Can weed killers be applied before you see weeds?

ASK THE HORT AGENT

Question Can weed killers be applied before you see the weeds?

Answer Assuming the term “weed killer” means herbicide and not an old hippie, yes, some of them should be applied before you see the weeds. Herbicides are pesticides which are specifically used to kill unwanted plants. There are two types of herbicide applications – preemergent and postemergent. Pre means before and emergent refers to the tip of the baby root poking out of the seed shell. Post means after and again refers to the baby plant having made its transition from a seed to a plant.

These applications are made with herbicides which are designed for these purposes. In other words, if you want to kill an existing plant then apply a postemergent herbicide. If you want to stop seeds from becoming plants, then use a preemergent herbicide.

Weed identification is the key to knowing which type of herbicide will do the job. Perennial weeds, like poison ivy, can live for several years. Some begin life as a seed and some simply sprout and spread from an existing plant. Most perennial weeds are identifiable plants before you point at them and say “look! A weed.” Therefore, the only option you have is a postemergent herbicide like RoundUp, Weed-B-Gone, Brush-B-Gone, etc....

While perennial weeds are typically the hardest to kill, annual weeds can be the most prolific. There are two types of annual weeds – summer and winter. Annual plants only live one growing season. The plant dies leaving seeds behind to come up next year. If a preemergent herbicide is applied before the seeds begin to emerge, then they will be killed. Obviously, the key is the timing of preemergent applications. Most winter annual weeds (chickweed, henbit, bluegrass, etc…) can be killed if a preemergent herbicide is applied in mid September and again in late November. Preemergent applications for summer annual weeds (crabgrass, sandspur, lespedeza, etc…) need to be made in late February and late April. Preemergent herbicides can be purchased in liquid or granular form. Examples of brand names include Pendulum, Surflan, Barricade, etc…).

For more info on herbicides, visit http://pubs.caes.uga.edu/caespubs/pubcd/C867-4.htm

If you don’t have internet access, then call my office at 893-7533 or email me at gpierce@harnett.org.

Manual removal of garden weeds is always an option as well. Sometimes law enforcement officers use postemergence herbicides to kill weed. Most of the time, they simply pull it up and burn it. Cops seem to have a lot more folks “monitoring” their weed fires than you will have while burning your garden weeds.

Gary L. Pierce

Horticulture Extension Agent

Harnett County