Both cheeseweed and mustard have strong taproots that might be expected to help break up zones of soil compaction. Winter annual weedy grasses, such as the wild oat (Avena fatua), ripgut brome (Bromus rigidus), and wild barley (Hordeum leporinum) have relatively superficial fibrous root systems that promote good soil structure, reduce crusting and increase water infiltration. Roots of various grasses are efficient at scavenging soil N, preventing its leaching as nitrate through the soil profile (Bugg and Miller, 1991a, b; Miller et al., 1989). Brassicaceaeous weeds and common fiddleneck (Amsinckia intermedia F & M, Boraginaceae) are also regarded as efficient nitrogen scavengers; residues of these decompose and release N faster than do grass residues (Bugg, pers. comm.).

Forcella and Gill (1986) showed that timing of tillage can be used to determine floristic composition of swards. Seeds of subterranean clover and annual ryegrass (Lolium rigidum) are less persistent than sorrel (Rumex acetosella) and common knotweed (Polygonum aviculare). Red brome (Bromus rubens), silvergrass (Vulpia spp.), and subterranean clover dominated only where tillage was in summer or early autumn. Annual ryegrass was most abundant where soil was tilled in late autumn and early winter. Sorrel and common knotweed were favored only by winter tillage.

Forcella and Gill (1986) showed that timing of tillage can determine floristic composition of swards. Seeds of subterranean clover and annual ryegrass (Lolium rigidum) are less persistent than sorrel (Rumex acetosella) and common knotweed (Polygonum aviculare). Red brome (Bromus rubens), silvergrass (Vulpia spp.), and subterranean clover dominated only where tillage was in summer or early autumn. Annual ryegrass was most abundant where soil was tilled in late autumn and early winter. Sorrel and common knotweed were favored only by winter tillage. Fall or winter tillage predisposes for common knotweed, which does not germinate during the spring or summer.

Understory weeds managed as cover crops become assets to which the euphemism "resident vegetation" is usually applied. The timing of mowing or tillage can influence the floristic composition of understory weed complexes (Altieri and Whitcomb, 1978/1979), as can soil conditions (Hill and Ramsay, 1977), or herbicide applications (Meagher and Meyer, 1990).

Weeds can be managed as de facto cover crops, and thereby become assets in the understories of orchards and vineyards. When so managed, the euphemism "resident vegetation" is usually applied (Bugg, 1992). The timing of mowing or tillage can influence the prevalent weed complex (Altieri and Whitcomb, 1978/1979).

Forcella and Gill (1986) showed that timing of tillage can be used to determine floristic composition of swards. Seeds of subterranean clover and annual ryegrass (Lolium rigidum) are less persistent than sorrel (Rumex acetosella) and common knotweed (Polygonum aviculare). Red brome (Bromus rubens), silvergrass (Vulpia spp.), and subterranean clover dominated only where tillage was in summer or early autumn. Annual ryegrass was most abundant where soil was tilled in late autumn and early winter. Sorrel and common knotweed were favored only by winter tillage.

As recounted by Bugg (1992) weeds play a paradoxical role in orchards and vineyards. Some species compete excessively for water and nutrients, and some can harbor ice-nucleating bacteria, thereby predisposing for frost damage. On the other hand, weeds have also been termed "the guardians of the soil (Cocannauer, 1950)," which is hardly an exaggeration in light of their tendency to quickly colonize and cover disturbed areas, reducing soil erosion.

Forcella and Gill (1986) showed that timing of tillage can be used to determine floristic composition of swards. Seeds of subterranean clover and annual ryegrass (Lolium rigidum) are less persistent than sorrel (Rumex acetosella) and common knotweed (Polygonum aviculare). Red brome (Bromus rubens), silvergrass (Vulpia spp.), and subterranean clover dominated only where tillage was in summer or early autumn. Annual ryegrass was most abundant where soil was tilled in late autumn and early winter. Sorrel and common knotweed were favored only by winter tillage.

At the Blue Heron Vineyard (Fetzer Vineyards), Hopland, Mendocino County, California, May, 1991, total above-ground biomass data (dry, cover crop + weeds) in weedy control plots was 4.917+/-1.010 Mg/ha, (Mean +/- S.E.M., r=4). The weed flora was dominated by chickweed, shepherds purse, and annual ryegrass (Bugg et al., unpublished data).

Cheeseweed (Malva parviflora) and various mustards (Brassica spp.) produce abundant biomass. McKee (1910) measured above-ground biomass production by cheeseweed (Malva rotundifolia) at 22.2 Mg/ha (fresh weight), whereas Bugg et al. (unpublished manuscript) estimated that black mustard (Brassica nigra) contained 10.9 Mg/ha in dry matter.

Residues of such grasses as wild oat (Avena fatua), ripgut brome (Bromus rigidus), and wild barley (Hordeum leporinum) are rich in lignin, which when decomposed by soil microorganisms is believed by some researchers to degrade to fulvic and humic acids, essential to the maintenance of soil fertility (Waksman , 1936; Flaig, 1964).

Some weeds are regarded as "dynamic accumulators" of nutrients (Kourik, 1986) and can help correct soil nutrient deficiencies.

In California, such volunteer nitrogen-fixing Fabaceae as woolypod vetch (Vicia villosa ssp. dasycarpa) can contain 250 kg N/ha or more, whereas burclover (Medicago polymorpha) can contain 140 kg N/ha or more (summarized in Bugg et al., 1992). Like many weeds, both of these species have fairly indeterminate growth habits (given adequate water), with seed production by late March for burclover and mid May for woolypod vetch; both can be managed to reseed as well as produce N for the associated cash crops.

Some weeds are regarded as "dynamic accumulators" of nutrients (Kourik, 1986) and can help correct soil nutrient deficiencies.

Weeds may also concentrate soil nutrients out of proportion to their apparent abundance; with proper management, this could conceivably lead to improved availability to cash crops, although the subject requires more study. As summarized by Kourik (1986), Chickweed (Stellaria media) is believed to accumulate P, K, and Mn; mustards (Brassica spp.) are rich in S, P, and Fe. Scarlet pimpernel (Anagallis arvensis) is thought to accumulate Ca, and sowthistle (Sonchus arvensis) Mg, K, and Cu.

Bugg et al. (1987) reported that common knotweed, Polygonum aviculare L. (Polygonaceae), a summer annual weed occurring in agricultural and urban settings in much of California, is attended by numerous predatory and parasitic insects, many of which fed on the exposed floral nectar. Representatives of 36 insect taxa were observed feeding at the flowers; 29 of these groups contain entomophagous species. Other entomophagous insects were associated with a honeydew-producing, host-specific aphid, Aphis avicularis Hille Ris Lambers. Among predators frequently observed feeding at the flowers were bigeyed bugs, Geocoris spp.

Surveys by Bugg et al. (1987) indicated that common knotweed plants harbored higher densities of bigeyed bugs than did hay alfalfa (Medicago sativa L.), prostrate pigweed (Amaranthus graecizans L.), or field bindweed (Convolvulus arvensis L.). Two predators which are also nectarivorous, Geocoris punctipes (Say) and adults of a species of softwinged flower beetle, Collops vittatus (Say), survived longer when caged on common knotweed as opposed to hay alfalfa, as did G. punctipes caged on common knotweed as opposed to prostrate pigweed. No difference in longevity was found when G. punctipes was caged on common knotweed with flowers versus without flowers.

As reviewed by Bugg and Waddington (1992), various parasitic wasps that attack orchard pests use nectar as a complementary food. In Ontario, Canada, Leius (1967) stated that mean parasitism of codling moth (Cydia pomonella [L.], Lepidoptera: Tortricidae) larvae and tent caterpillar (Malacosoma americanum [Fabricius], Lepidoptera: Lasiocampidae) pupae and eggs was higher in apple orchards with rich assemblages of flowering understory plants. Leius rated 15 orchards as possessing poorly, intermediately, or richly flowering understories. The rich orchards included the following spring flowers: strawberry (Fragaria sp., Rosaceae), buttercup (Ranunculus sp., Ranunculaceae), hawkweed (Hieracium sp., Asteraceae), clover (Trifolium sp., Fabaceae), dandelion (Taraxicum sp., Asteraceae), violet (Viola sp.), fleabane (Erigeron sp., Asteraceae), white mustard (Sinapis arvensis L., Brassicaceae), willow (Salix sp., Salicaceae), wild cherries and plums (Prunus spp., Rosaceae). Summer flowers included wild carrot (Daucus carota L., Apiaceae), wild parsnip (Pastinaca sativa L., Apiaceae), blue-eyed grasses (Sisyrinchium spp., Iridaceae), white daisy (Chrysanthemum sp., Asteraceae), milkweed (Asclepias syriaca L., Asclepiadaceae), sweetclover (Melilotus sp., Fabaceae), and alfalfa (Medicago sativa L., Fabaceae). Late summer and autumn flowers were goldenrod (Solidago sp., Asteraceae) and aster (Aster sp., Asteraceae). Hymenopterous parasites included: (1) for tent caterpillar larvae: the Ichneumonidae (Hymenoptera) Itoplectis conquisitor (Say), Glypta simplicipes Cresson, and Scambus hispae (Harris); (2) for tent caterpillar eggs: Telenomus sp. (Scelionidae), Ooencyrtus clisiocampe (Ashmead) (Encyrtidae), and Eupelmus spongipartus Foerster (Eupelmidae). Parasites of gypsy moth larvae were not identified. Leius (1967) presented only means, minima, and maxima for each host type under each flowering regime: No inferential statistics were calculated.

Bugg and Waddington (1992) re-analyzed Leius's (1967) data. Analysis of variance (ANOVA) employing maxima, minima and a composite value representing the mean of the remaining three plots in each treatment. Significant differences were found using untransformed % parasitism of codling moth larvae (P=0.0282) and tent caterpillar larvae (P=0.0119); there was no significant effect on parasitism of tent caterpillar eggs (P=0.2891). Fisher's protected least significant difference indicated that parasitism of tent caterpillar larvae was significantly greater in orchards with richly flowering understories than in those of poor or intermediate status. Parasitism of codling moth larvae was significantly greater in richly-flowering understories than in those of poor status. Simple regression analyses of the same data showed that increasing richness of flowering had a significant influence on parasitism of codling moth larvae (P=0.0057) and tent caterpillar larvae (P=0.0037), and a non-significant effect on parasitism of tent caterpillar eggs (P=0.1135). In all cases, slopes of the fitted lines were positive. These ANOVA and regression analyses effectively decrease the replication by two, because the data from three plots are effectively pooled and regarded as being from a single plot. From this standpoint, the reassessments are conservative.

Batra (1979) found that two introduced Eurasian chickweeds (Caryophyllaceae), chickweed (Stellaria media [L.] Vill.), and stitchwort (S. graminea L.) collectively had at least 156 species of associated plant-feeding and pollinating insects. Stellaia media is a winter annual commonly found in Californian orchards and is said by Munz (1973) to flower from February to September, although observations by Bugg and Waddington (1992) suggest February through May. Nectar-feeding entomophagous insects listed by Batra (1979) included numerous species of aphidophagous hoverflies (Diptera: Syrphidae), three species of Tachinidae (Diptera), and, among Hymenoptera, one braconid, two Ichneumonidae, and one vespid. There were also six species of ants observed feeding on the floral nectar. Aphids commonly encountered included green peach aphid on S. media.

Chickweed can be an important nectar source to parasitic Hymenoptera, as has been documented for a wasp that attacks pests of field crops. Meteorus rubens (Nees von Esenbeck) (Hymenoptera: Braconidae), which attacks black cutworm (Agrotis ipsilon Hufnagel, Lepidoptera: Noctuidae), survived and produced more offspring when caged and fed honey-water or given access to chickweed (S. media), wild parsnip (Pastinaca sativa L., Apiaceae), wild mustard (Brassica kaber [DC.], Brassicaeceae), shepherdspurse (Capsella bursa-pastoris [L.] Medic., Brassicaceae), or lady's thumb smartweed (Polygonum persicaria L., Polygonaceae) than when caged with water alone (Foster and Ruesink , 1984). Foster and Ruesink (1986) also found in a greenhouse cage study that chickweed led to an increase in density of the parasite due to better survival. Number of black cutworm larvae attacked per 24 hours by each individual wasp was not affected by chickweed, but net increase in parasitism was estimated to be 5% due to improved longevity. This improvement would not offset prefential egg-laying by the pest in cornfields with reduced-tillage management.

Toothpick ammi (Ammi visnaga [L.] Lamarck, Apiaceae) is an occasional warm-season annual weed in the Central Valley of California and is attractive to a broad spectrum of entomophagous insects (Bugg and Wilson, 1989). It is seldom found in orchards but could be encouraged in sunlit sections by seeding into prepared seedbeds during the autumn and avoiding tillage and close mowing until after seed maturation in August.

Wild carrot (Daucus carota L.) is a biennial weed that flowers from May through September (Munz, 1973). It is common in the northern coastal counties of California and widely recognized as a potent nectar source used by beneficial parasitic and predatory insects (Bohart and Nye, 1960; Judd, 1970; Hirose, 1966). This species can volunteer within a perennial grass matrix, if mowing regimes permit maturation of seed.

In California, sweet fennel (Foeniculum vulgare Mill., Apiaceae) is a common perennial weed that flowers from May through September (Munz, 1973).

In Massachusetts, the domestic form attracted 48 species of parasitic ichneumonid wasps (Hymenoptera: Ichneumonidae) (Maingay et al., 1991) including three that attack larvae of codling moth: Coccygomimus aequalis (Provancher), Coccygomimus annulipes (Brulle), and Scambus (Atleophadnus) pterophori (Ashmead), and several attacking other lepidopterous pests of apple. Also observed were 4 species of Sphecidae (solitary wasps) and 4 of Vespidae (social wasps). Such predatory wasps are commonly seen in Californian orchards, but their agroecological roles have not been documented. Sweet fennel is probably too tall to justify inclusion in orchard understory vegetation, but it could easily be tolerated or encouraged along margins.

Poison hemlock (Conium maculatum L.) is another common, tall-statured weed. It is abundant throughout the central and north coasts and its flowers are attractive to many beneficial insects. Poisonous plants such as this are unlikely to warrant encouragement in or near orchards, however (Bugg, pers. obs.).

Bugg and Waddington (1993) listed several annual asteraceous weeds as sustaining aphids or other Homoptera that in turn sustain beneficial arthropods. Annual sowthistle (Sonchus oleraceus L.) sustains several species of aphids during April and May; some of these are suitable prey for lady beetles, whereas others are toxic. Other asteraceous weeds harboring potentially-valuable alternate prey include mayweed (Anthemis cotula L.) (flowering April through August) and pineapple weed (Matricaria matricarioides [Less.] Porter) (flowering May through August). These weeds can all be abundant in Californian orchards and vineyards.

Bugg and Waddington (1992) wrote that several fabaceous plants volunteer in Californian orchards. Most commonly encountered is bur medic (Medicago polymorpha L.), a winter annual; this plant harbors abundant pea aphid and associated lady beetles, but it also sustains high densities of Lygus spp., as noted in southern Georgia by Bugg et al. (1990b). Therefore, this plant should probably be used with caution when cover cropping in orchard crops susceptible to lygus damage, e.g., pistachio (Rice, 1985; Michailides, 1989).

Various Vicia spp. are also sometimes encountered in resident orchard vegetation, particularly in foothill and northern coastal counties (Bugg et al., 1990). Common vetch (Vicia sativa L.) and hybrids thereof have stipular extrafloral nectaries that attract parasitic wasps (Bugg et al., 1990) but also may sustain Lygus spp. (Bugg et al., 1990a, b). Various vetches also harbor diverse aphids and associated aphidophaga, as detailed in the section on pecan (Bugg and Ellis, 1990; Bugg et al., 1990a, 1991).

Various grasses produce windblown pollen used as an alternate food by the predatory mite Euseius tularensis Congdon (Acari: Phytoseiidae) (Kennett et al., 1979). This predator and congeners attack several pests of California citrus and avocado, including avocado brown mite (Oligonychus punicae [Hirst], Acari: Tetranychidae), citrus thrips (Scirtothrips citri Moulton, Thysanoptera: Thripidae), citrus red mite (Panonychus citri [McGregor], Acari: Tetranychidae), and scale crawlers (Homoptera: Diaspididae) (Congdon and McMurtry, 1985). This mite can subsist and even reproduce on diets of various pollens (Zhimo and McMurtry, 1990). Deposition of windblown pollen during late winter and early spring has been shown to be particularly important in hastening the seasonal buildup of populations of these mites (Kennett et al., 1979).

In California, pollens from anemophilous tree species (e.g., valley oak, Quercus lobata Ne_, Fagaceae) are available as early as March (Munz, 1973), whereas annual and perennial grasses produce pollen mainly during April and May. Cool-season annual grasses that commonly volunteer in Californian orchards include annual ryegrass (Lolium multiflorum Lamarck), ripgut brome (Bromus rigidus Roth.), wild oat (Avena fatua L.), wild barley (Hordeum leporinum L.), slender wild oat (Avena barbata Brot.), rattail fescue (Vulpia myuros [L.] M.K. Gmel.). Flowering periods for these and other grasses are summarized in Table 3, based on records presented by Munz (1973).

Bugg (1992) wrote that weeds and wild plants within or outside orchards or vineyards can be important reservoirs for pest or beneficial arthropods (see reviews by Altieri and Letourneau, 1982 and Andow, 1988). Lygus spp. are well known to emanate from weed hosts (Fye, 1980), and under some conditions may damage orchard crops. Bugg (1992) mentioned that other Miridae and flower thrips (Frankliniella spp.) can also cause problems in this regard. Entomophagous arthropods can also benefit from weeds and other wild plants that provide alternate prey, alternate hosts, honeydew, pollen, or nectar.

As recounted by Doutt and Nakata (1965, 1973), Anagrus epos is an egg- parasitic wasp that is an important natural enemy of grape leafhopper (Erythroneura elegantula) and variegated leafhopper (Erythroneura variabilis), both vineyard pests. Wild blackberry (Rubus spp.) sustains the alternate-host leafhopper Dikrella californica and wild grape (Vitis californica) harbors D. cockerellii, both of which overwinter in the egg stage. These wild plants thereby provide overwintering refugia to the parasite. The parasite disperses to grapes during spring, and vineyards located near such refugia are believed to have better biological control of pest leafhoppers.

The ichneumonid wasp Exochus nigripallipus subobscurus is a larval-pupal parasite of orange tortrix (Agrotaenia citrana), itself a lepidopterous pest of grapes in the Central Coast region of California. Orange tortrix and another tortricid, Aristoteliae argentifera, can also infest coyote brush (Baccharis pilularis). Kido et al. (1981) suggest that high levels of parasitism of orange tortrix may occur in vineyards with abundant coyote brush nearby. Orange tortrix can infest various vineyard weeds, including mallow (Malva parviflora), curly dock (Rumex crispus), filaree (Erodium spp.), lupin (Lupinus sp.), mustard (Brassica sp.), pigweed (Amaranthus sp.), and California poppy (Eschscholtsia californica) (Kido et al., 1981). It is not clear what role these weeds or various cover crops could play in sustaining parasites.

Weeds and resident vegetation are identical (editor's note).

Above-ground biomass data (dry) were taken in a replicated study (r=4) at Blue Heron Vineyard (Fetzer Vineyards), Hopland, Mendocino County, California. Cover crops were seeded in late October, 1990; harvest was on May 15-16, 1991. Dominant winter annual weeds were chickweed, shepherds purse, rattail fescue, and annual ryegrass. Data were as follows: CONTROL: 4.917+/-1.010 Mg/ha, Mean +/- S.E.M. (Bugg et al., unpublished data).

Vegetational cover data were measured for cover crops in a replicated trial (r=4) at the Blue Heron Vineyard (Fetzer Vineyards), Hopland, Mendocino County, California, May, 1991: CONTROL: 100.00+/-0.00 % Vegetational Cover (Mean +/- S.E.M.) (dominated by chickweed, shepherds purse, annual ryegrass) (Bugg et al., unpublished data).