown it to inhibit weed germination. It is used in a similar way to chemical applications with similar requirements. It must be applied before germination and is not effective against actively growing weeds. It should not be used on new lawns, where it will interfere with roots of turfgrass.

Different strokes

ProMark Landscaping, Zieglersville, Pennsylvania, whose company uses an IPM approach to insect pests, relies primarily on chemical applications to meet customer expectations when it comes to weeds. Their mostly commercial clientele includes a mix of corporate centers, industrial facilities, apartments and homeowners associations.

“In this part of the country, that type of clientele has a low tolerance for weeds in the turf or the ornamental beds,” says Stu Preston of ProMark. “The weed control methods that we employ are of the traditional variety, granular Pre-M weed control. It enables us to bring a higher level of quality to our customers and meets their level of expectation for the fit and finish of their properties.”

He stresses using chemical products judiciously, however. “When pre-emergence is done correctly, it can lessen the need for straight weed control applications. You may be able to do spot applications as opposed to blanket applications.”

To help ensure that you are not using more chemicals than necessary, it’s important to treat each property individually. “Know your properties,” says Preston. “If you are making properly timed chemical applications and you have developed a good stand of turf, you may not need to blanket-apply the entire property. This reduces the overall chemicals needed at each application.”

In a different part of the country with a different client base and a different climate to work with, Kim Swearingen, manager for Interpretive Gardens, a Reno, Nevada-based landscape firm, uses an alternative approach. “We practice a holistic approach to landscaping,” says Swearingen. “We use a bioregionally appropriate plant palette, are 100% organic and incorporate perma-culture principles into design.”

For her, heavy mulch is the weapon of choice. “We’re proponents of deep mulch and of ‘sheet mulching’,” says Swearingen. “This is especially effective here in the desert where soils are lacking in organic matter.” First, a barrier is put down. A deep layer of mulch is placed on top. “We use cardboard. We put it down and get it really wet to hold it in place. It lasts long enough that any seeds that do germinate are blocked by the cardboard. Then it essentially composts in place.”

Though the company plants very little turf, their approach to weed control in turf is also organic. Emphasis is on promoting turf health versus treating weeds. “A layer of compost in the fall, a layerof compost in the spring, and the right amount of water,” says Swearingen. “The right amount of moisture is crucial.”

Swearingen has also tried corn gluten meal on a limited basis. “It works pretty well. You do have to re-apply it, but if you follow the instructions, it does work.”