1993 Proceedings of the Midwest Oak Savanna Conferences

HERBACEOUS WEED CONTROL IN ECOLOGICAL RESTORATIONS: AN ANALYTICAL APPROACH TO THE USE OF HERBICIDES 

The decision to use herbicides as a method of weed control should only be made after careful examination of several issues. The target species must first be analyzed as far as its life cycle, growth pattern, and physical characteristics. The overall quality of the site must then be considered, along with the density of the target species.

Once these biological factors have been assessed, alternative control methods should be given priority over herbicide use. Examples are burning, mowing, scything, and hand pulling. These alternatives will need to be evaluated in light of the work force availability.

When considering herbicide use within this framework, investigation should be made into the allowable herbicides and their chemical modes of action. Only when all of these issues have been examined, can you determine the optimum control method.

INTRODUCTION 

Depending on the circumstances, the word "weed" has varying definitions. Generally, it is a plant living where it does not belong. For the purpose of ecological restoration, a weed is defined as a plant that is out of place and grows in profusion. It crowds out native species and possibly may form a monoculture. It is generally alien, but may be an opportunistic native which thrives in disturbed areas.

The proper place of herbicides in the control of weeds is best enunciated by the Illinois Nature Preserves Commission: "Herbicides should be limited to situations in which no other reasonable means of control are available."¹ 

To apply this standard, certain characteristics of the plants and the site should be ascertained. Once the plants and site are understood, then available alternative means of control can be considered.

PLANT CHARACTERISTICS 

A plant's characteristics will help us to determine whether it will be a long-term problem and what control method to use on it. Specifically, we need to look at a plant's life cycle, its growth pattern, and its physical characteristics.

1. Life Cycle

a. Annual, Biennial, or Perennial

First, look at whether a plant is an annual, biennial, or perennial. This will help determine whether the plant will likely be a long-term problem. If it is an annual or biennial, it will more likely be pushed out in the long-term by the perennials. This is expected to happen to Queen Anne's lace (Daucus carota).

However, perennial weeds are not the only problem. Short-lived biennials must be opportunistic to survive as a species. They seem to thrive in disturbed areas. Unfortunately, the process of restoration frequently disturbs the land. Examples of biennials with extra-competitive characteristics are common burdock (Arctium minus) and teasel (Dipsacus sylvestris).

The classification of plants as annuals, biennials, or perennials can also be used in developing a control strategy for the weed. For perennials, the plant must be killed. This may require herbicide. However, stopping the seed production or distribution of an annual or biennial does not usually necessitate killing the plant. This is because they have a limited life spans.

Seed production of white sweet clover (Melilotus alba) can be controlled by cutting off the plant below its lowermost leaf after that leaf has dried out. However, it must be cut at the right time. If it is cut too early, it will regenerate that year. If it is cut too late, the seed may have set already. Thus, the plants must be cut andremoved from the site without knocking the seed off.

This method is not very successful with yellow sweet clover (Melilotus officinalis) or teasel. Yellow sweet clover plants are less synchronized as to when they bloom. Thus, it is difficult to find the best time to cut them. You may have to cut the same plants more than once in a growing season to be fully successful. This method is more labor intensive, but still an effective means of control.

Teasel has the ability to regenerate seed heads after being cut. The regenerated seed heads, hastily grown late in the season, are usually lower to the ground than the initial seed heads. Therefore, they are more difficult to locate and control.

b. Reproduction 

Reproduction is the name of the game for monoculture forming weeds. Many of these species have evolved in a manner resulting in high reproductive rates. Again, focusing on this characteristic of the plant not only helps predict its tendency towards monocultures, it also helps in formulating a control plan.

Some species simply produce massive amounts of seed. A single purple loosestrife (Lythrum salicaria) plant, for example, may produce 300,000 seeds per plant.² Other weeds concentrate their efforts by producing seeds with a high germination rate. Teasel germination rates, for example, can hit 80%.³ Others produce seed that remains viable for many years, until the right conditions for germination occur. Sweet clover seeds, for example, may remain viable for 30 years.⁴

Last, but possibly worst, are the plants that combine one of the above reproductive feats with the ability to produce new plants via rhizomes. Rhizomes are roots that travel horizontally, and from which new shoots are sent up to the surface. Examples are reed canary grass (Phalaris arundinacea), Canada thistle (Cirsium arvense), and tall goldenrod (Solidago altissima). Rhizomes give these plants a competitive edge both because they double their reproductive capability and because they tend to form a tight matrix that keeps out other plants.

With rhizomatic weeds you cannot merely attack some of the plants, since the remaining ones will quickly replenish the site via rhizomes. Further, pulling out plants usually leaves the rhizomes, which will send up new shoots. This situation may require the use of a systemic herbicide. Systemic herbicides are ones that, when applied to a plant (e.g., by a foliar application), are absorbed into the plant, killing the whole plant.

All reproductive traits should be considered in planning control of weeds. For instance, just killing the plants in one year is going to be ineffective if the weeds have established a seed bank that may remain viable for many years. However, if those seeds can be induced to germinate in the same year, you can control them. Fire can often be used to promote germination. This has been shown to be effective with white sweet clover.⁵

2. Growth Pattern 

The next characteristic to consider is the growth pattern of the plant. Some weeds obtain their competitive advantage by emerging earlier than other plants. Luckily, this gives weed control personnel an advantage. These weeds are more easily detected, thus, more readily controlled. For example, garlic mustard (Alliaria officinalis) is easily located and pulled before most other plants are half as tall.

Early detection and control doesn't always work well, however. For example, Canada thistle emerges early, but its small leaf surface at this stage, compared to the size and strength of its rhizomatic system, makes it less susceptible to a foliar systemic herbicide. Insufficient amounts of herbicide will be absorbed by the leaf surface.

Another growth characteristic to be aware of is the basal rosette. Basal rosettes are initial leaves that emerge and form a rosette of leaves on the ground. Examples are teasel and bull thistle (Cirsium vulgare). Basal rosettes give plants a competitive advantage by tightly covering the ground, thereby preventing other plants from surviving. This plant feature provides weed control personnel with a clue to the most effective control method. Because the basal rosette normally forms early, it is usually easy to find. If herbiciding is chosen as the control method, spray drift is minimized due to the relatively large, close-knit, leaf surface of the rosette. Further, vapor drift is minimized since the leaves are close to the ground where wind is not likely to be a problem.

3. Physical Characteristics

Other physical characteristics of weeds should be considered. For instance, burdock is able to form monocultures because its huge leaves shade out everything growing beneath it. However, these leaves offer a good target which helps to avoid spray drift. It should be sprayed early. Late in its second year it is usually so large and healthy that it requires much more herbicide per plant. By then, it will have killed off every other plant in a four foot diameter.

In contrast, plants such as the Canada thistle have small, widely-separated leaves. This makes herbiciding less efficient because it is hard to apply sufficient herbicide to kill the plant. Spray drift becomes a definite problem.

Pulling weeds as a method of control can be successful if the soil is moist enough. If the entire garlic mustard or white sweet clover root is not extracted, for example, the weed is likely to resprout. One drawback to this control method is that if pulling disturbs the soil too much, you make the area more receptive to other opportunistic weeds.

Certain physical characteristics limit control options due to worker safety. For instance, although wild parsnip (Pastinaca sativa) can be pulled if the ground is wet, its phototoxicity requires caution when handling. Similarly, bull and Canada thistles have thorns that demand a worker's respect.

SITE CHARACTERISTICS 

Site characteristics must also be taken into consideration when determining an appropriate method to control weeds. First, how many plants of a specific species are involved? Are you dealing with 500 or 5000? If the number is large, then strong consideration must be given to the timing necessary for the most effective control. If this window is relatively small, then the attack must be carried out with a method whereby many plants can be handled in a short period of time.

Next, what is the density involved? A plant that has already formed or is close to forming a monoculture might be effectively controlled by mowing (e.g., white sweet clover). If plants are spaced every 5 feet or every 50 feet then a hand control method would be more appropriate.

Site characteristics also include the assessment of the general quality of the surrounding flora. Is the area predominantly covered with other exotic species, or is it an area where native plants are in the majority? More conservative methods (i.e., ones less damaging to the surrounding flora) should be chosen for the higher quality areas if possible.

Plant and site characteristics should be considered together in determining the best methods to control weeds. An understanding of ecosystem interrelationships is a prerequisite to altering any one aspect.

CONTROL METHODS 

A variety of weed control methods are available. Once plant and site characteristics are understood, you can better consider alternatives. Each method will need to be weighed against your work force availability. Methods include controlled burning, mowing, hand cutting, hand pulling, and herbiciding. Use of herbicides poses risks to both people and the environment and should be your last choice.

Controlled burning is a method of weed control that needs to be coordinated with overall site management. Fire has an effect on all other life at the site, including plants, birds, insects, mammals, and herpetofauna. It can promote seed germination of certain species, like white sweet clover. This will not only aid in forecasting when you will encounter future weed control problems, but will also reduce the seed bank for future management. Fire can also be used to control some species. For example, annual burning for 5-6 years controls reed canary grass if native species are reintroduced to compete with it.⁶

Mowing is another method of weed control that can be very effective under the right circumstances. It is one of the most labor efficient methods available for monoculture areas that are mower accessible. Because the visual appearance of the area would be altered, public reaction should be taken into consideration when considering this method.

Hand cutting can take several forms. Loppers can be used for thick stemmed weeds such as tall goldenrod, whereas clippers work well on removing seed heads (e.g., teasel). Scythes and motorized brushcutters are efficient for removing larger stands of foliage. Each version of hand cutting requires a sufficient work force.

Hand pulling is the most labor intensive weed control method currently in use. If the weed population is dense and extensive, this may not be the best alternative. Soil disturbance as a result of pulling may encourage other aliens to invade after your work force has passed through. Tamping down the soil and/or reseeding will minimize this after-effect.

HERBICIDES 

The use of herbicides should be examined when there are no other reasonable methods available. The first step is to determine what restrictions apply to the site. Forest preserve districts may specify chemicals to be used, whereas the Illinois Nature Preserves Commission may have different guidelines. Private land owners may not allow herbicides, or may allow whatever is on the market for the existing problem.

Several different types of herbicides are on the market. Selective herbicides control only certain plants. For instance, Garlon 3A is a selective herbicide that controls broadleaf plants, with a lesser effect on grasses. Broadleaf herbicides are preferable for conditions where the herbaceous mix has both native grasses and broadleaf weeds.

Non-selective herbicides, such as Roundup, will kill any plants that they come in contact with. This type would normally be considered for the control of grasses (e.g., reed canary grass) or in areas where some drift would not cause regrettable damage.

Wetlands have safety needs specific to themselves. Herbicides have been designed to meet those needs. Rodeo is an example of an aquatic herbicide.

HERBICIDE APPLICATION 

Once an herbicide type is selected, then the application parameters on the label should be carefully examined and strictly observed. Even if all other factors suggest the use of herbicide, these parameters may prevent its use. For example, temperature restrictions with upper and lower limits are specified. All herbicides have precipitation guidelines for current and near future conditions. Roundup, for example, should not be used within six hours of expected (or suspected) precipitation. If the humidity is too high, then runoff of the applied spray will result. If humidity is too low, then the evaporation rate will decrease its effectiveness. If the application day is too windy, then spray drift or vapor drift will occur. 

The manufacturer's application parameters are listed on the labels and in the booklets that accompany every container of herbicide sold. This information should be used along with knowledge of plant and site characteristics in determining the proper control method.

Safety is the most important aspect of herbicide use. This includes safety for humans and for the environment. The decision to apply herbicides requires the full responsibility of the applicator to use it in the safest and most effective manner possible. State licenses are required for the applicators and additional safety classes are highly recommended. The subject of safety is broad and will not be dealt with here.

Once you have decided to use herbicide, and an herbicide has been selected, attention should be given to preparation for use. Most herbicides come in a concentrated form, and their labels and instruction booklets recommend various dilutions for control of various species. The herbicides should be mixed either with water or oil and an emulsifying agent, as per the instructions. Whatever medium is used, it should be clean to avoid clogging the nozzles of the application equipment.

In some circumstances, application will be enhanced through the use of a surfactant. Surfactants are liquids that increase the surface tension of the mixture. Increased surface tension decreases drift while spraying, reducing the likelihood of inadvertently killing neighboring plants. They also decrease runoff by helping the mixture remain on the plant surface. This increases intake by the plant, thereby increasing the effectiveness of the herbicide.

Dyes are another useful additive. They help the operator observe where he has sprayed. This increases uniformity of application and safety by making avoidance of sprayed areas easier.

Ingredients should be mixed in a bucket. A filter funnel should be used to avoid spillage and to keep the mixture clean. A measuring cup with ounce increments is useful for proper measurement of ingredients. The mixing rod should be nonporous for easy clean up. A wooden stick is not recommended due to particle contamination of the mixture and disposal problems.

When mixing the ingredients water should be added first, followed in turn by the herbicide, the dye, and the surfactant. This order insures proper mixing of the water and herbicide. The exact proportions should be calculated before mixing begins to avoid confusion and errors. Only the amount anticipated for immediate use should be mixed. Once the mixture is prepared, it should be poured into backpack sprayers, spray bottles or other application devices.

On site, the group leader should provide applicators with proper instructions prior to each session. Safety should be stressed. Plants to be herbicided should be identified in sufficient detail to avoid confusion with other similar species. Herbicide has no conscience and will eradicate the wrong plant as easily as the target species.

Application techniques should also be reviewed prior to each work session. The basic premise is to avoid drift and run-off as much as possible. To do this, all nozzle settings should be set (and readjusted periodically as necessary) to produce a uniform spray pattern without misting. The applicators should spray as close to each plant as possible to avoid drift. Applicators should be given frequent breaks to insure their careful attention.

CONCLUSION 

Herbicides can be an effective means of weed control. However, they should only be used after an exhaustive consideration of the weed's characteristics, site characteristics, alternative control methods, workforce size, landowner requirements, herbicide types and weather conditions. Vegetative management is constantly being researched and tested for new methods of control. Hopefully, in the future, safer and more effective techniques will be available.

Footnotes:

1. Management Guidelines for Illinois Nature Preserves, Volume 4, Herbicide Use and Application, p. 1, Illinois Nature Preserves Commission, 1990.
2. Management Guidelines for Illinois Nature Preserves, Volume 1, Vegetation Management Guideline, No. 17, p. 2, Illinois Nature Preserves Commission, 1990.
3. Management Guidelines for Illinois Nature Preserves, Volume 1, Vegetation Management Guideline, No. 24, p. 2, Illinois Nature Preserves Commission, 1990.
4. Management Guidelines for Illinois Nature Preserves, Volume 1, Vegetation Management Guideline, No. 23, p. 2, Illinois Nature Preserves Commission, 1990.
5. Management Guidelines for Illinois Nature Preserves, Volume 1, Vegetation Management Guideline, No. 17, p. 3, Illinois Nature Preserves Commission, 1990.
6. Management Guidelines for Illinois Nature Preserves, Volume 1, Vegetation Management Guideline, No. 19, p. 3, Illinois Nature Preserves Commission, 1990.