The African rue is one tough mother of a weed to kill

Q.) In Otero County we have been dealing with the African rue weed. What can the average homeowner and county property owners do or use to get rid of the African rue infesting their property. I am hoping you could write a column about it.

Duane B.

Alamogordo Daily News

A.) Leslie Beck, New Mexico State University Extension Weed Specialist provided the following information: African rue (Peganum harmala) is a perennial noxious weed that is rapidly spreading to multiple counties in southern New Mexico. Due to its ability to germinate and spread through both seed and vegetative roots, containment is the most effective means of trying to prevent this weed from invading the rest of N.M. African rue is extremely drought tolerant; however, growth can be very swift and robust in the presence of soil moisture, i.e., after rain events. This plant also contains at least four toxic alkaloids distributed throughout every part of the plant, as well as allelopathic chemicals to reduce or hinder the growth and development of surrounding vegetation.

One primary key to managing this weed is to prevent the production and spread of viable seed, which can be relocated by running water and attachment to animal fur, human clothing, shoes, pants, etc., or the tires of passing vehicles/machinery. African rue seed can persist in the soil prior to germination, though the approximate length of the dormant period is unknown. The other most important factor to consider when trying to manage African rue is the root system. This plant has a woody tap root that can grow to depths of more than 25 feet in the soil, as well as lateral roots that can extend up to 20 feet away from the central taproot in any direction. As a result, this extensive root system makes African rue notoriously difficult to control once a population is allowed to establish.

Control of African rue is difficult and often impractical with physical or mechanical methods alone, simply because you are unlikely to damage enough of the root system to prevent the recovery of the plant. Prescribed fire is also only a temporary solution to damage the above ground tissue with little impact or effects on the root system. After a short amount of time, the plant simply grows back. Damage or removal of as much of the root system as possible will help to provide temporary control since it will take longer for the plant to grow back. Continued removal of the plant may help to weaken the root system over time, but control may take years of constant effort to remove the plant and is not feasible in most cases.

Because of the root system, management of African rue should combine physical damage/mechanical removal with herbicide applications. One of the select few active ingredients that has been observed to effectively manage African rue is imazapyr (Arsenal), which is not labeled for use in residential or recreational properties. Imazapyr is a non-selective herbicide, controls both grassy and broadleaf plants, which is often used for total vegetative control in parking lots, bare-ground areas, rights-of-way, fence rows, industrial sites, and for the control of invasive rangeland woody weeds such as saltcedar (Tamarisk spp.). As a non-selective, it is important to avoid any contact with desirable vegetation when making an application. Additionally, imazapyr is persistent for an extended period of time in the soil (soil sterilant), thus care should be taken to avoid applications of this herbicide on areas where the roots of desirable plants (trees, shrubs, etc.) may extend or in locations where treated soil may be washed or moved into contact with these roots. Always read and follow the directions of the herbicide label prior to every herbicide application.

For homeowners, it is important to remember that the more mature the plant gets, management becomes less and less effective. After digging up as much of the taproot as possible, the regrowth tissue will be young and more susceptible to herbicides. Active ingredients labeled for the control of brush or other types of woody weeds, such as glyphosate (Roundup) or triclopyr (Turflon Ester) may cause injury to young African rue regrowth or germinating seedlings. More importantly, making these applications when the plant is actively growing/recovering is the best way to help the herbicides translocate throughout the taproot system as much as possible.

Allow at least 14 days for the herbicide to cause maximum amounts of injury, and then dig up the roots again. Continue to repeat the process whenever the plant recovers in order to cause maximum amounts of prolonged injury to the root system. This process may take several growing seasons and require constant diligence on the part of the landowner; however, this is the only management strategy available to address African rue infestations in areas where imazapyr is not labeled for use. Again, always read the label prior to application with any herbicide.

Early detection and rapid response is the key to prevent the further spread of African rue. Even though populations seem to be concentrated in the southern counties of N.M., it is highly probable that this plant will continue to move from county to county. Small isolated populations have a greater likelihood of being eradicated since efforts can simply focus on the plants in that particular area. Once populations become established and are spreading, it is more feasible for the goal to be adequate management of African rue over complete eradication.

Disclaimer: Herbicide active ingredients and trade/brand names appearing in publications are for information purposes only. The author, Alamogordo Daily News and New Mexico State University assume no liability resulting from their use. No endorsement is intended, nor is criticism implied of similar products not mentioned. By law, persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.

Send gardening questions to Yard and Garden, Attn: Dr. Curtis Smith at cwsmith@nmsu.edu or leave a message at https://www.facebook.com/NMSUExtExpStnPubs. Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist, retired from New Mexico State University’s Cooperative Extension Service.