Weed control in nursery production, especially container production, should be preventative. Therefore, this top 10 list will pertain to why preemergence herbicides fail. And just like David Letterman, we will start with the 10th most common reason and end with the number-one most common reason.

10) You failed to read the label.

Herbicide labels contain a lot of information about application timing, rates, spray volume, soil conditions, and how to incorporate or activate the herbicide. Many of these topics are addressed below in one way or the other. If you don’t read the label, you are very likely to fall victim to one of the conditions described in # 9 through #1 of this article. Effective herbicide management begins with reading the label.

9) You disrupted the chemical barrier.

Preemergence herbicides form a chemical barrier over the soil/container surface. The barrier is typically ½ to 1 inch thick (depending on product, soil type, and method of incorporation). Fortunately, most weed seed germinate in the top ½ inch of the soil. Herbicides do not prevent weeds from germinating; instead they control weeds as they germinate within the chemical barrier.

Spilling containers, poking fingers in the media while moving containers, and walking or dragging objects across treated fields are common ways we disrupt the chemical barrier. Weed seed are ubiquitous. A million or more weed seed can be found in just ½ cubic meter of soil. A disruption in the chemical barrier provides an opportunity for weed seed to germinate.

8) Your herbicide combination (or lack thereof) did not cover a broad weed spectrum.

No single preemergence herbicide provides control of all weeds. Some herbicides are effective in controlling broadleaf weeds but provide poor control of grasses. These products would include formulations of isoxaben, simazine, or oxyfluorfen. Other products provide effective control of grasses and small seeded broadleafs, but poor control of many larger-seeded broadleafs. These products include any herbicide classified as a dinitroaniline, as well as napropamide, metolachlor, and dichlobenil. When spraying herbicides, use a product effective in controlling broadleafs mixed with another effective in controlling grasses. Some herbicides are simply ineffective in controlling one or more common weed species. For example, oxadiazon does not provide adequate control of common chickweed and other weeds in the same family. Pearlwort, one of the most troublesome weeds in containers, is another weed in this family that is reported to be poorly controlled by oxadiazon. Using two herbicides simultaneously should prevent an uncontrolled species.

Many granular herbicides contain two active ingredients for broad-spectrum control, some contain only one. Generally, two or more active ingredients provide improved control compared to just one. Check the labels of your granular herbicides to see which chemicals are used. And be sure the chemicals used provide control over the species prevalent in your nursery.

7) You failed to incorporate the herbicide with irrigation.

This is a crucial step in herbicide management. The herbicide label will provide instruction for how much irrigation to apply after application.

Many field growers are switching to drip irrigation, with some placing the drip tape underground. Capillary movement of water is insufficient to adequately incorporate the herbicide. Others have switched to some form of micro-irrigation in their large containers. Spray stakes and other forms of micro-irrigation generally do not provide water over the entire surface of the container, thus they are generally insufficient for incorporating herbicides. Whether in the field or in containers, adequate incorporation of the herbicide must occur with overhead irrigation or rain.

The labels of several herbicides state that as long as the herbicide is incorporated within 3 weeks, the herbicide will be effective. The most ideal circumstance in using rainfall for incorporating the herbicide would be to have a single rain event that provided the complete volume of water needed for incorporation. Many will assume that as long as the cumulative rainfall over 3 weeks is greater than ½ inch, the herbicide will be adequately incorporated. However, research on sandy soils has shown that if the soil is allowed to dry after the initial irrigation, the herbicide will bind to the soil and will not be prone to moving thereafter. So, repeated cycles of small rain events followed by intermittent dry spells will cause the herbicide to bind tightly to the top layer of the soil without adequately moving through the soil. The net effect is that the chemical barrier is very thin, and not deep enough to prevent weeds from germinating. Under these circumstances, it is possible for weeds to germinate under the thin chemical barrier.

6) You routinely over- watered your plants.

Over-watering will reduce herbicide efficacy. This is most problematic in container production, where over watering plants is commonplace. The exact reason why over-watering reduces herbicide efficacy is unclear, but I have a few good ideas. First, increased irrigation will leach the herbicide away from the container surface more rapidly, though I doubt this is a major reason due to the chemical properties of herbicides. Second, increased irrigation results in the container surface being more wet, which also causes increased microbial activity and increased microbial degradation of the herbicides (though, microbial degradation is not a major pathway for herbicides commonly used in containers). Third, and most important, is that excessive water allows weeds that would otherwise suffer from sub-lethal herbicide toxicity to survive. I recall many times pulling weeds from containers with very large shoot systems, and remarkably small root systems due to herbicide activity. These weeds should have died, however, excessive irrigation allowed them to escape control.

5) You tried to save a few pennies by using a lower herbicide rate.

There is a concentration in the soil at which a herbicide will inhibit weed growth. After applying the recommended herbicide rate (assuming uniform application and incorporation), there should be a chemical barrier in the soil with sufficient herbicide concentration that weeds will be inhibited from growing for roughly two to three months (depending on the herbicide). When lower than recommended rates are used, the herbicide concentration in the chemical barrier starts out low. And under typical degradation processes, it is soon degraded to the point where weeds can successfully germinate and grow.

4) You failed to use proper sanitation.

If you neglect sanitation, especially in container nurseries, forget about controlling weeds. Herbicides alone will fail to provide sufficient weed control.

Think of it as a game of probability. Even under the best circumstances, using the most effective herbicides applied at the proper time and in the proper manner, the chemical barrier will not be perfect. There will always be some small areas where the chemical barrier is insufficient for controlling weeds. In addition, the chemical barrier begins to break down almost immediately after application. Over time the chemical barrier becomes more dilute, and gaps will start to form. The more weed seed in your production system, the higher the probability that seed will find a weakened area and exploit it.

3) You topdressed fertilizers.

Topdressing fertilizer is a common practice in container production. It is also a practice that will likely lead to poor weed control. I set up a demonstration/experiment that evaluated 15 granular herbicides. It included 7 new and experimental products, along with 8 commonly used products (including Snapshot, RegalKade, OH2, Rout, etc.). For each container, I topdressed just 6 grams of Osmocote 18-6-12 (½ the recommended rate) and overseeded each container with bittercress seed. By 45 days after applying the herbicides, many containers were overrun with bittercress. None of the herbicides worked! The highest labeled rate was applied for each product, the herbicides were applied well before any seed germination, and the herbicides were properly incorporated with irrigation. Despite meticulous attention to details, the herbicides performed poorly.

Why does topdressing fertilizer tend to decrease herbicide effectiveness? It might be that increased microbial activity around elevated nitrogen concentrations (caused by topdressing) leads to accelerated microbial degradation of the herbicide. There is also a scant amount of research that suggests increased salinity reduces herbicide activity.

2) You did not calibrate your equipment.

Improper herbicide calibration will result in improper herbicide rates being applied to your crops. See #5 for more discussion on this.

Without regular sprayer calibration, you cannot know what rates are being applied. Be sure all the nozzles are clean, operating properly, and that each nozzle is emitting the same volume of spray. This is difficult to see with the naked eye. It requires you to capture spray from each nozzle to verify uniformity. If one nozzle is applying more or less than the others, it may lead to stripes or bands in the field with too little or too much herbicide.

Calibrating granular herbicide applicators is easy; however, making applications uniform is not. As mentioned previously, too much herbicide may cause plant injury, and too little herbicide will result in poor weed control. To improve application uniformity, consider applying the total amount of herbicide for a given area in at least two or three passes.

1) You applied herbicides to soil/containers that already had weeds.

This is hands down the number one reason why herbicides fail to provide weed control. A common excuse is that “there wasn’t enough time” to weed the containers before applying the herbicide. Herbicides registered for use in nursery crops will not kill existing weeds (except for spray-applied Goal, which is only labeled for conifers or directed sprays). Many of the herbicides used in nursery crops work by inhibiting root growth. But even small weeds generally have a deep, well developed taproot, which when greater than one inch deep will not be affected by herbicide activity.

Containers and fields must be weed free before applying preemergence herbicides. This is why I recommend an herbicide application soon after potting or planting. Many weeds germinate within days of receiving optimal environmental conditions. That is, after you’ve planted your fields, and in the process brought weed seed to the soil surface, many weeds can germinate within several days.

Disclaimer: This article is for educational purposes only. Mention of a specific product should not be interpreted as an endorsement, nor should failure to mention a product be considered a criticism.

Dr. James Altland is a research horticulturist at the USDA-ARS Application Technology Research Unit in Wooster, Ohio; james.altland@ars.usda.gov

A noxious weed is any plant designated by federal, state or local government officials as injurious to public health, agriculture, recreation, wildlife or property. Once a weed is classified as noxious, authorities can implement quarantines and take other actions to contain or destroy the weed and limit its spread.

Of all the classifications of weeds, “noxious” weed is the most straightforward. It is a legal term defined in the Plant Protection Act. The law says that a noxious weed is any plant or plant product that can directly or indirectly injure or cause damage to crops, nursery stock, other plant products, livestock or poultry, or to other interests of agriculture, irrigation, navigation, natural resources, public health or the environment. There is a well-defined federal process for listing and delisting plants as federal noxious weeds through the USDA’s Animal and Plant Health Inspection Service. The Secretary of Agriculture is authorized to prohibit or restrict import, entry, export or movement in interstate commerce of any noxious weed in order to prevent its introduction or dissemination within the U.S. There are more than 110 weeds on the Federal Noxious Weed List. Most states also maintain their own state-level lists of noxious weeds.

Invasive weeds are weeds that establish, persist and spread widely in natural ecosystems outside the plant’s native range. When in a foreign locale, these invaders often lack natural enemies to curtail their growth – enabling them to overrun native plants and ecosystems. It should be no surprise that many invasive weeds are also classified as noxious weeds by government authorities.

Presidential Executive Order 13112 prohibits federal agencies from authorizing, funding or carrying out actions likely to promote the introduction or spread of invasive weeds – unless the benefits clearly outweigh the potential harm. The order also mandates that all feasible and prudent measures be taken to minimize risk of harm. The Invasive Species Advisory Committee of the National Invasive Species Council has issued a nonregulatory policy interpretation of the term invasive species. Some of the key conclusions:

• Invasive weeds are not native to the ecosystem under consideration and cause or are likely to cause harm to human, animal or plant health, the economy or the environment.
• Plant and animal species under domestication or cultivation and under human control are not invasive species.
• For policy purposes, to be considered invasive, the negative impacts caused by a non-native species will be deemed to outweigh the beneficial effects it provides.

Weeds must be controlled because they can cause direct or indirect reductions in crop growth or quality and because they are aesthetically objectionable. They directly compete with crops for water, nutrients, and light. Some weeds affect the growth of woody plants by producing chemicals that stunt their growth. Weeds indirectly affect crops or landscapes by harboring rodents that may eat the bark of trees and shrubs, causing severe damage or death. Many nursery practices, such as pruning, are complicated by the presence of weeds. Also, weeds slow air movement, which may result in a higher incidence of foliar diseases or frost damage.

For this issue, we’re focusing on sprayers.

In calibrating a sprayer, the speed and pressure at which the sprayer is operated, the nozzle being used, and the height at which the nozzle is held (which controls the width of the band sprayed) must be kept constant.

The parts of the spray system include the pump, pressure regulator, pressure gauge, screens (strainers), and nozzles. Many herbicides are made up of particles suspended in the spray solution. They cause a lot of abrasion and wear on piston pumps. The best pumps to use with these products are diaphragm or centrifugal pumps because they will last much longer than other pumps. Pressure regulators are needed to maintain a constant pressure during application. Herbicides should be applied at 15 to 40 psi. The pressure gauge makes it possible to reset the pressure regulator from one operation to another.

A 50-mesh in-line strainer should be used to prevent nozzle clogging, especially when wettable powders are being applied. Strainers with check valves are available that require 5 psi pressure to open them. This prevents the dripping of the spray solution that is in the system between the shutoff valve and the tip. The check valve in the strainer closes immediately when the shutoff valve is closed.

Sprayer nozzles

The nozzle tip is a critical component of a sprayer. Although relatively inexpensive, this part of the system must not be neglected. The nozzle affects the flow rate, breaks up the mixture into droplets, and disperses the droplets in a specific pattern. The proper nozzle must be selected for the desired job. No single nozzle can meet all spraying needs.

When buying nozzles, the grower must choose from a variety of angles and spray rates. Nozzles with angles ranging from 65 to 130 degrees are available for applying herbicides. Wide-angle nozzles can be operated close to the ground to minimize drift. Narrow-angle nozzles should be used where high clearance over some plants may be needed.

The flow rate of nozzles is measured under standardized conditions, such as a tractor speed of 4 miles per hour and a pressure of 30 psi. The rate of spray applied per acre by a particular nozzle can be increased by driving slower or by increasing the pressure. The rate of spray applied per acre can be decreased by driving faster or by decreasing the pressure.

An example of a nozzle used for applying herbicides is an 8004, which provides a spray angle of 80 degrees and has an output of 0.4 gallons per minute at 40 psi.

Different types of nozzles are commonly used to apply herbicides in nursery or Christmas tree plantings.

Flat-fan nozzles disperse droplets in a fan shape. The edges of the flat fan pattern have lower spray volumes than the center of the pattern, so they should be used only in combination on spray booms. To get uniform distribution of the herbicide across the width of the boom, the patterns of adjacent nozzles should overlap by 40 to 50 percent. With a 20-inch nozzle spacing, the proper overlap would be 8 to 10 inches. To get uniform spray patterns from flat fan nozzles, a pressure of at least 30 psi is required.

Low-pressure flat fan nozzles are for those who want to use lower pressures, as is often the case with backpack sprayers. They are designed to be operated at 10 to 25 psi. The lower operating pressures and larger orifices on these nozzles reduce clogging and provide larger spray droplets that reduce drift. A low-pressure flat fan nozzle is designated with the letters LP after the number (8004 LP). Flat fan and low-pressure flat fan nozzles are recommended for use only on boom sprayers. The nozzles on a boom should all be the same type and size to ensure even distribution. They should never be used for making band applications because, when used alone, the spray distribution pattern is very uneven.

Even-spray nozzles are used to make band applications. They are similar to flat fan nozzles but apply the herbicide uniformly across the entire pattern. Even-spray nozzles are available with 80- or 95-degree spray angles. Typical working pressures range from 20 to 40 psi. The width of the band can be adjusted by raising or lowering the nozzle. This nozzle should not be used on a boom for broadcast spraying. Even-spray nozzles are identified by the letter E after the number (8004 E).

Flooding fan nozzles produce a wide, flat spray pattern. The wide spray angle (110 to 130 degrees) allows wider nozzle spacing and lower boom heights. The larger orifice produces large droplets and makes the flooding fan nozzle resistant to drift and clogging. Optimum broadcast coverage is achieved by overlapping spray patterns 100 percent to obtain double coverage. Pressures of 10 to 30 psi are commonly used.

Off-center nozzles produce a pattern that is approximately half that of a flat fan nozzle. They are used for directing a spray at the base of a row of plants without getting the spray on a cover crop between the rows. Off-center nozzles are identified by the letters OC before the number designating rate of output (OC 04).

Nozzles are available in a variety of materials. Brass and aluminum are the least expensive, but the metal is soft and wears quickly when abrasive materials like wettable powders are used. Stainless steel and hardened stainless steel cost three to five times more than brass but wear 19 to 77 times longer. The cost, uniformity, and durability of nylon nozzles vary with the quality of the nylon used to make them. They can be as uniform and durable as stainless steel at one-half the cost, or they may be quite variable in rate of spray output. The operator should consult with the supplier before buying nylon nozzles. An excellent combination of products is a nozzle with a nylon body and a stainless steel orifice. It provides the durability of stainless steel with the low cost of nylon.

To prevent plugging and excessive wear of nozzles, screens should always be used to remove large particles from the spray mixture. Check the manufacturer’s recommendation for the nozzle to be used. A general recommendation is to use a 50-mesh screen. Screens with ball check valves are recommended for use when applying postemergence herbicides because they prevent dripping from the nozzle after the control valve has been closed.

A sprayer remains accurately calibrated only as long as the openings of the nozzles remain unchanged. As nozzles wear, the openings become larger and the rate and uniformity of herbicide applied become more variable. To minimize these problems, the operator should buy high-quality nozzles, inspect them regularly, and replace them as needed. To clean a clogged nozzle, use compressed air or a soft-bristled brush, such as a toothbrush. Never use wire or a nail because the nozzle orifice can be easily damaged.

Tractor-mounted sprayers

Many configurations and systems can be used with tractor-mounted sprayers. Centrifugal or diaphragm pumps are recommended. They can have their own engines for power or can be run off the PTO of the tractor. The tanks range greatly in size and may be mounted in the back or front of the tractor. Spray booms or individual nozzles may be mounted in the back, front, under the belly, or on the foot plates of the tractor. Flat booms or drop nozzles may be used. The system that is best for a particular situation will depend on the herbicide being used and the growth habit and size of the plants being grown.

Hand-carried sprayers

The most common hand-carried sprayers are the 2- or 3-gallon compression type and the continuous pump backpack sprayers. Either can be used to spray a small or irregular area like a landscape bed without worrying about calibration. Simply add the amount of herbicide needed to treat the area to enough water to spray uniformly over the area at least twice. Marker dyes also can be added to the spray solution to indicate the uniformity of application. The dyes break down in sunlight and disappear within 3 or 4 days.

For treating larger areas, sprayers must be calibrated. Backpack sprayers should be used because the pump can be operated continuously to maintain a uniform pressure. Sprayers with diaphragm pumps are far superior to sprayers with piston pumps because piston pumps wear out faster and tend to leak. Because these sprayers are mounted on the backs of the applicators, any leaks result in considerable direct exposure to the spray solution.

Some backpack sprayers have pressure regulators built into them. They can be used with no further adaptation. Sprayers without pressure regulators need some adaptation to make them more suitable for herbicide application. A pressure gauge is essential for uniform application. It should be mounted on the tank side of the shutoff valve so the tank pressure can be continuously monitored. If the gauge is mounted on the nozzle side of the shutoff, it will show the tank pressure only when the valve is open. For optimum control, a pressure regulator also should be inserted into the system somewhere between the pressure gauge and the nozzle.

Shutoff valves of some hand sprayers are often too slow to prevent dripping from the nozzle after the valve is closed. This can cause serious problems when nonselective herbicides such as Roundup Pro are being used. Such valves should be replaced with a quick, positive pressure shutoff valve or strainers with check valves.

A two- or four-nozzle boom can be adapted to apply herbicides to large plantings or turf.

Calibrating a sprayer

After a tractor-mounted or backpack sprayer has been customized to meet particular needs, it should be calibrated according to the following instructions:

On an area that best represents the average topography for the area to be sprayed, measure and mark off the calibration distance that coincides with the band width, if band applying, or with the nozzle spacing (width covered by a single nozzle) if broadcast applying (see Table 1).

Calibration distance____________ feet
Fill the sprayer with water only and record the number of seconds required to travel the calibration distance at a comfortable, steady speed. With a tractor-mounted sprayer, note and record the engine rpm and the gear selection so that the same speed is used during calibration and application. For backpack sprayers, it is important that the person making the application is the person that calibrates the sprayer, because the calibration is based on that person’s pace.

Gear ___________ rpm____________ time in seconds___________________
While maintaining the selected application pressure, collect the spray output from one nozzle for the same number of seconds needed to travel the calibration distance.

Amount of water collected in fluid ounces _________________________________
The number of fluid ounces collected equals the gallons per acre (GPA) output of the spraying system.

Example 1: With a 32-inch band, if it took 28 seconds to travel 127 feet, collect the nozzle discharge for 28 seconds. 16 ounces collected equals a system output of 16 GPA.

Example 2: With 20-inch nozzle spacing, if it took 35 seconds to travel 204 feet, collect the discharge of one nozzle for 35 seconds. 20 ounces collected equals a system output of 20 GPA.

If using a boom, repeat Step 3 twice more, collecting water from a different nozzle each time. The average number of ounces collected for each of the three nozzles is equal to the gallons of water applied per acre for that boom, speed, and pressure. If the difference in output of any of the nozzles tested varies more than 10 percent from the others, check all of the nozzles on the boom and replace as needed.

To determine the amount of chemical to add to the spray tank, divide the capacity of the tank by the number of gallons of water applied per acre to determine the area that can be covered with a tankful of spray.

Example 1: 3 gal tank ÷ 16 GPA = 0.188 acres covered per tank.

Example 2: 200 gal tank ÷ 20 GPA = 10 acres covered per tank.

Multiply the application rate of the product per acre times the area covered per tank. Add that amount of chemical to the sprayer tank.

Example 1: 2 qt per acre = 64 fl oz per acre x 0.188 acre per tank = 12 fl oz per tank.

Example 2: 5 lb per acre = 80 oz per acre x 0.188 acre per tank = 15 oz per tank.

Example 3: 2 qt per acre x 10 acres per tank = 20 qt, or 5 gal per tank.

Mixing herbicides

Concentrated herbicides should never be poured directly into an empty tank. Add one-half the necessary water to the tank, then the herbicide concentrate, and finally the remainder of the water. Mix the spray solution by operating the sprayer with the control valve in the closed position for a few minutes.

Never allow a sprayer with mixed chemicals to stand without agitation. Heavy wettable powders may clog nozzles and settle into the corners of the spray tank, becoming very difficult to remove.

When applying combinations of herbicides, add them to the tank in this order: wettable powders, flowables, water solubles, adjuvants, and emulsifiable concentrates. Constant agitation is important when combinations of pesticides are in a tank.

When using backpack sprayers, thoroughly mix the chemicals in a bucket with a pour spout; then pour the mixture into the sprayer. It is difficult to get proper mixing if the chemicals are added directly to the sprayer.

Herbicide compatibility

Two pesticides or a pesticide-fertilizer mixture sometimes separates when combined in a sprayer. Including a type of surfactant called a compatibility agent, or adjuvant, may help get them mixed. Compatibility agents available on the market include Compex, Co-mix, and Unite.

When the compatibility of the materials to be mixed in the tank is uncertain, the following jar test can be used to determine how well they mix:

Put 1 pint of water or chosen fluid fertilizer in each of 2-quart jars.

Using rates proportional to those that will be used in the field, combine the chemicals to be mixed in one of the 2-quart jars.

To the other quart jar first add a compatibility agent (0.5 teaspoonful) to water or fluid fertilizer; then add the herbicide mixture.

Close both jars and mix contents by inverting jars 10 times.

Inspect jars immediately.

Allow jars to stand undisturbed for 30 minutes and inspect them again.

If both mixtures remain uniform for 30 minutes, the combination may be used. If the mixture with the compatibility agent stays mixed but the one without the compatibility agent does not, be sure to use the compatibility agent in the spray tank.

Should either mixture separate after 30 minutes but remix readily with 10 jar inversions, the mixture can be used if good agitation is maintained in the tank. If nondispersible oil, sludge, or clumps of solids form, the mixture should not be used.

If tank-mixed chemicals are compatible, time and labor can be saved by applying herbicides in combination with fertilizers. When such mixtures separate, clogged tanks and lines and uneven, deficient, or excessive rates of application result. This can cause crop injury, poor weed control, and residue problems in addition to the expense and disposal required for mixtures that cannot be used.

Tracey Harpster is an Extension Educator, Penn State Pesticide Safety and Education; and Jim Sellmer is a Professor of Horticulture at Penn State. Read “Controlling Weeds in Nursery and Landscape Plantings” here: https://extension.psu.edu/controlling-weeds-in-nursery-and-landscape-plantings.