VELVETLEAF: PREVENTION BETTER THAN THE CURE?

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1 VELVETLEAF: PREVENTION BETTER THAN THE CURE? Robert F. Norris Section of Plant Biology, University of California, Davis, CA Velvetleaf has been a serious weed in many regions of the midwest and central USA, in southern Canada, and in the Mediterranean region. It is now increasing in importance as a weed in all low altitude regions of California. A 1991 survey of Cooperative Extension farm advisors showed that it is present from the Imperial Valley to the northern end of the Sacramento valley. It is most prevalent in the Sacramento valley and the delta region where it is already a major problem in crops like sugarbeets, tomatoes, dry beans, and safflower. It also occurs in com, and in limited infestations in cotton. The latter is of particular concern because velvetleaf is in the same botanical family, Malvaceae, as cotton and may be particularly hard to control in this crop. If it hasn't invaded your area yet then be thankful. When one reviews the population biology of velvetleaf, and our ability to control it in many of the crops grown in California the only logical conclusion is to not permit the weed to invade new areas. We have reached an era where new 'magic bullet' herbicides are unlikely to be developed. We also seem to have forgotten some of the age-old principles of weed management; the time has come to remember them in the case of velvetleaf. Or the weed is going to become a very serious threat to California agriculture if it isn't already. The topic that I will cover is population biology of velvetleaf. I will address several aspects including seed production, seed dispersal and spread, and seed longevity in the soil. I will also consider the overall population dynamics implications of permitting this plant to become established. Seed production: A single velvetleaf plant produces about 17,000 seeds if it is growing without competition in the midwest or southern Canada (Warwick and Black, 1988). In the Po valley in Italy (Zanin and Sattin, 1988) reported that velvetleaf produced about 10,000 seeds per plant without competition, decreasing to less than 250 seeds per plant at densities of over 60 plants/m 2 This translates to a peak seed production of about 40,000 per m 2 Growing velvetleaf in com in the Po valley reduced these values to 2000 seeds per plant and about 16,000 seeds/m 2 respectively. Recent work in the Sacramento valley by Scott Steinmaus (personal communication) has indicated that velvetleaf can produce in excess of 50,000 seeds/plant when growing without competition. Single plants only produced about 800 to 1000 seeds when grown in corn, but 2,500 to 3,500 seeds were produced per plant when grown with dry beans. It is apparent that velvetleaf can reproduce effectively under cultural and environmental conditions present in the Central Valley of California. These preliminary data for California also show 211

2 that seed production may be much higher under California growing conditions than in other regions. The com earworm (aka cotton bollworm, tomato fruitworm) will feed on and in the pods of velvetleaf (personal observation). The potential significance of this to seed production has not been determined. Seed dissemination/spread: Velvetleaf does not possess any special adaptation for seed dispersal. A few instances have been reported where pods are detached from the plant and spread by machinery. Under natural conditions the seed, which have no dispersal adaptations, are shed within a few meters of the parent plant. The spread of the weed is thus essentially by actions of humans. We therefor have the ability to regulate the spread of the weed. Velvetleaf thus becomes a weed about which we should pay special attention to sanitation practices. Do not move it in crop seed. Originally imported as a possible fiber crop. Although not specifically recorded as being a contaminant of crop seeds every effort should be made to assure that it does not get seeded. Do not move it on farm machinery. This method of spread of weeds is well documented. Velvetleaf is potentially so serious a threat that equipment being moved from an infested field should be cleaned before it leaves the field. The converse of this is that if custom machinery is being brought in to a non-infested farm it behooves the owner to request that the equipment be cleaned. Do not move it in farm animals. Substantial numbers of velvetleaf seeds survive digestion when eaten by animals (Harmon and Kiem, 1934). The average survival of seeds recovered from farm animals was 22.8%; recovery from chickens was only 1.9% but over 45 % of the seeds were recovered from calves. Animals that have been allowed to ingest velvetleaf seeds should be allowed to clear these from their digestive system before being moved to areas free of velvetleaf. use of manure that might contain velvetleaf seeds should be avoided. Do not move seeds in crop waste. Gin trash and sugarbeet tare/ soil are notorious for containing weeds seeds, pathogens, and nematodes. If these sources could be contaminated with velvetleaf seed they should not be returned to fields currently free of the weed. Do not move it in irrigation water. Seeds can float for considerable distances. Every effort should be made to ensure that velvetleaf seeds are not being spread into non-infested areas in the irrigation water. Do not let it grow in fence lines, ditch banks, etc. These areas serve as a source for seeds that can then be spread to adjacent crop land by machinery, animals and birds, irrigation water, and us! Donnancy and longevity in soil: Velvetleaf produces hard seed. The seed coat has a woody layer that is impermeable to water and to gases. This means that the seed are dormant at the time of shatter from the plant. The loss of hard-seededness is a long process over many years. The rate of seedbank decline in other regions of the world seems to be about 20 to 30% per year. Germination is sporadic even under ideal conditions and is typically only about 10 to 20% of the total seedbank (Weaver and Black, 1988). The loss rates are apparently much 212

3 lower under non-tillage systems (Lueschen et al. 1993) leading to much greater retention of the seedbank under a non-tilled crop like alfalfa. In Mississippi and Missouri a scentless plant bug (Niesthrea louisianica Sailer) feeds on velvetleaf seeds (Kremer and Spencer, 1989). This results in reduced seed viability and to substantially increased fungal attack of the seeds in the soil. Seedbank decline is much faster under conditions of such attack; viability of the seedbank was only 2 % after 24 months. The potential significance of this to population dynamics of velvetleaf in California is not clear. Population dynamics: If one takes these general values for seed longevity, seedbank decline rates, and germination it can be predicted that after about 12 years there will still be between 1 and 2 percent of the seedbank remaining, even if no new seed were produced during that period. These theoretical projections have been substantiated by work where velvetleaf seeds were buried and then exhumed after different lengths of time. Chandler and Egley (1983) reported that 36% of the velvetleaf seed remained in the soil after 5.5 years in Mississippi. Burnside et al. ( 1981) showed that about 5 % of the initial seedbank remained in Nebraska after 10 years. Recently Lueschen et al (1993) working in Minnesota demonstrated that 1 to 2 % of the initial seedbank was still present after 17 years of arable cropping (Figure 1). Chemical fallow increased the seedbank survival to 15 % and continuous alfalfa increased it to 24 %. Decreased tillage thus decreased the rate of seedbank decline. Velvetleaf seedbank dynamics under several different croppmg practices in Minnesota (redrawn from Lueschen et al., 1993) 1200 \1~ U) fil 800 w U) ~ 600 ~ ~ ~ \;~.. r:t \ '?. \. \ \ \-CherricalfalaN '\ \ \ \ \a \ :o.. AWa i6/~~ ~ \ ~- ~~---o.\... --cr _.~ v - - -v-- - -V...._..._.. \- Cormuous ~relation....::--o o... ~~ v 'O. ~ plow continuous fallow ~:.:..:..:..:::..:.;_ft...:..:.:..~ -----n TIM: (YEARS)

4 It is clear that once velvetleaf is established it is very difficult to eliminate. This has been noted by authors writing about velvetleaf in regions where it is already a problem. The universal conclusion of these authors is that velvetleaf should not be allowed to invade land that is not infested. The 'bottom line' of all this discussion is 'do not let this plant go to seed'. Diligence is required to achieve this. Rogue all plants that do become established, and make sure this is done prior to seed set. A safe rule would be to remove plants at or before they start to flower. The ultimate goal must be to ensure that no viable seed are produced. Conclusion: Without effective selective velvetleaf control in many California field crops (see paper by T. Lanini in this series) the bottom line of these discussions is do not let this weed become established. In a popular review of velvetleaf in the central region of the USA Roeth (1987) concluded 'Preventive practices to reduce the chances of its introduction into a clean field should receive high priority'. This same conclusion was reached by Zanin and Sattin (1988) in the Po valley in Italy, and I quote "Once A. theophrasti [velvetleaf] becomes established in a field, even intensive efforts cannot eradicate it. Even one plant per m 2 can counterbalance the annual reduction of the seed-bank. Preventive practices to reduce the chances of its introduction into a clean field should therefor receive high priority. " Is California agriculture doomed to suffer the same problems that other regions have experienced as a result of invasion by this weed? Or can we learn from their experiences? It is in your hands. From where I sit it would definitely appear that prevention is better than the cure. LITERATURE CITED. Burnside, 0. C., C. R. Fenster, L. L. Evetts, and R. F. Mumm exhumed weed seeds in Nebraska. Weed Sci., 29: Germination of Egley, G. H. and J. M. Chandler Longevity of weed seeds after 5.5 year in the Stoneville 50-year buried-seed study. Weed Sci., 31: Harmon, G. W. and F. D. Keim The percentage and viability of weed seeds recovered in the feces of farm animals and their longevity when buried in manure. Agron. J., 26: Kremer, R. J. and N. R. Spencer Impact of seed-feeding insect and microorganisms on velvetleaf (Abutilon theophrasti) seed viability. Weed Sci., 37: Lueschen, W. E., R. N. Anderson, T. R. Hoverstad, and B. K. Kann Seventeen years of cropping systems and tillage affect velvetleaf (Abutilon theophrasti) seed longevity. Weed Sci., 41:

5 Roeth, F Velvetleaf: coming on strong. Crop and Soils Mag.39(6):10-11 Warwick, S. I. and L. D. Black The biology of Canadian weeds. 90. Abutilon theophrasti. Can. J. Plant Sci., 68: Zanin, G. and M. Sattin Threshold level and seed production of velvetleaf (Abutilon theophrasti Medicus) in maize. Weed Res., 28: