Vow To Vanquish Weeds Among Vegetables
October 1, 2014 It is a scene that has been repeated time and time again. We are occasionally fortunate enough to witness it happening on the drive home or when checking the mailbox. You have probably even experienced it first-hand if you have been blessed with children or grandchildren. Thinking back we just might remember ourselves as children walking around our yards or through our neighborhood streets being attracted to and picking a fresh bouquet of flowers to bring home to mom. She smiles, gives thanks, a hug and a kiss before placing them in water with care and displaying them with love and heartfelt appreciation. An assortment of various shades of yellow, orange, pink and white commonly brightened our kitchen table and windowsills. A consequence of what I realize now was our very weedy yard.
Though there is simple beauty in the diversity of flowers and plants surrounding us. They come in a variety of sizes, shapes, colors and even scents. With just a glimpse or whiff of something once familiar, we can be transported back to that time of innocence. It is only after we mature a bit and come to understand any plant growing where it is not wanted is regarded as a weed. Some weeds are native but many were introduced as escapees into our fields, landscapes, and waterways as a result of their beauty or perceived function.
For example, this past summer, Florida Department of Agriculture (FDACS) officials added four new plant species to Florida’s list of noxious weeds making it illegal to cultivate, introduce, possess, move or release them without a state or federal permit. According to the Division of Plant Industry, Ligustrum sinense was introduced to the United States as an ornamental shrub in 1852. This Chinese privet as it is commonly called escaped cultivation by the 1930s. It is tolerant of low light, does well in poor soils, and has colonized abandoned lots, pastures, and forests. Though initially planted for the clusters of fragrant white flowers they produce, Chinese privet is now regarded as one of the major weeds of woodland habitats in the southeastern United States.
Ardisia crenata is another attractive ornamental that has escaped cultivation into Florida’s natural areas. It is now thoroughly naturalized and is replacing native species. More information on or coral berry, as it is often called, may be found in EDIS publication SS AGR 276.
Crested floating heart (Nymphoides cristata) and yellow floating heart (Nymphoides peltata) or fringed water lily were introduced to Florida through the aquatic plant trade and are also now thoroughly naturalized, replacing native plant species in Florida’s canals and lakes. Plant species on the noxious weed list have been determined by agricultural officials to pose a serious threat to agriculture, have a negative impact on protected plant species and/or disrupt naturally occurring native plant communities.
Taking a closer look at the blossoms I collected and gave to my loved ones, I see the blooms may have indeed been beautiful but were, at the same time, largely common weeds. One such staple of that fresh flower bouquet was Lantana camara, which is also poisonous to livestock and has caused the death of small children. This Lantana is however often confused with the endangered endemic native, Florida lantana (Lantana depressa). The bases of the native’s leaf blades though are tapered (cuneate), not truncate like its weedy relative (EDIS #SP 257). This exemplifies why correct pest identification is such an important principle in Integrated Pest Management (IPM). The University of Florida Herbarium maintains a plant identification service for the use by the public. Additional service details are in EDIS #RFSR013.
Though modern agriculture has become heavily reliant on chemical herbicides to manage weeds, it is always advisable to consider and implement pest management tactics in an integrated way so pesticide use is minimized and pesticide resistance is delayed. Scouting, accurate pest identification, and resistance management are all part of the broader concept of IPM where multiple, cost-effective efforts to decrease pest populations to tolerable levels are made with minimal environmental impact by utilizing plant, pest, and environmental information. The main principle of IPM is using the best scientific data to develop a site specific management plan.
We want to be mindful of any information we can collect when surveying fields and waterways that will aid in determining the best management decision for the weeds we see and those in the soil as seeds. This weed seedbank is the reserve of viable weed seeds present on the soil surface or in the soil profile. Because weed seeds are often small, they remain unseen until they germinate and foliage emerges. Limiting contributions to the weed seedbank however is the best approach to ease future weed management.
Such useful knowledge to collect while scouting are the types and identity of weeds present, their size, density, stage of the crop, field conditions, and any insect pests present on the weeds. The collected data helps determine the best course of action to take. Depending on the analysis of a particular situation, the best thing might be to do nothing at all at this time. Other times circumstances require some type of action or implementation of a management tactic.
Herbicide application should be considered a necessary last resort tool whose use is based on the current threat assessment and in conjunction with other management strategies in an integrated manner. An IPM plan that includes herbicides could also include exclusion efforts like barriers and using a clean seed source. Both of which can be useful in delaying weed development. Removal of weeds by mechanized cultivation or by hoe in hand may be appropriate at times. Some efforts in biological control of some weeds have also been utilized. Cultural methods via environmental manipulation may also be employed in an IPM plan. Such cultural practices may include using crop varieties that sprout and develop quickly, adjusting growing conditions so they are ideal for crop growth, and managing surrounding vegetation to eliminate weeds that serve as alternate pest hosts. More information on herbicide efficacy on common weeds in Florida vegetable production may be found in EDIS # HS706.
The red and pinkish hues of Florida Tassel-Flower also known as Cupid’s Shaving Brush (Emilia fosbergii) was often included in my collection of fresh flora but the plant often harbors aphids. Common Beggar’s Tick or Spanish Needle (Bidens alba) with its daisy-like yellow centered flowers and white petals was another favorite to collect for the weed bouquet. It does however serve as an alternate host for Bidens mottle virus, which can infect endive, escarole, and lettuce through aphid transmission. The danger here is managing the weeds in such a way that potentially damaging insect pests move into the crop and cause damage.
This often requires a tank-mix of herbicide and insecticide. For instance, Bidens mottle virus is no longer a very common disease in the Everglades Agricultural Reserve. Symptomatic plants are encountered in less than 1% of plants under typical growing conditions. An atypical situation was experienced one season where the planted field of escarole bordered a frequently mowed roadway. The incidence of Bidens mottle virus in that field was upwards of 15% by the end of the season. More information on weeds as alternate hosts may be found in EDIS # ENY-863.
In addition to serving as alternate hosts by harboring insects and plant pathogens, weeds are an important pest to manage because they can reduce yields by competing with the crop for moisture, nutrients and light. For these reasons it is advisable to develop a weed management plan before the start of the growing season and manage weeds when they are young.
Weed scientists often point out the importance of critical weed-free periods for a crop to establish and achieve maximum yield. This critical period of interference (CPI) is the specific time interval during crop growth that a crop must be maintained weed-free to prevent interference from reducing yields. CPI depends mainly on weed species, weed mixtures, weed density, crop species and crop planting method (direct seed or transplant).
The graph demonstrates the total yield of watermelon and muskmelon from amaranth interference. Bill Stall’s study shows that if the amaranth is removed (time of removal) one week after the crop is emerged or transplanted; the yield is 90% of the weed free crop (10% yield loss). If however the weed emerges after three weeks and grows in the crop the remainder of the season, less than 10% yield is lost. In this case the critical period to keep the crop weed free so less than 10% yield loss is seen was one to three weeks after the crop goes into the ground or emerges. Visit ipm.ifas.ufl.edu/resources/success_stories for more details.
Some weeds with a fast growth rate and high seed production are at risk of developing herbicide resistance. Pesticide resistance involves inherited genetic physiological and/or behavioral adaptations that confer a selective advantage in the presence of a pesticide and lead to control failures of a once susceptible pest population despite proper application. Pesticide tolerance is on the other hand, when physiological and/or behavioral adaptations lead to some toxicity baseline but is not the result genetics (Buss et. al. ENY-842). Resistance often develops as a result of repeated use of the same herbicide mode of action (MOA). In Florida, certain populations of Amaranthus palmeri or Palmer amaranth are one such example of an herbicide resistant weed. More information on this example may be found in EDIS # SS-AGR-336.
Herbicide resistant weeds have been around since at least the 1970’s when the first was reported in Washington State (Ryan 1970). Improper use of herbicides and the widespread adoption of genetically modified crops designed to withstand repeated exposure to the same pesticide has only exacerbated the problem in recent years.
It was not long before herbicide resistant weeds were detected in fields across the nation following successive plantings of Round Up Ready corn. Herbicide resistant weeds are not however unique to North America. There was an estimated 1190 global plant species with herbicide resistance in 2009 (Bhowmik 2010). Argentina and Brazil are just two countries with areas of transgenic glyphosate-resistant crops grown successively. Consequently glyphosate-resistance has evolved in populations (Pest Management Science Special Issue: Glyphosate-Resistant Weeds and Crops Volume 64, Issue 4, pages 360–365, April 2008).
It is often not enough to simply rotate between two different pesticide products as part of a resistance management plan to avoid development of resistance. This is because pesticide products may have the same active ingredient or have the same mode of action despite having different trade names. The Herbicide Resistance Action Committee (HRAC) was formed to communicate and educate agricultural producers and crop protection professionals across the globe by providing resistance management information. Groups have been created to do the same for fungicides (FRAC) and insecticides (IRAC). These Resistance Action Committees classify pesticides on the basis of their mode of action (MOA) which is the specific physiological activity of a toxin resulting in the death of a pest.
The herbicide mode of action poster may be downloaded free of charge by visiting hracglobal.com. Other useful weed management resources include your local county extension agent and University of Florida’s Institute of Food and Agricultural Sciences (UF/IFAS) Electronic Delivery Information Source (EDIS) found at http://edis.ifas.ufl.edu.
Depending on how you see it, flowers of all sorts may also bring smiles and beauty into the home much like highly prized roses, tulips, and orchids. It may however be time to reevaluate your weed management strategy if you are frequently receiving common flora collected from your own yard or field.
Resources
- Bhowmik, P.C. 2010. Current status of herbicide resistant weeds around the Globe. Journal of Crop and Weed 6(1): 33-43 (2010).
- Dittmar, Peter J. and William M. Stall. Estimated Effectiveness of Recommended Herbicides on Selected Common Weeds in Florida Vegetables. University of Florida-IFAS Pub HS706. http://edis.ifas.ufl.edu/wg074
- Florida Department of Agriculture Noxious Weed List 2014. https://www.flrules.org/gateway/ruleNo.asp?id=5B-57.007
- Gillett, Jennifer L., et. al. 2006. Grower’s IPM Guide for Florida Tomato and Pepper Production. http://ipm.ifas.ufl.edu/resources/success_stories/T&PGuide/pdfs/Chapter6/Weed_Mgmt.pdf
- Goyal, Gaurav, Harsimran K. Gill, and Robert McSorley. 2012. Common Weed Hosts of Insect-Transmitted Viruses of Florida Vegetable Crops. University of Florida-IFAS Pub ENY-863. http://edis.ifas.ufl.edu/in931
- Langeland, K.A., et al. 2008. Identification and Biology of Nonnative Plants in Florida’s Natural Areas. University of Florida-IFAS Pub SP 257. http://plants.ifas.ufl.edu/misc/pdfs/SP257/Lantana_camara(SP257-126).pdf
- Morichetti, Sergio, Jason Ferrell, and Ramon Leon. 2010. Amaranthus palmeri “Palmer amaranth”. University of Florida-IFAS Pub SS-AGR-336. http://edis.ifas.ufl.edu/ag346
- Pest Management Science Special Issue: Glyphosate-Resistant Weeds and Crops Volume 64, Issue 4, pages 360–365, April 2008.
- Ryan, G. F. 1970. Resistance of common groundsel to simazine and atrazine. Weed Sci. 18: 614-16.
- Sellers, B. A., Sarah Lancaster, K. A. Langeland, J.A. Ferrell, Michael Meisenberg, and J. Walter. 2007. Identification and Control of Coral Ardisia (Ardisia crenata): A Potentially Poisonous Plant. University of Florida-IFAS Pub SS AGR 276. http://edis.ifas.ufl.edu/ag281
- Williams, N.H. 1993. Plant Identification and Information Service. University of Florida-IFAS Pub RFSR013. http://edis.ifas.ufl.edu/sr013
- hracglobal.com. Mode of Action Poster 2010.
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