Those with unwanted plants or weeds often have little or no control. Many have asked why a recommended herbicide doesn’t work to get rid of their weed problem. This is a common problem for most home gardeners. Below are a few reasons why their weeds are still living after trying a herbicide.

• Incorrect mixtures — some feel if they use more than the recommended amount it will kill their problem easily. In most cases, over application of herbicides is wasteful and will not provide needed results.

• Old herbicides — Many herbicides that are left over in a sprayer lose their strength over a short period of time. Herbicides that have been stored away in original container for years have the same issue.

• Wrong time — Some plants will not respond to herbicide usage because it’s the wrong time of the year. Many broadleaf plants must be sprayed during the early growing season for satisfactory control. Older leaves on some plants produce a waxy surface in mid to late summer, preventing adsorption into the vascular system.

• Weeds are not healthy — It sounds weird, but some herbicides are not effective if the weeds are growing poorly and unhealthy. It may be a bit unusual, but the healthier the weeds, the better the adsorption of the pesticide, so irrigating weeds before spraying may help control them.

• Deep roots — many perennial weeds have a deep and expansive root system allowing them to survive over-the-top herbicide sprays and continue to grow later. Systemic herbicides work best on these types of plants.

• Weed seeds are present — many plants produce an abundance of seed. Removing the host plant does not eliminate seedlings from reappearing. Areas that have reoccurring weeds after the initial kill should also consider applications of a preemergence herbicide in early spring to control newly germinating seedlings.

• Poor timing with weather — if the weather is too hot or cold weeds will not adsorb and translocate herbicides. Applying before or after a rainfall will also reduce weed control.

• Some weeds are just difficult to control. There a few weeds in our landscapes that are just difficult. Some weeds require multiple or annual spraying for adequate control. Patience is the key for these weeds.

Darrell Blackwelder is the retired horticulture agent and director with the North Carolina Cooperative Extension Service in Rowan County. Contact him at deblackw@ncsu.edu.