White or coextruded white-on-black mulches can slightly lower surface soil temperatures by about 1ºC at 2 cm depth or 0.4ºC at 10 cm relative to bare soil because they reflect most incoming radiation . These mulches are used when lower soil temperatures may be desirable for planting vegetables in particular summer production windows. Clear mulches effectively retain much of the heat normally lost to the atmosphere by bare soil, increasing daytime soil temperatures from 4º to 8ºC at a depth of 5 cm , and 3º to 5ºC at 10 cm relative to bare soil . These clear plastics are the choice for soil solarization. Clear plastics, however, do not control weeds and require other weed management practices such as fumigation and herbicide application. Plastic mulches also influence nutrient levels and uptake. Wien and Minotti found plastic mulching increased shoot concentrations of nitrogen , nitrate , phosphorus , potassium , calcium , magnesium , copper and boron in transplanted tomatoes. Bhella , also working with tomatoes, found higher levels of ammonium , nitrate , and magnesium in plastic mulched soils. Hassan et al. found higher levels of nitrogen, phosphorus, potassium, and calcium in leaf tissue of chilies grown over plastic reflective mulch compared to those grown over bare soil. A wide variety of other colored plastic mulches, including red, yellow, silver, blue, gray, and orange, have been used in various efforts to achieve specific production goals. Each of these colors has distinct spectral reflectivity characteristics and thus modifies the radiation balance in and below a crop canopy. These colored mulches affect not only the microclimate around a crop but have also been shown to influence insect behavior. Yellow, for example, is generally highly attractive to insects and has been shown to increase green peach aphid and striped and spotted cucumber beetle populations compared to plants grown over bare soil .

White at times repels aphids and at other times attracts them, depending on the physiological state of the insect . Orange has been shown to repel various aphids and whiteflies , while pink and green attract aphids . Red has been shown to attract both aphids and whiteflies . Blue has been shown to be repellent in some studies and attractive in others . Highly reflective or shiny aluminum plastic mulches have been shown to repel certain aphids and thereby reduce or delay the onset of aphid-vectored mosaic viruses in zucchini squash and melons and tomato spotted wilt virus in tomatoes . These are the mulches of choice when insect and disease management are the principal objective. On balance, other than UV-metalized reflective mulches, repeated and consistent benefits in managing insects with most colored mulches have not been documented. These mulches, however, have produced mixed results . Mahmoudpour and Stapleton noted that “the influence of mulch colour on growth and productivity has been postulated to be highly specific, and may vary with plant taxa, climate, and seasonal conditions.” As is the case with insect management, aluminum mulches have provided the most positive and consistent findings on crop production .Using cover crops as mulches is a relatively recent management strategy that is currently being refined and evaluated in a wide range of vegetable production systems. The winter annual legume hairy vetch , for example, has been used successfully as both a cover crop and as a mulch in fresh market tomato production systems in the southeastern United States. As a cover crop, the vetch fixes nitrogen, recycles nutrients, reduces soil erosion, and adds organic matter to the soil. When mowed and converted to a mulch, the vetch reduces weed emergence,grow strawberry in containers lowers soil temperatures during the hot summer months, reduces water loss from the soil, and acts as a slow-release fertilizer .

This system, developed by USDA ARS researchers, eliminates tillage, reduces the need for applying synthetic fertilizers and herbicides, and reportedly adapts to both large- and small-scale tomato production in a low-input, no-tillage system. Recent work in Florida by Chellemi et al. has shown that although a cover crop surface residue mulch production system had lower yields than the standard black polyethylene plastic, the overall profitability of the alternative system was actually higher. Work in California’s Central Valley has shown that cover cropping increases water infiltration and reduced winter runoff and increases soil carbon . Additionally, cover crops, when cut and dried, have been shown to delay and reduce the incidence of aphids and whiteflies as well as the incidence of aphidborne viruses. Burton and Krenzer observed a reduction in green bug populations where surface residues of a previous wheat crop existed. Summers et al. found wheat straw mulch significantly delayed and reduced the incidence of alate aphids and several aphidborne cucurbit viruses in zucchini squash. The incidence of silver leaf whitefly and squash silver leaf was also significantly reduced. Similar results were obtained with cantaloupe grown over wheat straw residue. A number of other examples of the successful use of cover crop mulches have been reported in Georgia, Virginia, North Carolina, and Pennsylvania, but their potential in California’s vegetable crop production is only now beginning to be investigated, evaluated, and refined. Combining the potential benefits of surface residue cropping alternatives with those of conservation tillage is becoming increasingly attractive to row crop producers in many of California’s agricultural regions as shown by the following case studies.Beginning in 1995, a series of studies and demonstrations were initiated in the Central Valley to evaluate and develop conservation tillage and cover crop mulch production systems for tomato crop rotations.

While the immediate goal of these efforts was to reduce production costs, a longer-term objective was to develop information on the potential of reduced tillage to improve soil quality, store carbon in the soil, and conserve resources. Initial studies, conducted at the University of California West Side Research and Extension Center in Five Points, at the UC Davis campus, and in commercial production fields in Tracy, Vernalis, and Le Grand, evaluated the use of winter cover crops as surface mulches, the feasibility of no-till and strip-till transplanting, and options for in-season weed management. No-till transplanting requires the use of coulters or some form of residue manager to cut surface residues ahead of the transplanter shoe. Strip-till is a form of CT in which a set of coulter or shank implements tills a narrow band of soil 15 to 20 cm wide to a depth of about 8 to 36 cm only in the line into which transplants will be placed. Results from these preliminary evaluations have indicated that planting and harvesting both processing and fresh market tomatoes is possible and that yields comparable to those attained using standard winter fallow techniques may be achieved with certain reduced-till approaches that do not result in excessive cover crop regrowth or weed competition with the tomato crop. On-farm strip trial data for demonstrations conducted in 1999 in Tracy and in 2000 in Vernalis are given in table 3. This early work, and other experiments summarized by Herrero et al. 2001b, also revealed that in-season weed control by a surface cover crop mulch alone is not adequate. The authors of this publication have subsequently investigated and refined the use of a high-residue cultivator that is able to effectively slice through residues while cultivating weeds. Cover crop mulch species selection and mulch management must be optimized if organic mulch tomato production is to expand in California. Care must be taken to avoid the use of certain cover crops such as sorghum-Sudan hybrid as mulches because they are highly allelopathic to tomatoes and several other vegetable crops . More efficient and low-risk production protocols for managing cover crop mulches in vegetable crop rotations must also be developed.Aphid-transmitted virus diseases cause significant economic losses to California’s multi-million-dollar vegetable crop industries annually. Over the past few years, production of fall melons , squash , peppers , and tomatoes has been extremely difficult in certain regions of the San Joaquin Valley due to extensive virus epidemics and severe infestations of silver leaf whitefly . Spring crops, while affected to a lesser extent, have also suffered significant losses by aphid-transmitted viruses and whitefly infestations.Several plant viruses are responsible for these epidemics,hydroponic nft channel and most are capable of infecting all of the crops mentioned above. Among the most important viruses are cucumber mosaic virus , zucchini yellows mosaic virus , potato virus Y , and watermelon mosaic virus . These viruses are transmitted by aphids in a stylet borne, non-persistent manner. They are acquired and transmitted in as few as 15 seconds, and are transmitted by a large number of aphid species, all of which are abundant throughout California. Due to the rapidity with which the viruses can be acquired and transmitted, insecticides are of little value in preventing virus spread and under some circumstances may actually increase the rate of virus transmission and spread. This has not, however, dissuaded a large number of growers and PCAs from attempting to control the spread of these viruses by using insecticides. UV-reflective mulches consist of a polyethylene base to which a thin coat of aluminum ions has been adhered. The mulches are collectively referred to as metalized mulches. These mulches reflect UV wavelength , which confuses and repels incoming alate aphids, adult whiteflies, and leaf hoppers , reducing their incidence of alighting on plants .

UV-reflective plastic mulches have been used successfully to reduce the incidence of aphidborne virus diseases in squash and other crops . Brown et al. found silver plastic mulch superior to white, yellow, or black with yellow edges in repelling aphids in yellow crookneck summer squash. Plants grown on silver mulch produced significantly higher yields of marketable fruit than did those grown on bare soil. Mulches applied to the planting beds before seeding were effective in repelling alate aphids and delaying the onset of several virus diseases as well as the onset of silver leaf whitefly colonization and the appearance of squash silver leaf in spring and fall-planted zucchini squash in California’s San Joaquin Valley . Disease symptoms in plants growing over these mulches appeared 10 to 14 days later than in plants growing on unmulched beds. In spring-seeded squash, approximately 30 percent of the plants on unmulched beds were infected with one or more viruses by the first harvest, while only 10 to 15 percent of those grown over the metalized mulches showed virus symptoms. In fall-planted trials, 100 percent of the plants grown on unmulched beds, with and without insecticide applications, were virus-infected by the first harvest. Metalized mulches were generally more effective in repelling aphids and delaying virus onset than were white-pigmented mulches . Although plants grown over the metalized mulched plots eventually became infected, they continued to produce a significantly higher percentage of marketable fruit throughout the season than did the unmulched controls. In addition, squash, cantaloupes, cucumbers, and corn grown over reflective mulch produced marketable fruit 7 to 10 days earlier that plants growing over bare soil. Stapleton and Summers also showed than cantaloupe grown over reflective mulches yielded over 500 cartons of marketable fruit per acre compared to less than 50 cartons per acre from plants grown on bare soil. A delay of 4 to 6 weeks in infection by CMV in plants growing over reflective mulch allowed them time to mature and set a good crop of melon fruit before becoming infected. Even though the plants eventually became infected, the delay in infection permitted the harvest of a highly profitable crop. Summers and Stapleton have shown that tomatoes grown over reflective mulches averaged approximately 7 percent virus-infected plants, while plants grown over bare soil averaged in excess of 50 percent infection with the same viruses. This approach is currently the only viable means of managing virus disease in these production systems. Growers are cautioned to use only metalized reflective mulches when insect and disease management is the primary objective. Other colors lack the high degree of UV reflectance necessary to repel incoming insects. Also, the mulch must be applied prior to seedling emergence. Plants may be inoculated with aphidborne viruses in the cotyledon stage, and any delay in applying the mulch could lead to an extensive infection. Some crops are more susceptible to injury when planted over metalized mulches.