Catchweed bedstraws and false cleavers

Images above: Upper left: Catchweed bedstraw seedling (Antonio DiTommaso, Cornell University). Upper right: Catchweed bedstraw prickles on stem and fruit (Anurag Agrawal, Cornell University). Bottom: Catchweed bedstraw leaves (Scott Morris, Cornell University).

Identification

Other common names:

• Catchweed bedstraw:  cleavers, goose grass, spring-cleavers, scratch-grass, grip-grass, catch-weed, bedstraw, white hedge, valiant's cleavers
• False cleavers:  stickwilly, cleavers.

Family:  madder family, Rubiaceae

Habit:  Lax summer or winter annual herbs, typically scrambling over other plants.

Description:  The two species are very similar in appearance.   The seedling stem is green with purple or brown splotches.  The first true leaves are elliptical or narrow, spine-tipped and arranged in spoke-like whorls of four or more.   Cotyledons are egg shaped with notched tips and long stalks.  The upper cotyledon surface is green and hairy.  The leaf surface, margins and midvein on the underside have backwards curving, stiff hairs.

• Catchweed bedstraw:  Cotyledons are egg-shaped, 0.24-1.25” (0.6-3.2 cm) long by 0.2-0.5” (0.5-1.3 cm) wide.
• False cleavers:  Cotyledons are 0.2-0.4” (0.5-1 cm) long by 0.08-0.16” (0.2-0.4 cm) wide.

Mature plants have four-sided square, jointed stems that branch near the base, and have backward curving bristles clustered near the leaf whorls that cause the weed to cling to neighboring objects.  Leaves are in whorls of usually 8 but sometimes 6.  Leaves have a strong central vein, and have an indistinct stalk.  Leaf edges and midveins are rough and leaf tips are prickly.  Roots are fibrous. 

• Catchweed bedstraw:  Plants form loose, sprawling mats.  Stems grow up to 5 ft (1.5 m) long.  The base of the stem bristles is round.  Leaves are green, oval to lance shaped. and 0.5-3” (1.3-7.6 cm) long by 0.1” (0.25 cm) wide.
• False cleavers:  Plants are stiffer, rougher, and more branched than catchweed bedstraw, and have stems up to 6.6 ft (2 m) long.  The base of the stem bristles is flattened.  Leaves are linear, yellowish, and are 0.5-2.6” (1.3-6.6 cm) long by 0.1-0.24” (0.25-0.6 cm) wide.

Individually stalked flowers are grouped in clusters arising from leaf axils.  Flowers are four-petaled.  Fruits are split into two identical parts, each containing one seed.  The dry fruit (the apparent seed) is gray-brown, a rounded kidney shape, and is usually covered in short hooked hairs that cling to clothing and fur. 

• Catchweed bedstraw:  Flowers are white and 0.08” (0.2 cm) in diameter.  Seeds are 0.08-0.16” (0.2-0.4 cm) long.
• False cleavers:  Flowers are pale yellow to yellow-green but occasionally white and are 0.04-0.06” (0.1-0.15 cm) in diameter.  Seeds are 0.1” (0.25 cm) long.

Similar species:  Smooth bedstraw (Galium mollugo L.) is a rhizomatous perennial weed that largely lacks hairs except for the rough leaf margins.  It has smaller leaves [0.4-1.1” (1-2.8 cm) long by 0.08-0.16” (0.2-0.4 cm) wide] than catchweed bedstraw.  Many other, non-weedy species of bedstraw are present in most regions of North America.  Carpetweed (Mollugo verticillata L.) has smooth leaves and stems, lacks square stems, and has leaves that vary in size and shape within each whorl.

Taxonomic note:  These two species are difficult to distinguish morphologically and their weedy behavior is considered similar by farmers and weed professionals (Deroo et al. 2019, Malik and Vanden Born 1988, Reid and Van Acker 2005).  However, there are sufficient differences in flower and fruit size, chromosome number, and molecular markers to justify classification as separate species (Deroo et al. 2019, Malik and Vanden Born 1988).  Most of the research that supports the descriptions presented in this chapter was conducted on catchweed bedstraw, and will be identified as such, but should be understood as generally applicable to false cleavers as well.  Where research specifically distinguished between these species, this will be noted explicitly in the section under discussion.

Management

Deep moldboard plowing (8-10” = 20-25 cm) can help suppress populations of these species.  After plowing, seeds will be buried too deeply to emerge during the current season, and since the seeds usually die off quickly, most will be gone before they return to the surface with subsequent tillage.  If you wish to avoid annual moldboard plowing, use this practice to bury seeds the spring after years with heavy seed production; then use conservation tillage practices in following years.  This will allow extra years for the buried seeds to die.   Because the seeds do not survive long in the soil, rotation into a hay crop like alfalfa can also greatly decrease seed populations (Benaragama et al.2022, Ominski et al. 1999).

Since catchweed bedstraw responds strongly to P (Blackshaw et al. 2004), avoid accumulating excessively high P levels in the soil.  If P levels are higher than necessary for good crop production due to regular use of manure or compost as an N source, increase use of legume cover crops and decrease applications of compost or manure. 

Cultivation with a harrow or tine cultivator tends to increase germination of both species (Reid and Van Acker 2005) and can be used to help flush out seedlings that are then killed during preparation of a seedbed.  Because seedlings often emerge from two or more inches, rotary hoeing is relatively ineffective, and tine weeding should be aimed at burial of seedlings just after emergence.  In cereal grains a tine weeder can be used to comb these weeds out of the crop just prior to stem elongation of the grain (Wilson et al. 1993).  When soil is dry, set the weeder to just graze the soil surface; although much of the plant will be left, the practice can shift the competitive balance in favor of the crop.

Since most seeds are retained on the plant at harvest of spring-planted crops (Burton et al. 2016, 2017; Tidemann et al. 2017), equipment designed to harvest and destroy weed seeds during crop harvesting operations will effectively reduce populations of these species.

Ecology

Origin and distribution:  Although catchweed bedstraw is native to coastal areas of eastern and western North America to the Aleutian Islands, the weedy races in agricultural fields appear to have been introduced from Europe.  It presently occurs in most of the U.S.A. and southern Canada.  It also is native to Eurasia and has been introduced into temperate areas worldwide (Defelice 2002, Holm et al. 1977, Malik and Vanden Born 1988, Taylor 1999).  False cleavers is native to Europe, and is widespread in Asia, Africa, and North America.  In North America, it occurs throughout the U.S.A. southern Canada and into Alaska (Deroo et al. 2019, Malik and Vanden Born 1988).  Molecular analysis reveals that the predominant Galium species found in western Canadian fields is false cleavers and that these populations are closely related to each other and to European populations of the same species (Deroo et al. 2019).  

Seed weight:  Catchweed bedstraw: 3-6 mg  (Malik and Vanden Born 1988), 3.7 mg (from shade-tolerant woodland plants) to 7.3 mg (for hedgerow plants) (Taylor 1999), 4.6-8.5 mg (Royo-Esnal et al. 2012), 6.6 mg (Gaba et al. 2019), 6.7 mg (Milberg et al. 2000), 7.4 mg (Gardarin et al. 2010), 9.1 mg (from summer grown plants) to 10.7 mg (from winter grown plants)  (Mennan and Ngouajio 2006), 11.41 mg (Benvenuti et al. 2001), 16.9 mg (Lutman et al. 2002).  False cleavers: 1.6-2.0 mg (Royo-Esnal et al. 2012), 2.8 mg (Malik and Vanden Born 1988).

Dormancy and germination:  Germination behavior varies greatly depending on the local race, habitat, and time of year the seeds are produced. Many, and sometimes all, of the seeds produced in agricultural fields are capable of germination immediately after falling from the plant (Taylor 1999).  Seeds germinate in darkness just as well as in alternating light/dark conditions (Cussans et al. 1996). Unlike most weeds, light inhibits germination, and exposure to light induces dormancy (Malik and Vanden Born 1988, Milberg et al. 2000).  In some races drying leads to dormancy.  In others, dry seeds lose dormancy with time (Malik and Vanden Born 1988).  Cold, wet conditions will break dormancy (Slade and Causton 1979) as will nitrates (Malik and Vanden Born 1988).  Optimum temperature for germination of catchweed bedstraw varies considerably with country of origin from 33-68 °F (0.5-20 °C) (Malik and Vanden Born 1988).  In a comparison of Canadian populations, catchweed bedstraw had a higher base temperature for initiating germination than false cleavers, 39 °F versus 36 °F (4 °C versus 2 °C), but all populations achieved maximum germination at approximately 50°F (10 °C) (De Roo et al. 2022).  Catchweed bedstraw seeds produced in agricultural settings in Great Britain germinated at temperatures of 41-59 °F (5-15 °C) with optimum germination at 48-54 °F (9-12 °C) (Taylor 1999).  False cleavers had an optimum temperature requirement of either a constant 72 °F (22 °C) or alternating daily temperatures between 50 and 75 °F (10 and 24 °C) (Malik and Vanden Born 1988).  Germination of catchweed bedstraw was optimum at 40-60% soil moisture holding capacity, whereas false cleavers required a slightly higher optimum of 50-80% (Malik and Vanden Born 1988).

Seed longevity:  Few seeds of catchweed bedstraw last longer than 2 years in the soil (Barralis and Chadoeuf 1988, Malik and Vanden Born 1988, Taylor 1999).  In two six-year experiments in tilled grain fields, catchweed bedstraw seeds declined at 51% and 65% per year (Lutman et al. 2002).  In France, the annual mortality rate was 41% in untilled soil (Gardarin 2010).  One study showed the usual pattern of increasing survival with depth (Jensen 2009).

Season of emergence: Catchweed bedstraw exhibits a wide range of potential emergence periods, primarily in the fall and spring but sometimes in summer as well as in mid-winter in warmer regions (Malik and Vanden Born 1988, Taylor 1999).  Populations of false cleavers in western Canada were considered facultative winter annuals that could emerge in both fall and spring, but significant rainfall and moist soils were usually required for emergence (De Roo et al. 2022).  A dry autumn could delay peak emergence until spring (Royo-Esnal et al. 2010).  Emergence patterns of false cleavers was generally similar to that of catchweed bedstraw, except that false cleavers was more sensitive to dry soil conditions than catchweed bedstraw, which is adapted to a wider window of soil moisture conditions (Royo-Esnal et al. 2010).  Once emerged, early growth rate of several populations of both species responded similarly to temperature and moisture conditions (Royo-Esnal et al. 2012).  Seeds from two populations were collected in Turkey.  In one population, seeds were collected in June and seedlings from these seeds had two peak emergence periods per year, one in November and a second in April to June.  Seeds from the second population were collected in September and seedlings from these had one peak emergence period in June to August (Mennan and Ngouajio 2006).

Emergence depth:  Seedlings emerge best from anywhere in the top 2.4” (6 cm) of soil but many can emerge from 4” (10 cm) or more (Barralis et al. 1988, Benvenuti et al. 2001, Cussans et al. 1996, Froud-Williams et al. 1984, Malik and Vanden Born 1988, Mennan and Ngouajio 2006).  Emergence from 0.8 to 2.4” (2 to 6 cm) depth was higher than from the soil surface, partly because emergence of both species is enhanced by uniform soil moisture and declines with intermittent drying conditions as occurs at the soil surface (Malik and Vanden Born 1988, Reid and Van Acker 2005).

Photosynthetic pathway:  C3

Sensitivity to frost:  Before flowering begins the plants are very cold hardy and one study showed they tolerate temperatures of at least -13 °F (-25 °C) (Malik and Vanden Born 1988), though another showed severe damage or death at 14 °F (-10 °C) or lower (Cici and Van Acker 2011).  The threshold temperature for frost tolerance appears to fluctuate with the season, decreasing from 19 °F (-7 °C) in October to 1 °F (-17 °C) in December and then increasing to 28 °F (-2 °C) by April (Taylor 1999).  After flowering begins, the plants are killed by a hard frost (Malik and Vanden Born 1988).

Drought tolerance:  Catchweed bedstraw thrives in moist habitats and is not drought tolerant.  One report indicated that false cleavers plants are more drought tolerant than catchweed bedstraw, but both species are rare in areas with low summer rainfall (Malik and Vanden Born 1988).

Mycorrhiza:  Both presence and absence of VA mycorrhiza have been reported (Harley and Harley 1987, Pendleton and Smith 1983, Taylor 1999).

Response to fertility:  Catchweed bedstraw is highly responsive to both N and P and its dry matter production continues to increase up to very high P levels (Blackshaw et al. 2003, Blackshaw et al. 2004).  It responds most, however, when N and P are both applied (Pigott and Taylor 1964, Taylor 1999).  Increasing N fertility rates increases this weed's competitiveness with wheat.  It grows best at pH from 5.5-8.0 (Taylor 1999).

Soil physical requirements:  Both species thrive in a wide range of soil textures, but prefer moist, fertile soils (Defelice 2002, Malik and Vanden Born 1988).

Response to shade:  Catchweed bedstraw grows best in open conditions, but can survive and reproduce in woodland habitats. It scrambles up onto crops by means of the prickles on the stems and leaves, thereby avoiding shade (Defelice 2002, Malik and Vanden Born 1988, Taylor 1999).  This weed is highly adaptable to shade conditions by lowering respiration sufficiently to maintain a positive carbon balance (Taylor 1999).  Plants produce more leaves at the expense of root production under shade conditions (van der Weide 1992).  Shade of an individual branch leads to death of the shaded branch with little shift in the growth pattern of other branches (Kemball et al. 1992).

Sensitivity to disturbance:  Uprooted plants readily re-root on moist soil.  Little regrowth or seed production occurs if stems are removed during or after flowering (Mohler personal observation).

Time from emergence to reproduction:  Plants establishing in late summer and fall overwinter and flower from May to September and set seed from June to October (Malik and Vanden Born 1988, Taylor 1999).  Plants establishing in the spring begin flowering about 6 weeks later (Doll 2002).  Generally, as the combination of temperature and day length increase, the number of days between emergence and flowering decreases (van der Weide 1992).  Seeds mature about one month after the flower opens (Malik and Vanden Born 1988).

Pollination:  The flower structure normally ensures self-pollination (Taylor 1999), but a wide variety of insects visit the flowers (Batra 1984, Malik and Vanden Born 1988) and occasional cross pollination may occur.

Reproduction:  Plants typically produce 300-400 seeds but under favorable conditions may produce more than 1,500 seeds (Malik and Vanden Born 1988).  When subject to competition from a vigorous grain crop, seed production is as low as 9 seeds per plant in one study.  Seed retention at spring wheat harvest in western Canada was variable ranging from 62% (Tidemann et al. 2017) to 95% (Burton et al. 2016), and 90% of seeds were situated at a height on the plant that could be harvested by combine (Tidemann et al. 2017).

Dispersal:  The hooked hairs on the fruit coat help the seeds disperse on fur and clothing.  Seeds are a persistent contaminant of crop seed, particularly canola in the Canadian prairies, and have been spread by humans worldwide.  These species also commonly move in straw from infested crops and in combines.  The fruits float and disperse along streams and irrigation canals.  The seeds pass through livestock without loss of viability and are dispersed in droppings and with the spread of manure.  (Defelice 2002, Malik and Vanden Born 1988)

Common natural enemies:  Larvae of the sawfly Halidamia affinis, the mirids Criocoris piceicornis, Polymerus nigritus, and P. unifasciatus, and the gall forming aphids Aphis galiiscabri and Galiomum langei are restricted to Galium species (Batra 1984).  Harvester ants (Messor barbarus) consumed 49% of catchweed bedstraw seeds on the soil surface between grain harvest and fall in Spain (Baraibar et al. 2017), but the rate of seed predation in North American fields is unknown.  Catchweed bedstraw was one of only two out of 42 weed species not consumed by ground beetles (Gaba et al. 2019).

Palatability:  Although catchweed bedstraw has been used as a potherb, it is also considered to be poisonous and unsuitable for people or livestock (Defelice 2002).  The plant is very coarse, and consumption can cause a low-level poisoning (Malik and Vanden Born 1988).  It is preferred by geese and can be used as a poultry feed (Defelice 2002, Jones 1991, Malik and Vanden Born 1988).  A coffee substitute can be made from seeds (Defelice 2002, Malik and Vanden Born 1988).

References:

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