Mustards used as cover crops include several members of the genus Brassica, including brown mustard (Brassica juncea [L.] Coss.), black mustard (Brassica nigra [L.] Koch), field mustard Brassica campestris L., and white mustard (Brassica hirta Moench) (Munz, 1973).

Mustards include several members of the genus Brassica, including brown mustard (Brassica juncea [L.] Coss.), black mustard (Brassica nigra [L.] Koch), field mustard Brassica campestris L., and white mustard (Brassica hirta Moench) (Munz, 1973).

Several species of mustard are available and usable. The most common is a form of black mustard, Brassica nigra, known locally as Trieste. A brown-seed species, Brassica juncea, is also used. Most of the mustards available, however, are a mixture of types and species. Unfortunately, there has been no systematic evaluation of the relative merits of the various types and species (Madson, 1951).

Most, if not all species, produce some hard seed (Madson, 1951).

According to Fischer et al. (1978), black mustard seedlings have cotyledons broad, kidney shaped, with a deep notch at the tip or apex. The first true leaves are bright green on the upper surfaces and pale beneath.

The following are based on accounts of Munz (1973):

Brown mustard (Brassica juncea [L.] Coss.) is a pale subglabrous annual, 3-12 dm high; lower leaves runcinate-pinnatifid and crenate, petioles 3-12 cm long, the upper nearly sessile, lanceolate or linear, entire or dentate; pedicels slender, divergent, 8-12 mm long; sepals 4-5 mm long; petals yellow, spatulate, 7-8 mm long; siliques 3-4 cm long, ascending, the beak 5-8 mm long seeds subglobose, ca, 1.5 mm in diamter, red-brown to yellowish, weakly reticulate.

Black mustard (Brassica nigra [L.] Koch) is an erect anual, branched above, 5-25 dm high, sparsely pubescent or subglabrous; lower leaves 1-2 dm long, deeply pinnatifid, with large terminal lobe and few small lateral ones; cauline leaves gradually reduced but not clasping, the uppermost pendulous; sepals 3.5-4.5 mm long; petals bright yellowm 7-8 mm long; pedicels 2-3 mm long, erect; siliques appressed, 1-2 cm long, the beak subulate, empty, 1-3 mm long; seeds ca 1-1.3 mm thick, dark red brown, finely reticulate.

Field mutsard (B. campestris is an erect annual, 3-12 dm tall, with slender roots, glaucous and quite glabrous except for the scattered hairs on the lower leaves; these petioled, +/- pinnatifid or lobed, 1-2 dm long; upper leaves, sessile, auriculate-clasping, lance-oblong, subentire, glabrous; pedicels spreading, 1-2 cm. long; sepals narrow-oblong,yellowish, 4-5 mm long; petals yellow, spatulate, 6-8 mm long; siliques not torulose,terete, 2-5 cm long, stout with a stout beak an additional 1-1.5 cm long; seeds 1.5-2 mm thick, dark reticulate; n=10 (Karpechenko, 1924). Common weed, especially in orchards and waste places, widely distributed in Calif.; natur. from Eu. Jan.-May also in other months. (B. Rapa L. the Turnip, very like this, but with thickened roots, sometimes referred to the same sp.)

White mustard (Brassica hirta Moench) is an annual 3-7 dm high, more or less hirsute; lower leaves board, lyrately pinnate or pinnatifid, 10-20 cm long, petioled, the terminal lobe or leaflet large; upper leaves short-petioled, lanceolate or oblong; pedicels spreading in fr., 5-15 mm long; sepals 5-6 mm long; petals yellow, 8-11 mm long, 4-5 mm wide; siliques white-bristly, 2-3 cm long (including beak), the valves prominently 3-nerved, few-seeded, the beak flattened, broad, ca as long as the rest of the silique; seeds pale yellow, subglobose, 1-2 mm thick, minutely alveolate.

Most brassicas can tolerate minor freezes (Horn, 1985); mustards show moderate resistance to cold (Madson, 1951) and are not winter hardy in New England (Brinton, 1989).

The following account is paraphrased from that of Munz (1973). The genus Brassica is derived from Europe, Asia, and North Africa. In California, black mustard is common on dry grassy slopes, in grain fields, and in waste places through much of the state. Brown mustard is scattered through the state in grain fields and waste places. Field mustard is widely distributed in California in orchards and waste places. White mustard is cultivated as a green and is naturalized in widely scattered locales.

Williams (1966) found that various grasses grown as green manures improved infiltration of irrigation water on a deep, well-drained Yolo loam soil, but that black mustard did not.

N fertilizer is often applied when seeding an orchard cover crop of mustard to stimulate mustard growth and compensate for any competition that otherwise might affect orchard trees. The N taken up by the cover crop will become available to the trees as the organic matter decays (Madson, 1951).

Mustards do well in loam to heavy soils (Madson, 1951).

Mustards are sensitive to glyphosate as well as to 2,4-D and various other broadleaf herbicides (John H. Anderson, pers. comm.).

Per the account in Munz (1973), black, brown, and white mustards are all annuals. Flowering periods were given as brown: June-September; black: April-July; white: March-August.

Seeding rates for mustard have been suggested as 5-7 lbs/acre for small-seeded cultivars (Miller et al., 1989), 10-12 lbs/acre for large-seeded cultivars (Miller et al., 1989.), and generally as 20 lb/a (Brinton, 1989).

Mustards should be seeded at a depth of 0.4-0.8" (Brinton, 1989).

Mustard can be drilled or broadcast to one-half inch. (Fred Thomas, pers. comm.)

In California, mustard should be seeded from October to November (Madson, 1951).

Kloen and Altieri (1990) conducted a trial at Albany, CA, comparing broccoli in monocultural plots to three schemes of intercropping broccoli with mustard (three planting dates for mustard: simultaneous with the broccoli and 2 and 3 weeks after). There was no significant effect of cropping scheme on densities of aphids on broccoli, nor for aphidophagous syrphid (hover fly) larvae, but the ratios of syrphid larvae to aphids were significantly higher for broccoli with mustard sown one week later. This may have been due to attraction of adult syrphids by the flowering mustard.

Mustards are not legumes and require no inoculation, so far as is known (R.L. Bugg, pers. comm.).

Brown, black, and white mustards are widely available in the seed trade.

Mustards flower from 3-5 weeks after sowing (Brinton, 1989).

Sinapis alba (white mustard) is one of the fastest growing green manures (Pears et al., 1989).

Mustards are stemmy and produce dense, heavy growth (Madson, 1951).

Black mustard attained a height of 171.45+/-11.00 (cm, Mean +/- S.E.M.) at the Blue Heron Vineyard (Fetzer Vineyards), Hopland, Mendocino County, California, May 2, 1991 (Bugg et al., unpublished data). Munz (1973) listed heights as field mustard: 3-12 dm; black mustard: 5-25 dm; brown mustard: 3-12 dm; and white mustard: 3-7 dm.

Mustard taproots extend to depths of 1-3 ft (Brinton, 1989).

From data presented by Jackson et al. (1993b), for mid-November-planted cover crops in March, approximate values for N contained in root systems obtained by subtraction were as follows in kg N/ha:

• Annual ryegrass: 20
• White mustard: 35
• Oilseed radish: 58
• Phacelia: 57
• Merced cereal rye: 19
• White senf mustard: 13

Jackson et al. (1993b) stated that, for mid-November-planted cover crops in March, root biomass figures in kg/ha were:

• Annual ryegrass: 883
• White mustard: 2,273
• Oilseed radish: 4,128
• Phacelia: 1,502
• Merced cereal rye: 950
• White senf mustard: 592
• Standard error: 680.8 (d.f.=10)

Jackson et al. (1993b) stated that, for mid-November-planted cover crops in March, root length measurements yielded the following figures during March (m/m2):

• Annual ryegrass: 20,700
• White mustard: 22,200
• Oilseed radish: 15,300
• Phacelia: 19,800
• Merced cereal rye: 19600
• White senf mustard: 13,100
• Standard error (d.f.=10)=2,818.8

Kutschera (1960) reported that mustard (Sinapis arvensis) generally roots to a depth of 140 cm.

Fall-seeded mustards mowed in early February generally re-grow fairly well. Flowering mustards will re-grow modestly after mowing and will often flower and set some seed. (Mark Van Horn, pers. comm.)

Muehlchen, Rand, and Parke (1990) found that fall-planted white mustard (Brassica hirta) reduced root rot severity and increased yield in pea after two successive rotational cycles. However, there was also some evidence that white mustard can reduce emergence of pea seedlings and nodulation of pea by rhizobial bacteria. The presumed mechanism for suppression of pathogens is that, after plough-down, sulfur-containing compounds are produced by the breakdown of glucosinolates in crucifer tissues; these may act as a soil fumigant which causes death of fungal propagules.

Mustards are often used as cover crops in orchards (Madson, 1951). The mustards are not used in general farming or in rotation with annual crops because most types produce some hard seed which will remain dormant in the soil and can cause weed problems in following crops (Madson, 1951).

Mustards winter kill in the northeastern United States making desirable mulch to accomodate spring-sown vegetables (Brinton, 1989).

Liebman and Robichaux (1990) found that a long-vined variety of field pea ('Century') was better than a short-vined variety ('Alaska') at suppressing mustard growth by shading. 'Century' also showed a greater yield.

Liebman and Robichaux (1990) found that intercropped barley and field pea were no better at suppressing weed mustards (Brassica kaber) and white mustard (B. hirta) than was a densely-seeded monoculture of barley. The main mechanisms of weed suppression were shading (especially by the pea) and competition for nitrogen (especially by the barley).

Bialy et al. (1990) found that black mustard and brown mustard show the most active allelopathic inhibition of other plants. Compounds involved probably include various isothiocyanates (ITC), which suppressed wheat germination and growth. 2-phenethyl ITC and Allyl ITC showed particularly high activity.

Black mustard yielded a mean above-ground biomass of 11.9+/-1.4 Mg/ha (Mean +/- S.E.M.) at the Blue Heron Vineyard (Fetzer Vineyards), Hopland, Mendocino County, California, May, 1991 (Bugg et al., unpublished data).

Jackson et al. (1993b) stated that, for mid-November-planted cover crops in March, above-ground biomass figures in kg/ha were:

• Annual ryegrass: 2,070
• White mustard: 5,913
• Oilseed radish: 4,128
• Phacelia: 4,552
• Merced cereal rye: 4,410
• White senf mustard: 5,893
• Standard error (d.f.=10)=201.3

Mustards make no net contribution of nitrogen but can be used as catch crops to retain nitrogen already in the soil (Brinton, 1989). When mustards are used as catch crops, it is important to note that their residues break down more rapidly than do those of most grasses; thus, associated N may be liberated more rapidly (Bugg, pers. comm.).

In replicated studies in Salinas, CA, Jackson et al. (1993b) reported that November-planted cover crops had attained the following total-plant N content (kg N/ha) figures by March; approximate above-ground N contents were read from a graph and are given parenthetically:

• Annual ryegrass: 85 (65)
• White mustard: 205 (170)
• Oilseed radish: 200 (142)
• Phacelia: 182 (125)
• Merced cereal rye: 129 (110)
• White senf mustard: 161 (148)
• Standard error: 20.2, d.f.=10

From data presented by Jackson et al. (1993b), for mid-November-planted cover crops in March, approximate values for N contained in root systems obtained by subtraction were as follows in kg N/ha:

• Annual ryegrass: 20
• White mustard: 35
• Oilseed radish: 58
• Phacelia: 57
• Merced cereal rye: 19
• White senf mustard: 13

According to Williams (1966), a mustard cover crop did not significantly improve water infiltration, whereas barley, cereal rye, annual ryegrass, and soft chess (all grasses) did.

Mustard incorporated as a green manure failed to significantly increase infiltration of irrigation water, although barley, cereal rye, annual ryegrass, and 'Blando' brome did so (Williams, 1966).

Brassicas are important fodder crops, especially as late-fall or emergency feeds. (Horn, 1985)

In a trial in southern Georgia that concerned 20 cover-cropping regimes and associated insects, convergent lady beetle (Hippodamia convergens Guerin-Meneville) and seven-spotted lady beetle (Coccinella septempunctata [L.]) first were found in substantial numbers on rye (early February), then on crimson clover and lentil (mid February through late March), later on subterranean clover (early through late March), still later on narrow-leafed lupin (late March through early June), then hairy vetch (late March through early June), and lastly on mustard (mid April through late May) (which harbored turnip aphid [Hyadaphis erysimi {Kaltenbach}], cabbage aphid [Brevicoryne brassicae {L}], and, upon flowering, various thrips (Frankliniella spp., Thysanoptera: Thripidae), and collard (late April through early June) (Bugg et al., 1990).

As reported by Flexner et al. (1990), in southern Oregonian pear orchards, certain understory weeds can harbor high densities of twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae). This mite is mainly a secondary pest and a creature of pesticide-disrupted or stressed agroecosystems. Among the plant species suitable for use as cover crops, black mustard appeared least favorable to outbreaks of the mite.

Kloen and Altieri (1990) conducted a trial at Albany, CA, comparing broccoli in monocultural plots to three schemes of intercropping broccoli with mustard (three planting dates for mustard: simultaneous with the broccoli and 2 and 3 weeks after). There was no significant effect of cropping scheme on densities of aphids on broccoli, nor for syrphid larvae, but the ratios of syrphid larvae to aphids were significantly higher for broccoli with mustard sown one week later. This may have been due to attraction of adult syrphids by the flowering mustard.

Mustard is not recommended as a cover crop in apple orchards where orange tortrix (Argyrotaenia citrana (Fernald); also known as apple skinworm) can be a problem because the pest overwinters on the mustard (Pickel, pers. comm.).

Senph mustard and related species are used for their nematicidal properties. Becky Westerdahl at UC Davis has shown that the German cultivars developed for nematode resistance and suppression are not effective on the races in California. (Fred Thomas, pers. comm.)

White mustard (Sinapis alba) is susceptible to clubroot disease (Pears et al., 1989). By contrast, Muehlchen et al. (1990) found that fall-planted white mustard reduced root rot severity and increased yield in pea after two successive rotational cycles. However, there was also some evidence that white mustard can reduce emergence of pea seedlings and nodulation of pea by rhizobial bacteria. The presumed mechanism for suppression of pathogens is that after plough-down, sulfur-containing compounds are produced by the breakdown of glucosinolates in crucifer tissues; these may act as a soil fumigant which causes death of fungal propagules.

In studies at the Blue Heron Vineyard (Fetzer Vineyards), Hopland, Mendocino County, California, May, 1991, weed above-ground biomass data in plots seeded to black mustard was 1.015+/-0.437 Mg/ha (Mean +/- S.E.M.). This was 20.6% of the weed biomass in control plots. In early May, vegetational cover by black mustard was 91.25+/-5.54 % (Mean +/- S.E.M.) (Bugg et al., unpublished data).

A long-vined variety of field pea ('Century') was better than a short-vined variety ('Alaska') at suppressing mustard growth by shading. 'Century' also showed a greater yield (Liebman and Robichaux, 1990).

Liebman and Robichaux (1990) also found that intercropped barley and field pea were no better at suppressing weed mustards (Brassica kaber) and white mustard (B. hirta) than was a densely-seeded monoculture of barley. The main mechanisms of weed suppression were shading (especially by the pea) and competition for nitrogen (especially by the barley).

Black mustard and brown mustard allelopathically inhibit other plants. Compounds involved probably include various isothiocyanates (ITC), which suppressed wheat germination and growth. 2-phenethyl ITC and Allyl ITC showed particularly high activity (Bialy et al., 1990).