Ohio State University Extension Fact Sheet
Generally, herbicides are categorized for use as post-emergence or pre-emergence. The former refers to products sprayed directly on the weed. These types, such as liquid 2, 4-D and Amitrol-T, cannot be used safely in flower gardens due to the likelihood of injury to desired plants. Pre-emergence herbicides are applied to the soil before germination of weed seeds. This type of herbicide kills germinating seeds or young seedlings but will not affect existing weeds. Pre-emergence herbicides will be the subject of the remainder of this fact sheet.
At this time no herbicide will control all weeds, nor is one safe to use around all flowers. The user must read the label and follow directions with respect to type of weeds that will be controlled and ornamentals tolerant of the herbicide. Too often, applicators believe that once an herbicide has been applied, all weeds will be controlled. This is usually not true, particularly with perennial weeds such as dandelion, bindweed, quackgrass and thistle. Select the herbicide for your yard depending on weeds normally present and flowers to be grown.
Several cultural factors are important for successful results with pre-emergence herbicides. Knowledge of one's soil type is helpful. Certain products, such as those containing trifluralin and oryzalin, are more effective in lighter, sandy soils, while chloramben is most active in clay loam soils. The soil should be moist before application or the application followed by rain or irrigation. Herbicides applied to dry soils without rain or irrigation for 7 to 10 days will often fail as weeds will germinate before the herbicide becomes active. The soil should be weed free at the time of application and preferably freshly tilled, cultivated or hoed. Effectiveness of herbicides is improved if the product can penetrate the soil with water 1/2 to 1 inch deep. Mulching following application of an herbicide enhances effectiveness. Should some weeds germinate prior to the expected weed control period of four to eight weeks, shallow cultivation can be done. Cultivation or hoeing up to 2 inches deep often extends the weed control period; however, avoid disturbing the soil more than 2 inches or the weed killer becomes too diluted within the soil.
Most herbicides are suggested for use on an area basis. Thus, it's important to know about how much area is included in the beds to be treated. Once the area has been calculated, weigh or determine the amount of herbicide needed for a bed and apply with the equipment suggested on the label. In certain instances, it's advisable to rent sprayers or granular applicators from tool rental agencies rather than applying with equipment that cannot be calibrated properly. Always make certain all application equipment is calibrated properly prior to use. Following use, thoroughly clean the herbicides from the equipment.
The underlined products represent one trade name; there may be many for each product. In parentheses is the common name, which must appear on the label. If the trade name listed is not available, ask for it by its common name. For example, DCPA at one time was the ingredient in 24 products, Dacthal being just one.
NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.