Sicklepod

Senna obtusifolia (L.) Irwin & Barneby = Cassia obtusifolia L.

Images above: Upper left: Sicklepod flower (John Gwaltney, SoutheasternFlora.com). Upper right: Sicklepod plant with curved pods (John Gwaltney, SoutheasternFlora.com). Bottom: Sicklepod pod (John Gwaltney, SoutheasternFlora.com).

Identification

Other common names:  coffeebean, javabean, coffeeweed

Family:  legume family, Fabaceae.  Some botanists separate this genus and its relatives, most of which are woody, into the caesalpinia family, Caesalpiniaceae (Cronquist 1988).

Habit:  Upright or spawling summer annual herb or marginally perennial shrub with compound leaves.

Description:  Seedling cotyledons are large and round, 0.6-0.8” (1.5-2 cm) wide, with 3-5 prominent, pale veins radiating from the base in a hand-like pattern.  The first true leaves are divided, with 2-3 pairs of short-stalked, oval to egg-shaped, hairless leaflets.  Mature plants are 1-6 ft (0.3-1.8 m) tall and sprawling to upright.  Stems are green, angular, hairless, and occasionally branching.  Older stems may become woody at the base.  Leaves similar to those of young plants, with 3-4 pairs of leaflets and two 0.6” (1.5 cm) long, leafy bracts at the base.  Each leaflet is 1-3.5” (2.5-9 cm) long by 0.5-1” (1.3-2.5 cm) wide; leaflets at the tip of the leaf are generally larger than those at the base.  A 0.03-0.1”(0.08-0.25 cm) long orange/brown gland is present at the base of the lowest pair of leaflets.  The branching taproot can reach up to 3.3 ft (1 m) in length.  All parts of the plant give off a distinctive odor when crushed.  Yellow, showy flowers with 0.3-1” (0.8-2.5 cm) long stalks develop singly or in pairs from the upper leaf axils.  Flowers are 5-petaled, and 0.4-1” (1-2.5 cm) wide.  Petals are 0.3-0.8” (0.8-2 cm) long, narrowest at the base, and have at least three prominent veins.  The seedpod is four sided, curved downward, hairless, and 3-8” (8-20 cm) long by 0.12-0.25” (0.3-0.6 cm) wide.  Seedpods split when ripe, releasing 20 to 40 seeds.  Seeds are 0.1-0.25” (0.25-0.64 cm) long, brown, glossy, and rectangular to irregular in shape.

Similar species:  Coffee senna [Senna occidentalis (L.) Link] and partridgepea [Chamaecrista fasciculata (Michx.) Greene] have similar flowers and foliage to sicklepod.  Coffee senna leaflets are larger, up to 3” (7.6 cm) long and 1.5” (3.8 cm) wide, and more numerous (3-7 pairs per leaf) than those of sicklepod, and have pointed rather than rounded tips.  Coffee senna seedpods are cylindrical or flattened rather than angular, and are curved upward.  Partridgepea cotyledons are spade shaped rather than round, and have tapered, bluntly-pointed ends.  Mature partridgepea leaves have 8-15 pairs of leaflets that are much smaller, up to 0.7” (1.8 cm) long by 0.3” (0.8 cm) wide, than those of sicklepod.  Partridgepea pods are only slightly curved.

Management 

Rotation to winter and spring cereal grains helps suppress sicklepod populations since the grain will be highly competitive by the time the sicklepod germinates and will be harvested before the weed can produce seeds.  A cultivated fallow after grain harvest can flush seedlings out and kill them. 

If siclepod produces seeds, leave them on or very near the soil surface in the fall to encourage seed predation and weathering of the resistant seed coat.  When preparing a seedbed for a spring row crop, till shallowly; deep plowing will bury seeds for the current year, but many will survive and emerge in subsequent years.  If deep tillage is necessary, rip with as little inversion as possible.  Note also that shallow seedbed preparation will put the sicklepod seeds closer to the soil surface where they are more vulnerable to in-row weeding after planting.

Plant spring row crops as early as will allow good establishment.  Soybeans and corn, in particular, can emerge from substantially cooler soil than sicklepod and this gives these crops a head start over the weed.  High density planting of soybeans, for example, 395,000/A (976,000/ha) rather than 264,000/A (652,000/ha), can help to competitively suppress this weed (Nice et al. 2001).  Reports vary on the effectiveness of narrow row spacing for suppressing sicklepod, but at least one study has shown substantial reduction in weed density and dry weight with 7.5” (19 cm) row spacing as compared with 38” (97 cm) row spacing (Norsworthy et al. 2007). 

Since many sicklepod seedlings emerge from below the normal planting depth of row crops, in-row weeding should focus on breaking or burying young sicklepod, rather than uprooting them.  Consequently, a shallow sweep cultivator, rolling cultivator or tine weeder with relatively stiff tines will be more effective than a rotary hoe or tine weeder with highly flexible tines.  Cultivate shallow and close to the row, but throw enough soil toward the row to bury late emerging plants if the crop will tolerate this.  Cotton rows can be flame weeded to eliminate late emerging sicklepod.  Late emerging weeds will not be very competitive, except perhaps in a drought year, but the seeds they produce can pose problems for future crops in the rotation.     

The seeds provide sufficient energy to allow seedlings to penetrate even dense layers of straw mulch, but synthetic mulches are effective barriers against sicklepod in vegetable crops.  Distillery waste water applied as a fertility amendment at 1 gal/ft2 (40 L/m2) suppressed sicklepod emergence by 87% and post-emergence survival by 65% (Soni et al. 2014).  Although sicklepod seeds were the most tolerant to heat of species tested, most would be killed by narrow-windrow burning of soybean residue (Norsworthy et al. 2020).

Sicklepod can be a problem in poorly managed pastures and a severe problem in high traffic areas such as around hay rings or feed bunks, but a vigorous sward will outcompete the weed (Anning et al. 1989).  Trampling during intensive rotational grazing will kill sicklepod even though they are not eaten.  Mowing the pasture when sicklepod is about to flower will eliminate or greatly retard seed production (Anning et al. 1989).

Ecology

Origin and Distribution: Sicklepod is native to tropical and subtropical parts of South and North America (Irwin and Barnaby 1982).  It is widely introduced in warmer parts of Africa, Asia and Australia.  In the U.S.A. it is common throughout the southeast as far north as Tennessee and Virginia, and occurs sporadically northward to Wisconsin and New York, westward to Nebraska, and in southern California (USDA Plants). It is most problematic in the
southeast where high day and night temperatures prevail in mid-summer (Sosnoskie et al. 2021, Teem et al. 1980).

Seed Weight:  23-28 mg (Clay and Griffin 2000).

Dormancy and germination: Sicklepod seeds have a hard, waxy seed coat that prevents absorption of water, thereby preventing germination of most seeds.  On average, only about 10% of seeds with intact seed coats will germinate (Retzinger 1984, Creel et al. 1968), but germinability varies. Damage to the seedcoat promotes germination (Sosnoskie et al. 2021). In one experiment, incubation of the seeds in wet sand for 12 months increased germination from 5 to 15%, indicating that the seed coat slowly breaks down over time (Creel et al. 1968). Fire cracks the seed coat and can prompt high germination following the next rain (Anning et al. 1989).  Scarified seeds germinate well from 68-97 °F (20-36 °C), but poorly below 59 °F (15 °C) or above 104 °F (40 °C) (Creel et al. 1968, Norsworthy and Oliveira 2006, Teem et al. 1980, Wright et al. 1999). Development of a soybean canopy greatly reduced mid- to late-season emergence of sicklepod seedlings, possibly due to lower soil temperatures or decreased temperature fluctuations under the crop canopy (Norsworthy 2004). Rapid seedling emergence occurs at temperatures from 81-97 °F (27-36 °C) (Creel et al. 1968, Teem et al. 1980). At high
temperature, sicklepod will germinate well under moisture stress, conditions that frequently occur in the southeastern states (Sosnoskie et al. 2021). Light does not affect germination if the seed coat is damaged, but light may further inhibit germination of intact seeds (Norsworthy and Oliveira 2006). Seeds germinate best at pH 5-8, but a few will germinate at pH 3 and 9.

Seed longevity:  Due to its hard seed, sicklepod forms a persistent seedbank, and as much as 270 lb/A (300 kg/ha) of seeds have been recovered from soil samples (Anning et al. 1989).  Sicklepod seeds had 46% mortality over a 2-year period when soil was tilled annually in late fall and early spring, but only 28% mortality when the soil was undisturbed (Bararpour and Oliver 1998).  Seed mortality was computed to be 40% per year based on seed burial studies for 5.5 years in Mississippi (Egley and Chandler 1983).  Annual spring tillage followed by repeated disking through the summer depleted the seedbank more rapidly than annual fall tillage followed by a chemical fallow without soil disturbance during the summer (Bridges and Walker 1985).

Season of emergence:  In Arkansas, seedlings emerge from late May to August (Bararpour and Oliver 1998).

Emergence depth:  Sicklepod emerges relatively well from below the planting depth of most crops.  In one study, emergence was high down to 3” (7.6 cm) and about 15% of seeds buried at 5” (13 cm) produced seedlings.  Emergence was faster, however, from shallow depths (Teem et al. 1980).  In another study seedling emergence was high down to 1.5” (4 cm), and a few seedlings emerged from 4” (10 cm), but the emergence response to burial depth varied with soil texture (Norsworthy and Oliviera 2006).  

Photosynthetic pathway: C3.

Sensitivity to frost:  Day/night temperatures of 84/70 °F (29/21 °C) or lower inhibit growth of sicklepod relative to higher temperatures, and growth ceases at mean daily temperatures below 57 °F (14 °C) (Patterson 1993, Wright et al. 1999).

Drought tolerance:  Sicklepod is able to germinate at low moisture potential.  Although initial root elongation is slowed by low moisture (Hoveland and Buchanan 1973), enough growth occurs under drought conditions to allow roots to grow into deeper soil layers with more moisture.  Sicklepod tends to have a higher root:shoot ratio than soybean, which probably gives it a competitive advantage under drought conditions (Wright et al. 1999).  Several physiological characteristics related to photosynthesis and transpiration are intermediate compared to other C3 weeds (Patterson and Flint 1983).

Mycorrhiza:  Sicklepod is mycorrhizal (Tungate et al. 2007).

Response to fertility:  Although sicklepod is a legume it does not host nitrogen-fixing bacteria (Sosnoskie et al. 2021).  Application of 200 lb/A (224 kg/ha) of N to an N deficient soil increased dry weight of sicklepod by 59% and seed production by 114%.  Seeds produced under low N fertility were smaller and grew into less competive plants than seeds produced under high N (Tungate et al. 2006).  Its response to N-P-K fertility is greater than soybean and about equal to cotton (Creel et al. 1968).  Although sicklepod growth is somewhat reduced at low and high pH, it grows reasonably well at soil pH as low as 3.2 or as high as 7.9 (Creel et al. 1968, Buchanan et al. 1975). 

Soil physical requirements:  The species commonly occurs in soil with textures that range from loams to gravels.  It seems particularly associated with coarse soils like sandy agricultural fields and the gravel of railroad ballast (Hilty 2010, Norsworthy and Oliveira 2006).  It is highly tolerant of compacted layers in the soil (Place et al. 2008).

Response to shade:  Partial shading of 47 or 65% decreased dry weight of sicklepod, but induced plant height to increase by 32%, showing that this species can often grow out from under a partial crop canopy.  Shade of 80% reduced dry weight more than 75% and 95% shade suppressed sicklepod almost completely (Nice et al. 2001).

Sensitivity to disturbance:  If sicklepod plants in a subtropical climate are cut, they may persist as short lived perennials (Mackey et al. 1997).

Time from emergence to reproduction:  Flowering is hastened when days are shorter than 12 hours (Mackey et al. 1997).  Flowering occurs 6-12 weeks after emergence, with northern populations (e.g. Tennessee) flowering more quickly than southern populations (e.g. Florida) (Retzinger 1984).

Pollination: Although the flowers are heavily visited by bees (Retzinger 1984), self pollination usually occurs before the flowers open (Irwin and Barneby 1982).

Reproduction:  In a comparison of populations grown with minimal competition from across the South, the mean number of seeds per pod was 26 and varied little between populations and years (Retzinger 1984).  The number of pods per plant varied from 63-591 with substantial variation between years and with more pods produced by southern (e.g., Louisiana, Florida) than northern (e.g., Tennessee, North Carolina) populations.  Consequently, number of seeds per plant varied from 1,500-16,000 (Retzinger 1984).  Sicklepod grown without interference produced 11,420 seeds per plant in Arkansas (Bararpour and Oliver 1998).  Plants growing in conventionally tilled soybeans in Alabama averaged 176 pods per plant (Bridges and Liker 1985).  From 8 to 62% of seeds were shattered by the time of soybean harvest (Schwartz-Lazaro et al. 2021).

Dispersal:  The pods can throw seeds up to 16 ft (5 m) as they open (Mackey et al. 1997).  Seeds are carried by stream water, overland flow and in mud attached to the feet and fur of animals. They also move in contaminated mulch and in mud on machinery, vehicles and footwear (Mackey et al. 1997).  Although the species is generally unpalatable, livestock nibble on the pods, and seeds will pass through the animals (Anning et al. 1989) and disperse when the animals are moved.

Common natural enemies:  Sicklepod seeds are attacked by the beetle, Sennius fallax.  Caterpillars of several species of sulphur butterflies feed on the foliage, including little supgur (Eurema lisa), sleepy orange (Eurema nicippe), and cloudless sulfur (Phoebis sennae cubule) (Hilty 2010).  Alternaria cassiae causes seedling blight in sicklepod and is a potential mycoherbicide (Cock and Evens 1984, Walker and Riley 1982).  A mixture of spores of the anthracnose fungi Colletotrichum truncatum and C. Gloeosporioides in corn oil and a surfactant effectively controls sicklepod (Boyette and Hoagland 2010). See also Sosnoskie et al. (2021).

Palatability:  Sicklepod is cultivated in Africa for the young shoots, which are eaten as a pot herb (Bosch 2004).  Sicklepod is unpalatable to livestock (Anning et.al. 1989), and can cause poisoning (Mackey et al. 1997).  The seeds are toxic and pose a serious threat to chickens and pigs when they contaminate feed (Burrrows and Tyrl 2006). See also Sosnoskie et al. (2021).

Notes:  Sicklepod residue inhibits the germination and seedling growth of cotton and other crops (Creel et al. 1968­).

References:

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  • Tungate, K. D., M. G. Burton, D. J. Susko, S. M. Sermons, and T. W. Rufty.  2006.  Altered weed reproduction and maternal effects under low-nitrogen fertility.  Weed Science 54:847-853.
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