How They Work
Nonselective herbicides don't discriminate, killing nearly any plant they touch. Selective herbicides, in contrast, kill only specific groups of plants, such as grassy weeds or broadleaf varieties. Nonselective versions take the guesswork out of killing the weeds; you don't have to worry about whether you bought the right product. The systemic varieties are the most effective because they kill the plant from the inside out rather than soaking into the leaves and killing just the top of the plant, which is how many contact weed killers work. Systemic herbicides soak into the plant's leaves and into the vascular system, which moves the chemical throughout the plant along with sugars and other nutrients. This enables systemic herbicides to kill all areas of the plant, including the roots. Such herbicides are more effective at killing perennial plants.
How and When to Apply
In the simplest sense, all you must do to apply a nonselective, systemic herbicide is to spray it on the plant leaves. A few tips make it more effective, however. Don't spray while the leaves are wet, and try to time the application so the leaves stay dry for at least 90 minutes afterward. Cover all the leaves with the spray, even bottom-layer leaves that might be blocked by the outside leaves to ensure the most thorough penetration. The herbicides work best when the plants are actively growing; when they are drought- or heat-stressed or are dormant, they might not transport enough of the herbicide throughout the plant to kill it. For tough perennials, late summer or fall is best, when the plant is storing as much food in the roots as possible to help the plant survive the winter.
Keeping yourself, your family, your pets and your favorite plants safe requires a small amount of preparation. Wearing protective gear keeps the chemical off your skin and out of your eyes, nose and mouth. Put on safety goggles, a dust mask, long sleeves, gloves, long pants and closed-toe shoes before you spray. After the herbicide dries, it's safe for your kids and pets to play outside. Herbicide drift, where droplets travel away from the desired plant, can kill nearby plants such as flowers in your garden, so pick a day with little or no wind to apply the herbicide. This also reduces the number of drops that might fly back toward you. Even a few drops can harm or kill nearby plants, but covering the plants you want to keep with plastic or cardboard helps protect them from drift issues.
When you prefer not to use chemicals in your landscaping, you can still kill unwanted weeds with products from your kitchen. No organic, homemade remedy works as well as a nonselective, systemic herbicide, but many prove remarkably effective. For areas where you don't want weeds, or any other plants, to regrow, such as cracks in your driveway, pour 2 cups of boiling water mixed with 1 cup of salt over the weeds. The boiling water burns the exposed areas of the plant, and the salt changes the soil's salinity to make it unpalatable for most plants. Using boiling water alone works in flowerbeds or areas where you don't want to change the soil's salinity, but cover nearby plants as you would with a chemical herbicide to keep the water from burning their leaves accidentally. Vinegar contains acetic acid, which also helps burn weed leaves and kill the plants without leaving harmful residue in the soil. Applying it on a sunny, hot day provides the best results.