Hemp Dogbane Biology and Management

Jerry Doll

Hemp dogbane (Apocynum cannabinum L.) is one of several dogbane species native to North America. In Canada it is known as Indian hemp; both names suggest the plant was used as a source of fiber and indeed native Americans used the root to make ropes (Becker 1981). Hemp dogbane has been a serious weed in several midwestern states for many years. Among the perennial broadleaf weeds in corn and soybeans in Ohio, hemp dogbane is second in importance only to Canada thistle (Loux and Berry 1991). It was ranked as equally important in zero, reduced and conventional tillage systems. In Wisconsin, dogbane's importance as a weed has gradually increased. In 1977, 6% of the agricultural agents ranked it as serious or very serious in row crops and small grains (Doll 1978). By 1994, 15% ranked dogbane as a serious or very serious problem and it was most serious in the southwest and south central regions of the state (Doll, unpub. data).

A significant reason for this shift is the change in tillage and cropping practices farmers are making. This is particularly apparent in no-till soybeans which have increased from 10,000 acres (5% of 200,000 acres) in 1980 to 280,000 acres (30% of 860,000 acres) in 1994 (Oplinger, per. comm. 1994). Without primary tillage before planting and with no cultivation after planting, infested areas have increased significantly. Hemp dogbane often spreads within a field and from field to field because farmers transport a piece of root section on the machinery and "plant" it in new locations, producing new hemp dogbane colonies. The shift from moldboard to chisel plows and from disks to field cultivators may also enhance its spread as toothed tools drag root segments more than other types.

Hemp dogbane infestations are more common in some environments than others. In Nebraska, dogbane was most common in oats and soybeans, moderately common in corn, sorghum, fallowed fields and roadsides, and least common in alfalfa and winter wheat (Schultz and Burnside 1979a). The increase in soybean and decrease in alfalfa acreage has undoubtedly contributed to the rise of hemp dogbane importance in Wisconsin.

Description

Hemp dogbane is a deep-rooted perennial capable of reproducing by seed or by vegetative buds in the crown region and on the horizontal lateral roots. All plant parts have sticky, milky sap and this is why dogbane may be confused with common milkweed while in the vegetative stage. The figure on the next page shows the key parts of hemp dogbane.

Roots: Dogbane has an extensive, branched root system with vegetative buds irregularly placed on the lateral roots. The buds can form shoots. Vertical roots may penetrate 8 feet or more but do not have buds.

Stem and Leaves: Hemp dogbane stems are smooth and grow 3 to 5 feet tall. They are woody near the base and, unlike common milkweed, hemp dogbane stems branch near the top, giving them a "bushy" look. The leaves are smooth, opposite, ovate to lanceolate, and have a very short petiole. They are generally smaller, lighter green in color and more pointed than the leaves of common milkweed. Hemp dogbane leaves are bright green during the growing season and turn a yellowish-brown to orangish-yellow in the fall.

Flowers and Seeds: From late June to August, hemp dogbane produces clusters of small, greenish-white, bell-shaped flowers at the end of each branch. Each flower may produce two slender, slightly curved, pencil-like pods 3 to 4 inches long. Pods have up to 200 reddish-brown seeds, each with a tuft of silk on one end, similar to milkweed seeds. However, common milkweed seeds are much wider and flatter than hemp dogbane's spike-shaped seeds.

Biology and Ecology

Hemp dogbane can become established from seed. Seeds are highly viable, and although germination is greater in the light, few seeds on the soil surface produce established seedlings. Most emergence and seedling establishment occur when seeds are half an inch below the surface (Robinson and Jeffery 1972). True seedlings are sensitive to soil disturbance and if cultivated before they are 10 inches tall, few if any will be capable of forming new shoots from root buds.

A detailed study of the growth and development of individual plants for two seasons was done in Kansas (Frazier 1944). Seeds planted in early to mid May emerged 10 to 12 days later. With no competition from other plants, the primary root grew 7 feet deep in one season. No shoot-forming bud developed on the primary vertical root. Ten weeks after emergence, the general pattern of root development was apparent. Many branch roots developed from the primary root and most of these were feeder roots. Branch roots grow horizontally to the soil surface and seldom are deeper than 7 inches. Some become thickened roots with buds. These lateral roots also branch. Initial growth of secondary branches is at a right angle from the parent branch, but after 2 to 8 inches of growth, they bend and grow away from the primary vertical root Additional vertical roots develop from the lateral branches. They normally grow 2 to 12 inches horizontally before bending and growing downward. By 15 weeks after emergence, buds had formed on the permanent lateral roots, and at 20 weeks, shoots developed from these buds. Buds seem to develop randomly on the permanent branched roots and they are not found on the primary or secondary vertical roots. Buds either sprout and develop into shoots or remain dormant.

Plants do not flower the year they start from seed, but flower readily the second season (Frazier 1945). After two seasons, the primary vertical root was nearly 14 feet deep and the longest permanent lateral root was nearly 20 feet long. Thus single plants arising from seed and growing without competition or disturbance could invade an area nearly 40 feet in diameter in two seasons.

Hemp dogbane shoots do not emerge until soil temperatures reach 65 degrees F. After emergence, they grow and develop very rapidly. Plants reach the bud stage 3 to 4 1/2 weeks after emergence in Nebraska (Schultz and Burnside 1979a). Flowers appear 5 to 11 days after the buds and full bloom occurs 1 to 3 weeks after flowering starts. Seed pods may or may not be formed; this variability is well not understood.

Hemp dogbane grows much more rapidly than corn or soybeans. Table 1 illustrates this point with examples of corn and hemp dogbane heights in May and June from several states and years. Hemp dogbane is at least as tall as and usually much taller than corn. Thus directed spray applications with drop nozzles are not practical. The Wisconsin data also show the prolonged period of emergence of hemp dogbane. Some plants emerge early while others (probably arising from buds on roots deeper in the soil) appear 10 to 20 days later. Foliar applications made when the early emerging shoots are 10 to 12 inches tall will have little effect on small shoots with few leaves (and of course no effect on shoots yet to emerge). More uniform emergence would be expected in no-till systems because roots are not disturbed nor repositioned by tillage. This is not so. The 1994 Wisconsin data are from a no-till field and on June 1, shoots range from less than 10 to 20 inches tall. However, hemp dogbane emerges sooner and is usually much taller than corn or soybeans in no-till fields.

Hemp dogbane can be attacked by several leaf-feeding insects. In 1976, 80% of the infestations examined in Nebraska had feeding that caused 2 to 95% defoliation (Schultz and Burnside 1978). The insects noted in Nebraska were the woolly bear moth (Cycnia tenera), leaf roller moth (Clapsis pallorana), milkweed tiger moth (Euchaetras egle), and Indian hemp moth (Saurcobotys futialis). All four species exist in Wisconsin (Pelliterrii 1994, per. comm.) and in 1994, we observed both zebra caterpillar (Melancra picta) and Indian hemp moth on hemp dogbane in southern Wisconsin. The zebra caterpillar did minimal damage, but the Indian hemp moth resulted in 100% defoliation of hemp dogbane in a field in western Walworth Co. (near Richmond) in mid-July and no new leaves formed later. This field has a very large hemp dogbane infestation that has not been adequately controlled for several years. It appears that the epidemic level of the insect followed the epidemic level of the weed. Hemp dogbane in a Dane Co. field had little if any significant defoliation in 1994. Sometimes, heavy leaf feeding will reduce the effect of late-season foliar herbicide applications.

Importance

Hemp dogbane is generally found in patches. In Nebraska, 60% of the fields with this weed averaged 1 to 10 plants/acre, 25% averaged 11 to 100 plants/acre and 15% had 12 to 1000 plants/acre. Thus it is critical to attack hemp dogbane when the overall population is low to prevent the buildup of high densities and wide spread infestations throughout a field (Schultz and Burnside 1979a).

Competition data from hemp dogbane are limited. Researchers in Nebraska sampled crops infested with hemp dogbane and adjacent non-infested areas of the same field for several seasons (Schultz and Burnside 1979a). Average hemp dogbane densities in the infested areas ranged from 16,000 to 30,000 plants/acre. Densities were 10,000 plants/acre higher in irrigated than dryland fields, suggesting that hemp dogbane grows best in moist soil conditions. Yield losses in irrigated corn were 9% while no statistical differences were detected between infested and non-infested corn in dryland conditions. Thus it appears that corn yield may not be greatly affected by hemp dogbane. However, dryland soybean and sorghum yields were reduced 30 and 40%.

Besides yield losses, hemp dogbane affects the harvestability of several crops. This weed is usually in full bloom during wheat harvest and the milky sap in stems and leaves coats combine parts and grain, causing dirt and chaff to adhere to them. While dogbane is nearing maturity at soybean harvest, harvestability problems can also occur. The milky juice in hemp dogbane contains a cardiac glycoside toxin but plants are very unpalatable to livestock and cases of poisoning are unlikely.

Control

Cultural

Alfalfa competes well with hemp dogbane and the large acreage of harvested forages in Wisconsin is probably why hemp dogbane is less common here than in states with large cash grain acreages. While growing alfalfa may not eradicate hemp dogbane, the frequent mowings reduce root reserves, limit lateral root growth, prevent flowering, and avoid spreading root segments on tillage equipment. In the final year of hay production, avoid harvesting in August and apply herbicides to infested areas in mid to late Sept. Winter wheat also competes well with hemp dogbane because it is established in the fall and grows rapidly in the spring before soils are warm enough to stimulate hemp dogbane growth. Winter wheat also provides an excellent opportunity to use mechanical and chemical measures after wheat harvest in late summer or early fall.

Hemp dogbane occurs in fence rows and roadsides. Try to prevent seed production in these patches to eliminate additional sources of infestation. A single mowing or 2,4-D application should be adequate to achieve this goal in these areas.

Mechanical

Soil disturbance readily controls hemp dogbane seedlings. Six to 8 weeks after germination hemp dogbane is considered "established," as then it can reproduce vegetatively from buds on the root. Thus if seeds have germinated, it is important to till the field before seedlings are well established. A good rule of thumb is that hemp dogbane seedlings are most easily killed if they are cut off or cultivated before they are ten inches tall, about 6 to 8 weeks after seedling emergence.

Usually, hemp dogbane infestations are already established. Theoretically it is possible to till the infestation as needed to deplete the food supply in the roots and achieve eradication. This means disking or field cultivating the area every two to three weeks during the season. Tilling during warm, dry conditions kills root pieces near and on top the soil surface by desiccation. Cool, wet conditions are unfavorable for tillage in general, and tilling in such weather kills few roots.

It may appear that tillage increases the density of hemp dogbane. This is because tillage cuts up the root system, breaking apical dominance in the root. Each section of root with buds then forms one or more new shoot, producing more shoots per unit area than before the field was tilled. Tillage may also increase the size of the infested area by dragging root pieces beyond the edge of the infestation. If these root sections becomes established, then the patch will appear bigger. After tilling hemp dogbane infestations, clean the machinery before moving to non-infested fields.

Row cultivation alone will have limited long-term effect on hemp dogbane. Most plants in the row will escape control because they are as tall as or taller than corn or soybeans. Nevertheless, killing shoots between rows will reduce possible yield losses. Cultivators with sweeps kill more hemp dogbane than those with points or narrow shovels. Cultivation in conjunction with herbicide use is highly recommended to further weaken the infestation.

Chemical

2,4-D can be used in grass crops, pastures, fence rows and roadsides. Hemp dogbane is particularly sensitive to 2,4-D when appropriate rates are used at the proper growth stage. Iowa State Univ. researchers observed 97% hemp dogbane control 1 year after applying 1 lb ae/acre of 2,4-D amine when plants were in the bud to early flower stage. Banvel and Roundup gave significant dogbane control but were no more effective than 2,4-D (Becker and Fawcett 1980). Trials in Illinois found that 0.5 lb ae/acre of 2,4-D applied when hemp dogbane was in the early flower stage (33 inches tall) and corn had 7 to 8 leaves, reduced the dogbane infestations 60% or more 12 months after application (Orfanedes and Wax 1991).

Hemp dogbane is listed on many brands of 2,4-D. However, labels always require the use of drop nozzles once corn is 8 inches or more in height. For example, one manufacturer states, "Treat corn over 8" using drop nozzles. Dogbane should be in flower bud stage." Their recommendation recognizes that dogbane should be in the bud to early flower stage for best results because treating earlier is ineffective since (1) many dogbane shoots are yet to emerge and (2) little translocation into the roots occurs at this time. When most hemp dogbane has emerged and plants are in the bud stage, the weed is usually taller than the crop and using drop nozzles will not give effective control. Dogbane is more likely to be at or near the bud stage when no-till corn is 8 inches or less in height that it is in tilled fields at this stage of corn growth.

Fall applications of 2,4-D are generally more effective than spring treatments. Several brands of 2,4-D allow for preharvest applications of 1 lb ae/acre when corn silks are brown. Accurate field maps of where hemp dogbane is found and access to a high-clearance sprayer are essential for this approach to succeed. Be sure dogbane is still actively growing when preharvest treatments are made. If leaves are turning yellow after a frost or because of drought, control will be greatly reduced. Do not treat after a freeze even if leaves are still green (Furrer et al., 1983).

Banvel/Clarity (dicamba). Either formulation of dicamba can be used in corn, but only Banvel can be used in other grass crops, fence rows and pastures. Less published data exists on the use of dicamba for hemp dogbane control than for 2,4-D. Results usually show that it is similar to 2,4-D but sometimes it is less effective. Many states recommend a combination of dicamba and 2,4-D. In Wisconsin trials, 2,4-D alone has been more consistent that Banvel/Clarity alone on hemp dogbane. Banvel and Clarity can be tank mixed with Beacon. Dicamba can not be applied as a preharvest treatment in corn.

Beacon (primisulfuron). Beacon alone suppresses hemp dogbane but, as with 2,4-D, the problem is one of timing. The Beacon label says that applications should be made when hemp dogbane is 2 to 6 inches tall which is long before most hemp dogbane has emerged. We have had good to excellent suppression of dogbane by applying Beacon in postemergence at 0.38 ounce/acre with either 0.5 to 1.0 pint/acre of 2,4-D or Banvel/Clarity. Dogbane suppression of 80 to 90% is achieved when treating relatively small plants (5 to 12 inches tall) and control seems to improve if dogbane is more developed (12 to 24 inches tall) when treated. At this later time, most plants have emerged and all plants are more effective in translocating herbicides to the roots.

In two of three trials, 2,4-D with Beacon was more effective on hemp dogbane than Banvel or Clarity with Beacon. In the 1993 trial, corn was slightly injured by over-the-top applications of 2,4-D and Banvel on June 23. The second-year effects of treatments applied at two locations in 1994 will be monitored in 1995.

Eradicane (EPTC with safener). Eradicane often suppresses hemp dogbane for 30 to 40 days after application (Schultz and Burnside 1979b). This allows corn to get a competitive advantage and makes row cultivation more effective because if not suppressed, hemp dogbane grows much more rapidly than corn. However, the effect of Eradicane on this weed is of little long-term benefit and hemp dogbane is not listed on the product label.

Roundup (glyphosate). Roundup at 3 to 4 qt/acre when hemp dogbane is in the late bud to early flowering stage significantly reduces the infestation. Combinations of 1 qt/acre 2,4-D and 2 qt/acre Roundup would also be effective (Becker and Fawcett 1980; Schultz and Burnside 1979a). Such treatments can only be done as spot applications, in fallowed fields, or following the harvest of wheat, barley or early season vegetable crops. Treat when dogbane plants are actively growing.

Roundup can be applied in selective applicators when hemp dogbane is taller than soybeans. This height advantage of the weed will be greater and its growth stage more advanced in no-till than tilled beans. Sponge applicators may give effective of control with a single pass. Growers have reported noticeable reductions in dogbane infestations the year after such applications.

Roundup can be applied preharvest in soybeans. The effectiveness of this approach requires that the hemp dogbane still be actively growing but the soybeans must be completely defoliated and all pods must be yellow. Do not use this method on beans harvested for seed as Roundup may affect seed germination or seedling vigor. Use a back pack sprayer to make preharvest treatments to avoid crop damage by driving through the field.

Herbicide/options of the future. Starane (fluroxypyr) is an experimental pyridine herbicide from Dow/Elanco. It is registered for use in small grains in several European countries and may be registered in small grains in the US in a few years. However, it will not be registered for corn in the foreseeable future. Starane is particularly effective on hemp dogbane and several species in the buckwheat (Polygonaceae) and morning glory (Convolvulaceae) families, including wild buckwheat and field bindweed. It has very little risk of corn injury even when applied over the top to corn 12 to 24 inches tall. Symptoms of Starane injury to hemp dogbane are similar to those of 2,4-D, but 0.06 lb ae/acre of Starane gave the same level of control as 0.5 lb ae/acre of 2,4-D (Orfanedes and Wax 1991).

The era of glyphosate tolerant crops is just around the corner. Soybeans will be one of the first crops with "Roundup Ready" seeds on the market. When this occurs, a new avenue of attack on hemp dogbane will be available. These seeds have been genetically engineered to be unaffected by Roundup applied postemergence so hemp dogbane can be treated at the right stage (late bud to early flowering) with the appropriate rate of Roundup (3 to 4 qt/acre) while growing soybeans. The effects of such treatments will be evident for several years.

Integrated Strategies

As always, the best program is one that integrates all the control measures into a planned program. Review the tillage system and crop rotation in each field with hemp dogbane. Plan rotations so that effective treatments can be made at least every other year. Hemp dogbane may be more readily controlled in no-till systems than when tillage is done. Without tillage, the weed's rapid growth allows it to become taller than corn or soybeans sooner than if tillage is done. This means the weed should (1) reach the more ideal time for treatment (bud stage) before corn is too tall to use over-the-top applications and (2) be considerably taller than soybean when a selective applicator is used. Where possible, cultivate row crops following herbicide use to further weaken the weed. Make weed maps annually and plot the results of current management efforts to see if changes are needed. Wheat and alfalfa are very competitive with hemp dogbane. These crops and early season vegetables also allow for late summer treatments which are often very effective.

Bibliography

Becker, R. 1981. Today's Weed: Hemp Dogbane. Weeds Today. 12(4): 15-16.

Becker, R. and R. Fawcett. 1980. North Cen. Weed Cont. Conf. Res. Rept. 37: 54.

Doll, J. 1978. A survey of perennial weed problems in Wisconsin. Proc. North Cen. Weed Cont. Conf. 33: 72-74.

Fraizer, J. 1944. Nature and rate of development of root system of Apocynum cannabinum. Bot. Gazette 105: 463-470.3

Fraizer, J. 1945. Second-year development of root system of Apocynum cannabinum. Bot. Gazette 10: 332.

Furrer J. A. Martin and F. Roeth. 1983. Hemp dogbane. NebGuide G83-665. Univ. of Nebraska, Lincoln. 4 pp.

Loux, M. and M. Berry. 1991. Use of a grower survey for estimating weed problems. Weed Tech. 5: 460-466.

Orfanedes, M. and L. Wax. 1991. Differential response of hemp dogbane (Apocynum cannabinum) to clopyralid, Dowco 433, and 2,4-D. Weed Tech 5: 782-787.

Robinson, L. and L. Jeffery. 1972. Hemp dogbane (Apocynum cannabinum) growth and control. Weed Sci. 20: 156-159.

Schultz, M and O. Burnside. 1978. Defoliation of hemp dogbane by insects. North Central Weed Contr. Conf. Res. Rept. 35: 40.

Schultz, M and O. Burnside. 1979a. Distribution, competition and phenology of hemp dogbane in Nebraska. Weed Sci.
        27:565-570.

Schultz, M and O. Burnside. 1979b. Control of hemp dogbane with foliar and soil applied herbicides. Agron. J. 71: 723-730

November, 1994