How Long Does It Take for Roundup to Work?

Roundup Basics

• As a non-selective herbicide, Roundup can cause damage to any plant sprayed. It moves through the plant from the area that was sprayed down to the root system, hindering the plant's metabolism. Symptoms of Roundup damage include leaves turning yellow, wilting and turning brown. The underground growth will begin to decline, leading to the eventual death of the plant. Any plants sprayed with Roundup directly or through drift are likely to die, but the speed at which this happens varies.

How Much?

• Roundup can be purchased as a concentrate or pre-mixed. The concentrate should be mixed with water before application. If too much water is used, the product will become diluted and will not be as effective. Too much concentrate will result in waste and excessive chemicals in the landscape. Using water that is dirty, from a pond for example, can reduce the efficacy of the herbicide as well. The proper ratio of Roundup to water depends on the type of Roundup you are using and the amount of mix you need to prepare.

Type of Plants

• Roundup damage typically appears in younger plant tissue first. Shrubs that are sprayed during the summer or fall may not show symptoms until spring. At that time, plant leaves will be smaller than normal and yellow. Annuals will often show signs of efficacy within two to four days while perennials can take seven or more days. Do not to mow the area you plan to spray with Roundup before application. This can make the herbicide ineffective.

Ideal Weather

• The ideal temperature for applying Roundup is between 70 and 85 degrees Fahrenheit. If Roundup is sprayed during extremely cool or cloudy weather, the plants may take longer to show symptoms. If a heavy rain occurs immediately after Roundup is applied, it may wash the chemical off the plant leaves and make it ineffective. Roundup should never be applied on a windy day. Wind can spread the herbicide to other sections of the landscape, causing damage to the wrong plants. Dispersing the herbicide during windy weather can also reduce the amount of chemical being applied to the intended plants, and slow or eliminate the effects.