7.1 Weed Management Options

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 an 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.