Nursery Crop Science

College of Agriculture and Life Sciences

Early Detection and Rapid Response

Recognize Invasive Weeds Before They Can Take Over Your Nursery!

Dr. Joseph C. Neal

Department of Horticultural Science

North Carolina State University

When one of your nursery staff tells you that he found thickhead on the property – he may not be talking about your brother-in-law – he could be telling you about a new weed that you need to control!  Almost every year I meet a grower who remembers a time (many years ago) when bittercress was not in his or her nursery.  Well, little is accomplished by dreaming about what nursery life would be like without bittercress or spurge or other common weeds.  We have them and we manage them through sanitation, regular use of preemergence herbicides and, of course, lots of hand weeding.  While we cannot eliminate all weeds, perhaps we can do something to prevent the establishment of the next imported weed. 

Where Are They Coming From?

When in comes to weeds, we are sometimes our own worst enemies.  Many of our most troublesome weeds have been introduced with nursery stock purchased from other regions (and other countries).  As nursery crop production diversifies, we are moving plants farther and faster every year.  Un-rooted cuttings from overseas are rooted in one region, shipped for finishing in another and shipped again to retail centers across the country.  And, these crops acquire new weeds at each stop along the way.  Familiar examples include the introduction of common groundsel (Senecio vulgaris) and liverwort (Marchantia spp.) into east coast container nurseries in contaminated nursery stock from the west coast.  Once established in a nursery, these species are there to stay!  Current practices in the nursery industry involve the transport of liner stock and finished plants across the country, and in some cases around the globe -- a system that facilitates the rapid spread of weeds and weed seeds.  As a result, the common weed species found in container nurseries are similar throughout the US, and even overseas. 

Once introduced into our nurseries, weeds spread by various mechanisms including wind dispersal, water dispersal (by floating or splashing) and self-dispersal via forcefully discharged seeds.  Furthermore, once established in nursery crops, these weeds are subsequently distributed to landscapes. 

So, Why Should We Worry?

Many of these newly introduced species are somewhat tolerant of currently registered herbicides.  When weeds are not well controlled by current weed management programs, hand weeding costs go up.  Furthermore, if the introduced weed is a regulated species, your crop might be at risk of quarantine or destruction. 

OK, So What’s New and Interesting?

            There are the new species and then there are the ones you probably have not yet seen (and probably don’t want to).  In recent years the prevalence of new spurges, phyllanthus, doveweed, mulberry weed and others have been increasing.  Additionally, we continue to have introductions of field yellowcress (a NC noxious weed) in herbaceous perennials from the Netherlands.  Also introduced from the Netherlands was Inula britannica, a perennial aster that may soon be listed on the federal noxious weed list.  Both field yellowcress and Inula are nearly impossible to control in nursery crops.  Infested crops will likely need to be destroyed.  Asiatic hawksbeard has been around for about a decade but is now spreading rapidly in landscape plantings, roadsides and woodlands.  Although it is still relatively rare in NC nurseries, it is a common nursery weed in other countries.  Some that you probably have not seen, but are spreading in the industry include thickhead, tasselflower, and petty spurge.

Management of Introduced Species

             The two most important components of an effective management program for introduced species are (1) early detection and identification, and (2) sanitation.  While no nursery manager can know every plant that could possibly infest his or her nursery – each nursery has a spectrum of weeds that are common.  Learn to recognize these weeds that are already on-site, AND be able to recognize when a new species in introduced.  If at all possible, eradicate infestations of new weeds by intensively managing small infestations (yes, that may sometimes mean frequent hand weeding and disposal).  If this is not possible, conduct a little research to determine if this species has the potential to spread and become more troublesome in your nursery.  Preemergence herbicides are available to control many of these species; although, they are so new to the industry that they do not appear in many herbicide labels or extension publications.  Table 1 provides rankings on the effectiveness of some common nursery herbicides on these weeds.  These rankings are based on results of research conducted at NCSU and elsewhere over the past three years.  Using this (and similar) information, plan a weed control program that will control these invaders.  Your local cooperative extension office can provide assistance in developing this management plan.

Table 1.  Relative effectiveness of common nursery herbicides on some new weeds (based upon recent research conducted at N.C. State and other Universities).

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