This guide is intended to help the commercial grower and landscaper choose a safe and effective weed management program for herbaceous ornamentals. Every attempt has been made to provide updated information on the currently registered herbicides. It is the applicator’s responsibility, however, to check the most current state and federal registration information and to read and follow label directions.

Weed management is an integral and important part of all commercial production of herbaceous ornamentals. Weeds compete and interfere with plant growth and devalue the yield and quality of landscape-, container-, and field-grown ornamentals. It is important to develop a weed control strategy that uses all the available options at your disposal. These include preventive measures such as organic and inorganic mulches, preemergence herbicides, and sanitary practices that prevent weed seeds and vegetative parts from spreading. This is especially important in container operations where the potting medium is often soilless and initially weed-free.

Several pictorial guides and botanical identification keys are available to identify the most common weeds. It is essential to know the correct names to understand herbicide labels and control guidelines. Most weeds that infest ornamentals have one of four life cycles: summer annuals, which emerge in the spring, flower, and set seed before the first frost; winter annuals, which germinate at the end of the summer and overwinter as small dormant but green plants; biennials, which are similar to winter annuals but germinate earlier in the summer; or perennials, which survive more than two seasons and can propagate by seed or vegetative reproduction. Knowing the weed life cycle is key to determining the optimal timing of a herbicide application or cultural practice. It is important to scout the weed population during and after the growing season to assess the success of the weed control program. For instance, at the end of the season in the fall, escaped summer annuals and some perennials will be dead but can be identified by their characteristic “skeletons.” Escaped winter annuals, biennials, and most perennial weeds will survive the winter as dormant rosettes, crowns, or underground rhizomes.

Several herbicides are available that can be used safely and legally to control weeds in herbaceous ornamentals. Herbicides are commonly classified by their mechanism of action and use pattern. Preemergence herbicides are applied before weeds emerge and generally provide residual control of weed seedlings for several weeks.

Postemergence herbicides, applied after the weeds have emerged, are of two types. Contact herbicides kill only the portion of the plant with which the herbicide actually comes in contact. Good spray coverage is important when using contact herbicides. Systemic herbicides are absorbed and move through the plant. These are useful for controlling the creeping roots and rhizomes of perennial weeds. With systemic herbicides, the weeds must be actively growing so that the herbicide can be fully translocated. The post emergence herbicides that are labeled for herbaceous ornamentals are nonresidual and have little or no soil activity.

In many situations, herbicides cannot be used or are not effective in controlling all the weeds. In these cases, cultivation and hand pulling are often the only available options. There are two important facts to remember about mechanical cultivation. Hoeing and tilling will control small annual weeds fairly well. However, successive flushes of germinating weeds, stimulated by the cultivation itself, need to be controlled on a two- to three-week cycle. Once residual herbicides are applied and activated with water, they need to be in intimate contact with the germinating weed seedlings to work well. Mechanical cultivation will often destroy this contact.

Hand pulling is often an important, if backbreaking, component of a weed management program. It should be considered when no other cultural or herbicide options are available and when weeds are present, that will disperse their seed by wind to weed-free areas.

7.2 Types of Herbaceous Ornamentals

Plant species that are listed on herbicide labels have been tested by independent researchers and approved or registered by state and federal agencies. Because of the great number of herbaceous species, it is possible to test only a small fraction of all plants that are commercially grown. Table 7.1 contains information regarding herbicides that are currently registered on herbaceous ornamentals, and Tables 7.2 and 7.3 contain information about efficacy.

Spring-flowering bulbs that are planted in the fall can be treated with preemergence herbicides shortly after planting and again in the spring. If the planting is late, herbicides can be applied in early spring before summer annual weeds germinate.

Annual bedding plants are generally seeded in the greenhouse and transplanted in the landscape bed in midspring. In most cases, preemergence herbicides should be applied after transplanting to weed-free soil and then irrigated in. Research has shown that cultivars of a species can respond differently to the same herbicides. If possible, always test any herbicide on a small area first.

Perennials are propagated in several ways—e.g., seed, transplants, vegetative division—and are grown in the landscape as well as containers and the field. Most preemergence herbicides should be applied soon after transplanting.

Cut flowers are usually started from transplants, divisions, or tubers but sometimes are grown in the field from seed. For the most part, preemergence herbicides should be applied after transplanting. Research has shown that most field-seeded flowers are not as tolerant of the same herbicides that are safe on transplants. To achieve the same level of safety, the herbicide usually should not be applied until after plants emerge and are established.

Herbaceous and semi woody groundcovers are generally fairly tolerant of preemergence herbicides. In the landscape, it is crucial that weeds be controlled for the first two years of establishment. Using organic mulches in combination with pre- and postemergence herbicides is usually the most successful strategy. The mulch material chosen must not inhibit rooting or spread of the groundcover.

7.3 Growing Situations

A chemical weed control program in a commercial or home landscape is complicated by the diversity of plants being grown. Bulbs, annual bedding plants, perennials, and ground covers are often planted in a single bed. Good record keeping—of weeds, herbicides, and ornamentals—is important for site preparation, planning the planting, and avoiding injury to sensitive species.

A herbicide program should be devised for multispecies container operations so that ornamentals that are tolerant of similar herbicides are grown in the same area. Because preemergence herbicides will not control emerged weeds, containers should be treated shortly after the plants are potted or the weeds are removed from established plants. For control of winter annual weeds in the fall, preemergence herbicides should be applied at least two weeks before container houses are covered. Never apply preemergence herbicides in heated or unheated covered houses or greenhouses. Several herbicides that are otherwise safe can volatilize under these conditions and cause injury.

When growing any herbaceous ornamentals in the field, the most important weed management jobs are done before planting. Good site preparation includes scouting for perennial weeds and controlling them with cultivation and herbicides the season before planting. If the field is heavily infested, soil fumigation should be considered—especially if no effective herbicides are registered for the crop.

7.4 Herbicide Application

7.4.1 Injury Symptoms

Most of the preemergence herbicides registered for herbaceous ornamentals act by inhibiting the normal root development of small weeds before they emerge. In some cases, the ornamental species are inherently tolerant of the chemical, but more often selectivity and safety are attained by placement. Because most weed seeds germinate in the upper half-inch of the soil, surface herbicide applications control them without injury to the ornamental, which has roots normally growing well below the treated zone. When injury does occur it is often manifested by stunted and malformed roots and general failure to thrive. This may be difficult to detect sometimes if, for instance, all the plants in a bed are uniformly injured. Any stress to the plants will exaggerate the symptoms and worsen the injury.

7.4.2 Formulations and Application

For several preemergence herbicides, a choice of formulations is available. The sprayable formulations (emulsifiable concentrates, wettable powders, dry flowables, water-dispersible granules) are usually less expensive than granular formulations. These can be applied through a tractor-mounted sprayer or by a hand-held backpack sprayer equipped with a spray boom. When using backpack sprayers it is important to apply as uniformly as possible by maintaining a constant foot pace and even spray pressure and using uniform nozzle orifices.

Granular formulations are often used in landscapes and containers where spraying is not practical. Rates for granules should be calculated on an area basis and applied uniformly over the entire area. Granular herbicides should not be applied with a tablespoon to individual containers. This will concentrate the herbicide, which increases the chance of plant injury and decreases weed control.

7.5 Herbicide Resistance

Any weed population may contain or develop plants resistant to a herbicide or class of herbicides (herbicides with the same mode of action or the same target site of action). Weed species with acquired resistance may eventually dominate the weed population if herbicides from the same class are used repeatedly in the same field or in successive years as the primary method of weed control. This may result in partial or total loss of control of those species by the group of herbicides. In some species, repeated use of one herbicide class may lead to the selection of naturally occurring biotypes that are resistant to these products.

If poor performance occurs and cannot be attributed to adverse weather or application conditions, a resistant biotype may be present. This is most likely to occur in fields where other control strategies such as crop rotation, mechanical removal, and other classes of herbicides are not used from year to year.

To delay herbicide resistance:

· Use best practices, including IPM (integrated pest management) to develop an overall weed management strategy,

· Monitor weed populations, especially treated weeds for loss of herbicide efficacy,

· Avoid the consecutive use of the same herbicide or herbicides from the same class,

· Use tank mixtures or premixes with herbicides from different classes (as long as all products are registered for the same use and are effective at the mix rate on the weed of concern).

7.5 Description and Characteristics of Herbicides Registered for Nursery and Landscape Use

Acclaim Extra

Trade Name: Acclaim Extra Common Name: fenoxaprop Formulation: 0.57EC

Uses: Postemergence control of annual and perennial grass weeds in established turfgrass, nursery crops, and landscape ornamentals, including many trees, shrubs, herbaceous perennials, and annuals.

Amount of active ingredient Amount by formulation

0.57EC

Per Acre 0.1 to 0.3 lb. 3.5 to 39 oz.

Per 1,000 sq. ft. 0.08 to 0.90 oz.

Growth stage oz./A oz./1,000 sq. ft.

seedling (untillered) 13 0.30

1–2 tillers 20 0.46

3–4 tillers 28 0.64

Major Weeds Controlled: Annual grasses such as crabgrass, goosegrass, barnyardgrass, foxtails, and panicums.

Major Weeds Not Controlled: Annual bluegrass, broadleaf weeds, or sedges. Most perennial grasses are tolerant.

For Best Results: Apply to young (seedling to 3-tiller) actively growing grasses. May be tank mixed, following label directions with other pre- and postemergence herbicides. Thorough spray coverage is essential for optimal results. Flat fan nozzles are recommended. Addition of a surfactant is generally not recommended.

Cautions and Precautions: Do not use on Bar Harbor juniper, Salvia, Philodendron, Podocarpus, or Pittosporum. Check label for other species restrictions. Weed and crop tolerance may vary according to environmental conditions, and tolerance should be determined before extensive use. Do not apply more than a total of 138 oz. per acre per growing season. Do not apply more than 28 oz./A to Kentucky bluegrass or zoyziagrass.

Residual Activity: Up to two weeks of residual control has been reported.

Volatility and Leaching Potential: Loss from volatility is minimal. Leaching is negligible.

Symptoms and Mode of Action: Growth inhibition occurs within 48 hr. Meristems turn black shortly thereafter. Yellow to red foliage develops in about 7 to 10 days, leading to death within about 14 days. Mechanism of action involves inhibition of lipid synthesis at the root and shoot meristems.

Manufacturer: Bayer Environmental Science EPA Reg. No.: 432-950

*Barricade

Trade Names: *Barricade Common Name: prodiamine Formulation: 65WG, 4FL

Uses: Selective preemergence residual control of many annual grasses and broadleaf weeds in landscapes, field-grown nursery stock, container-grown nursery stock, Christmas trees, and established turfgrass (excluding golf course putting greens), lawns, and sod nurseries.

Amount of active ingredient Amount by formulation

65WG 4FL

Per Acre 0.325 to 1.5 lb. 0.5 to 2.3 lb. 21 to 48 oz.

Per 1,000 sq. ft. . 0.185 to 0.83 oz. 0.5 to 1.1 oz.

Major Weeds Controlled: Annual grasses such as crabgrass, goosegrass, foxtails, barnyardgrass, and johnsongrass (from seed). Some annual broadleaf weeds such as carpetweed, chickweed, shepherdspurse, prostrate spurge, lambsquarters, and pigweed.

Major Weeds Not Controlled: Established weeds, perennial grasses, and large-seeded broadleaf weeds. Weak on galinsoga, common groundsel, ragweed, nightshades, and velvetleaf. High application rates are required for prostrate knotweed control.

For Best Results: Incorporated with ½ inch of rainfall or irrigation or with shallow cultivation as soon as possible after application. Incorporation should not be delayed more than 14 days after application. Irrigation or rainfall soon after application is necessary to activate herbicide and to wash residual off foliage. May be applied to shadehouses and uncovered polyhouses (must remain uncovered for 7 days).

Cautions and Precautions: On landscape ornamentals, no more than 1.5 lb. AI per acre may be applied. In newly planted nursery stock or landscapes, delay applications until the soil has settled around the base of the plants. This is especially important in transplant beds.

Residual Activity: Season-long annual grass control is provided in many situations. Control of many broadleaf weeds will be shorter.

Volatility and Leaching Potential: Photodecomposition and volatilization occur when the product remains on the surface for prolonged periods without incorporation. Strongly adsorbed by soil; not readily leached.

Symptoms and Mode of Action: Absorbed through roots and inhibits root and shoot growth through interference with cell division.

Manufacturer: Syngenta EPA Reg. No.: 65WG: 100-834

4FL: 100-1139

* Restricted use pesticide. May be purchased and used only by certified pesticide applicators or by someone under the direct supervision of a certified applicator.

Basagran T/O

Trade Name: Basagran T/O Common Name: bentazon Formulation: 4L

Uses: Postemergence control of seedling broadleaf weeds, yellow nutsedge, and annual sedges in field-grown ornamental trees, shrubs, and ground covers, established turf, and noncrop sites.

Amount of active ingredient Amount by formulation

Per Acre 0.75 to 1 lb. 1.5 to 2 pt.

Per 1,000 sq. ft. 0.55 to 0.75 fl. oz.

Major Weeds Controlled: Annual sedges, yellow nutsedge, groundsel, ragweed, purslane, smartweed, wild buckwheat, wild mustard, and Canada thistle. Control of spring-germinating horseweed has also been observed: Fall-germinating horseweed was not controlled.

Major Weeds Not Controlled: Grasses, perennial broadleaves, and mature weeds are not controlled. Lambsquarters, galinsoga, pigweed, shepherdspurse, spurge, and woodsorrel are not well controlled.

For Best Results: Apply as a directed spray to small and actively growing weeds under good soil moisture. For yellow nutsedge control two applications are recommended, the first in late June and the second 7 to10 days later. For Canada thistle control, apply bentazon when the weed is between 8 inches tall to the bud stage; make a second application 7 to 10 days later. The addition of 1 qt./A of crop oil concentrate is recommended for yellow nutsedge, Canada thistle, and several other species (see label for details). Do not cultivate or mow five days before or after application. Rainfall or irrigation within eight hours of application may reduce effectiveness.

Cautions and Precautions: Do not apply more than 1 lb. AI per acre per application or 2 lb. AI per acre per season. Addition of crop oil concentrates will increase activity however crop injury may be a problem when humidity and temperature are high. Spraying near the base of rhododendron and sycamore may result in injury. See label for additional restrictions and limitations.

Residual Activity: No soil residual activity should be expected.

Volatility and Leaching Potential: No loss from volatility or photodegradation. Not absorbed by soil but rapidly incorporated into organic matter and rapidly broken down by microbes, therefore leaching potential is low.

Symptoms and Mode of Action: Contact burning action. Translocation is minimal. Mechanism involves inhibition of photosynthesis, therefore symptoms may take up to several days to develop. Higher temperatures and addition of crop oil may accelerate and increase the incidence and severity of burn.

Manufacturers: Micro Flo Company EPA Reg. No.:7969-45-51036

BASF Corporation 7969-45

Corral

Trade Names: Corral Common Name: pendimethalin Formulations: 2.68G

Uses: Preemergence control of grasses and certain broadleaf weeds in established liner-, field-, an

Pest Management Guidelines A Cornell Cooperative Extension Publication	Cornell Guide for Pest Management of Herbaceous Perennials
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