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White mustard mulch is ineffective in weed control

Sirkka Jaakkola

MTT Agrifood Research Finland, Plant Production Research, Plant Protection, FIN-31600 Jokioinen, Finland.


Incorporation of allelopathic cruciferous tissues into the soil can suppress weeds and soilborne pests. Biomass production of early sown white mustard, Sinapis alba L., was evaluated and application rates determined for its use as green manure to control weeds in peas, Pisum sativum L. and spinach, Spinacia oleracea L. White mustard yielded 1260-1590 g m-2 shoots and 148-154 g m-2 roots in June, corresponding to 0.2% dry soil weight. When pea was drilled one day after mustard incorporation, emergence of peas was substantially decreased. The amount of white mustard green manure incorporated did not affect weed numbers or biomass. When mustard was incorporated at 2.5 kg m-2 and 5.0 kg m-2 in the greenhouse, spinach emergence was decreased by over 90%, but scentless mayweed (Tripleurospermum inodorum L.) by only 63% and 67%. There was no inhibition of emergence when spinach and mayweed were sown two weeks later. In growth chamber experiments, dried mustard shoots were incorporated at rates of 0.5, 1.0, 2.0 and 4.0% of dry soil and imbibed or dry seeds of scentless mayweed were sown immediately afterwards. White mustard delayed emergence at all rates of incorporation but was more toxic to imbibed than to dry seeds. Emergence decreased by 90% only at application rates above 2%. However, biomass of scentless mayweed decreased by over 50% at 1% incorporation (equivalent to 6.9 kg fresh biomass m–2). It is concluded that control of annual weeds with white mustard green manure needs more biomass than can be produced in situ. White mustard appears to be more toxic to spinach and pea than to scentless mayweed and other annual weeds.

Media summary

Early summer white mustard green manure was used in an effort to control weeds in spinach and pea. It was more toxic to these crops than to weeds.


Allelopathy, green manure, glucosinolate, Sinapis alba, spinach, pea


Weeds constitute a major problem in organic vegetable growing. Generally weed management in organic farming is more critical than in conventional farming, and combinations of methods that prevent weed germination and control weeds must be used (Rasmussen 2004). Weeds can decrease yield and reduce the value of crops such as spinach and garden pea for the food processing industry. No weeds are allowed within harvested vegetables for frozen food. Farmers use a false seedbed technique to control weeds before sowing peas and spinach. The soil is cultivated sufficiently early to allow weed seeds to germinate before sowing the crop. Harrowing or cultivating before sowing then eliminates emerged weeds. Alternative management methods are needed however.

Incorporation of crucifer green manure into the soil suppresses weeds, soil-borne pathogens and pests (Brown and Morra 1997). The ‘biofumigant’ properties of crucifer tissues are suggested to be highly toxic isothiocyananates (ITC), and mildly toxic non-glucosinolate S-containing compounds released from tissues damaged during incorporated into the soil (Bending and Lincoln 1999). ITCs are the major products of hydrolysis of glucosinolates that are released when myrosinase, a degradative enzyme comes into contact with glucosinolates in damaged tissues. In addition to ITCs, other less toxic breakdown products of glucosinolates (e.g. nitriles, thiocyanates, and oxazolidinethiones) can occur, depending on various factors (Bones and Rossiter 1996). Myrosinase-glucosinolate system in cruciferous plants is involved in defence against insects and phytopathogens, in sulphur and nitrogen supply and growth regulation (Bones and Rossiter 1996). Brassica juncea L., B. nigra L. and S. alba are among the crucifers that yield the most isothiocyanates (Kirkegaard and Sarwar 1998). Petersen et al. (2001) noted that isothiocyanates released from Brassica rapa L. var rapa ssp. oleifera mulch strongly suppressed germination of five common weed species. Petersen et al. (2001) estimated that turnip rape yielded about 0.5 g m-2 of isothiocyanates, when about 540 g m-2 (dw) shoots and 66 g m-2 roots were incorporated into the soil. The amount is about 20% of the ITC concentration released from the commercial fumigant, metham sodium applied at the label rate of 450 kg ha-1. White mustard seed meal and green tissues were shown to be especially toxic to emerged seeds (Vaughn and Boydston 1997; Laitinen et al. 1994). Using early summer white mustard as green manure could represent an alternative measure to suppress weed germination when incorporated before sowing vegetable seed.

Experiments were carried out in the field, in greenhouse and growth chamber in Finland in 2002-2005 to quantify the biomass of early summer white mustard and to evaluate the effect of its incorporation on weeds and crops.

Materials and methods

Field experiments in 2003

Two field experiments were conducted to evaluate biomass production of early sown white mustard and its effect on weeds and crops. Experiments were carried at the experimental farm of MTT Agrifood Research Finland. Experiment 1 was done in a conventional field in Jokioinen and experiment 2 in an organic field in Kokemäki. The experimental layout was a randomised complete block with four replicates in both cases. Plots with and without white mustard were included. The experiments had the same amount of incorporated white mustard biomass, but there were time differences in incorporation of the mustard and sowing the crop. White mustard (cv. “Jo03”, germination 70%) was drilled at 880 seeds m-2 (52 kg ha-1) at a row width of 12.5 cm into sandy soil in May. Experiment 1 also included an oat (Avena sativa L. cv. “Freja”) green manure drilled at 100 seeds m-2 (35 kg ha-1). Plots were 1.25 m wide and 8 m long. Soil was fertilized with 80 kg N, 12 kg P and 32 kg K ha-1 and micronutrients in experiment 1, and with organic fertilizer of 40 kg N, 10 kg P and 20 kg K ha-1 in experiment 2. Insecticide was sprayed against flea beetles twice in experiment 1. White mustard was chopped at flowering at the end of June with a hay chopper and incorporated into the topsoil at 10 cm depth with a rotary cultivator. Incorporated m-2 fresh biomass of shoots was 1.6 kg m-2 and roots 0.15 kg m-2. At the same time 2.6 kg m-2 oat shoots and 0.9 kg m-2 roots were incorporated. Spinach (cv. “Laska”, germination 90%) and garden pea (cv. “Avola”, germination 97%) were sown five days after incorporation at 400 and 121 seeds m-2 in experiment 2. Experiment 1 included field pea (cv. “Perttu”) sown at 167 seeds m-2 one day after mustard and oat incorporation.

Total fresh biomass was recorded. A 1.0 m-2 sample of white mustard shoots and a sample of 0.25 m-2 of roots from each plot was weighed and returned to the plots. For dry matter content determination, four samples of 1.0 kg shoots and 100 g roots were collected from the margins in experiment 2. Roots were washed and all parts were dried at 70°C and weighed. Emerged white mustard plants were counted three weeks after sowing. Three 1m inner rows of each plot were assessed. Emerged weeds and crop plants were counted three times at 2 x 0.25 m-2 from each plot. Weeds were collected and dry biomass was weighed at 2 x 0.25 m-2. Spinach was harvested from 2 x 0.5 m-2. Pea was not harvested as it did not ripen.

Greenhouse experiment in 2002

A greenhouse experiment was conducted between February and May 2003 in raised beds 1m-2 and 9 cm high. White mustard was sown into fertilized soil (N 6 g, P 5 g and K 20 g m-2 and micronutrients) at mid February at 25 kg and 35 kg ha-1. Plots without mustard served as controls. At flowering mustard shoots and roots were chopped into 1 cm pieces and incorporated into the soil at 2.5 kg m-2 (0.4% kg-1 of dry soil) or 5.0 kg m-2 (0.8% kg-1 of dry soil). Spinach (cv. “Cézanne” F1, germination 92%) was sown in beds one day or with 14 days after incorporation in four rows at 140 seeds m-1 and scentless mayweed was sown in three rows between the spinach at 20 seeds m-1. Ambient temperature was kept at 18±5 °C. The experimental design was an incomplete block with three replicates of 2.5 kg m-2 and with two replicates of 5.0 kg m-2 and an untreated replicate.

Emergence of plants in each row was recorded four times in two weeks. Spinach was not harvested because of severe aphid infestation.

Dose response experiments in a growth chamber in 2004-2005

Dried white mustard green biomass was incorporated into sandy soil at 0, 0.5, 1.0, 2.0 and 4.0% of soil dry weight. Soil was then wetted to 20% and potted. Each pot weighed about 115 g. Five imbibed or dry seeds of scentless mayweed were sown in pots in the three first experiments and 20 seeds in the following three experiments. Seeds were imbibed in water agar plates in a growth chamber for two days.

Pots were kept under a day/night regime of 16/8 hr at 18/15 °C in a growth chamber (Arc Test CKS-2000). The pots were watered when necessary. The emergence was recorded daily and after 7-10 days one seedling was left in each pot. Seedlings were allowed to grow for one week more until they were cut and weighed. Experiments were completely randomized with six replicates. Imbibed and dry seed trials were repeated three times. Seedlings were considered as germinated when cotyledons were open.

Plant material and soil

White mustard (cv. “Jo03”) was grown in the greenhouse in fertilized garden peat. Plants were maintained under a day/night regime of 16/8 hr under high-pressure sodium lamps (Philips Green Power 400 W) at 20/18 °C. Shoots were harvested at flowering, oven-dried at 50 °C and milled. The green biomass was kept in sealed glass jars. Seeds of scentless mayweed were collected in autumn 1999. Before use, seeds were sterilized in sodium hypochlorite solution, rinsed with distilled water and dried. Germination was over 95%.

For greenhouse and growth chamber experiments soil (40% coarse sand, 30% fine sand, pH 6.6 and org C 2.19%) was collected from the top 20 cm of the field of experiment 1. The soil was stored at 4 °C and sieved before use.

Table 1. Equivalence of dry and fresh green biomass (dry matter 16%) incorporated into 10 cm deep soil (A soil bulk density of 1.1 g cm-3).

Dry mass % of dry soil

Fresh biomass kg m-2











Statistical analysis

Analysis of field data was done with the Mixed Models procedure of SAS V8 statistical package (SAS Institute 1999). The UNIVARIATE procedure of SAS was used to check that the data were normally distributed before analysis. Field weed data was log transformed to achieve homogeneity of variances and normal distributions.


The biomass of spring sown white mustard

White mustard emerged evenly. There were 97-105 plants in one meter. In experiment 1, flea beetles damaged plants despite spraying. White mustard produced fresh shoots 1260 g m-2 and 148 g m-2 roots in experiment 1 and 1590 g m-2 (dry weight 16%) shoots and 154 g m-2 (dry weight 21%) roots in experiment 2. Shoot biomass accounted for 89% and 91% of total biomass. The incorporated fresh biomass in experiment 2 was equivalent to 0.23% (10 cm upper layer) of dry soil.

Effect of dose and time between incorporation and sowing on weeds and crops

In field experiments white mustard was applied at 1.6 kg m-2 and crop seeds were drilled the next day (experiment 1) or five days afterwards (experiment 2). When drilled soon after white mustard incorporation emergence and growth of pea was severely inhibited. Oat green manure also inhibited emergence of pea. There was a significant difference in plant number between plots with and without white mustard. In untreated plots there were 8.9 seedlings m-1, in plots with white mustard 4.1 seedlings m-1 and in oat plots 3.6 seedlings m-1. Seedlings in treated plots were small and had only few branches.

In experiment 1 about 295 weeds m-2 emerged in June. However, after mustard incorporation only few weeds emerged and there were no differences between treatments. In experiment 2 many weeds emerged, but no effect was recorded between treated and untreated plots (Table 2). There were no significant differences in the numbers of spinach seedlings, yield and weed biomass.

Table 2. Number of weeds m-2 in pea and spinach plots after white mustard incorporation.

Greenhouse experiment

Incorporation of fresh, chopped white mustard at 2.5 kg m-2 (0.8%) or 5.0 kg m-2 (1.6%) reduced the emergence of spinach and scentless mayweed substantially (Figure 1) when sown one day after mustard incorporation. No toxic symptoms were noted when spinach and scentless mayweed were sown two weeks after incorporation.

Figure 1.Effect of incorporated white mustard green manure on the emergence of spinach (a) and scentless mayweed (b).

Dose-response trials

The emergence of scentless mayweed was decreased at all rates of white mustard incorporation (Fig. 2). Imbibed seeds were more susceptible than dry seeds. High rates of incorporation were needed to decrease the emergence by over 90%. In four experiments germinated seeds responded clearly to white mustard biomass incorporation, but in two trials only minimally. White mustard reduced the weed biomass by 50% at a rate of 0.5% when seeds were imbibed and at rate of 1.0% when seeds were not imbibed. A rate of 2% was needed to decrease the biomass by over 90% (Fig 3). Seedlings grown in soil at high rates of mustard incorporation were pale and had brown, dead primary roots. The soil surface in these pots was often mouldy.

All rates of white mustard incorporation delayed emergence of scentless mayweed when compared with the untreated control. Germination of imbibed seeds of scentless mayweed began about 2 days later than for dry seeds. At high rates of white mustard incorporation some seeds emerged after 10 days, when plants were already thinned.

a) Imbibed seeds

b) Dry seeds

Figure 2. Effect of white mustard green manure on emergence of imbibed (a) and dry (b) seeds of scentless mayweed in three experiments.

a) Imbibed seeds

b) Dry seeds

Figure 3. Effect of mustard green manure on the biomass of scentless mayweed grown from imbibed (a) and dry (b) seeds in three experiments.


Production of toxic compounds of white mustard depends on biomass and concentration (Kirkegaard and Sarwar 1998). Glucosinolate concentration is known to vary with plant ontogeny (Smith and Griffiths 1988). Above ground glucosinolates in vegetative material usually reach a maximum around flowering (Fieldsend and Milford 1994). Daylength and temperature also influence growth of white mustard (Schuster and Klein 1978). Under northern long day conditions white mustard has a short vegetative period and thus low biomass production. In these experiments white mustard flowered in 48-50 days . The shoot biomass of white mustard ranged from 202 g m2 (dw) to 254 g m2 and root biomass from 31 to 32 g m2. In our experiments 97-105 plants emerged in one meter (800 plants m-2). Kirkegaard and Sarwar (1998) planted 100 plants m-2 and harvested 140-387 g m2 shoot and 30-78 g m-2 roots after 125-126 days at mid-flowering.

In greenhouse experiment 2.5 kg m-2 fresh biomass quantity decreased spinach emergence for > 90%, but over 5.0 kg m-2 was needed to weed suppression for 70 % . In bioassays, the dry biomass needed to decrease emergence for 70 % was 2-4 % of soil dry weight, which is equivalent to 13.7-27.5 kg m-2 . However, growth of scentless mayweed was retarded much already at rate of 1 %. The toxicity of dried, milled cruciferous plants has been greater than of fresh chopped plants (Ramirez-Villapudua and Munnece 1987; Angus et al. 1994). This depends on faster liberating rate and thus higher concentrations. The difference in our experiments is probably because some toxic compounds are lost when plant material is dried, milled and stored. In the field and in greenhouse white mustard roots were also incorporated, which could increase toxic effects. Gardiner et al. (1999) suggested that roots of brassicas are a more important source of toxic compounds than shoots. They reported that green tissues of rapeseed cultivars produced very little ITC, whereas root tissues produced a pronounced flush of ITC during the first 4 days after application. Rapeseed mainly liberates less volatile 2-phenylethyl-ITC from root tissues and very volatile n-butyl and butenyl-ITC from shoots (Petersen et al. 2001). White mustard liberates less volatile p-hydroxybenzyl-ITC and high volatile benzyl-ITC from both shoot and root tissues (Kirkegaard and Sarwar 1998).

Under Finnish conditions it may not be possible to harvest the required quantities of early white mustard to suppress weed seed germination and growth. Furthermore, to avoid phytotoxicity the time between incorporating and sowing is decisive. There are only few crops that can mature when sown in July. For example, spinach can be harvested but not peas. It is concluded that for control of annual weeds with white mustard needs more biomass is required than can normally be harvested in situ under Finnish conditions. It also seems that white mustard is more toxic to spinach and pea than to scentless mayweed and other annual weeds and therefore cannot be recommended for weed control in these crops.


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