The effects of mulching on soil temperature are determined by the ability of the mulch to reflect, absorb, and transmit solar radiation (light). Clear mulch transmits the most radiation to the underlying soil, followed by red, green, yellow, blue, and black, with black mulch blocking almost all of the radiation. Black mulches absorb most UV, visible, and infrared wavelengths but reradiate much of that energy back into the atmosphere.^1,4 Black mulch does transfer energy to the underlying soil through conduction, meaning that the mulch needs to be in direct contact with the soil to be most effective for soil heating. Soil temperatures under black mulch tend to be about 5°F (2.8°C) higher at a two-inch depth than similar non-mulched soil. 

Soil temperatures tend to be highest under clear plastic mulch because most of the radiation energy is transmitted to the soil directly. The formation of water droplets on the underside of the mulch helps prevent the energy from reradiating back through the mulch. Soil temperatures under clear mulch tend to be 8° to 14°F (4.4 to 7.8°C) warmer at a depth of two inches as compared to non-mulched soil. Clear mulches tend to be used primarily in northern regions where higher levels of soil heating are desirable early in the season.^1,2,4 

White, white-on-black, and silver (reflective) mulches reflect solar radiation back into the canopy and transmit little to the soil. These types of mulches can lower soil temperatures by several degrees compared to non-mulched soils. The lower temperatures can help seedlings of cool-season crops to become established when planted in the warm summer months.^1,2 

Infra-red transmitting (IRT) mulches and other wavelength-selective mulches tend to warm soils to levels between those under clear and black mulches. Visible light wavelengths, especially those used by plants for photosynthesis, are reflected or blocked from reaching the soil, which helps inhibit weed growth. Infra-red and near-infra-red light is transmitted to the soil and contributes to soil warming.⁵ 

Plastic mulches help reduce water evaporation from soil surfaces, increasing water use efficiency and more even moisture distribution. Differences in soil temperature effects of different colored mulches impact the amount of water evaporating from the plant holes in the mulch layer, with higher temperatures resulting in increased evaporation. Reflection of photosynthetically active radiation (PAR) from the mulch back into the plant canopy can affect plant growth and transpiration. Cooler canopy temperatures and lower PAR levels result in lower transpiration rates, while increased canopy growth from higher PAR levels results in increased transpiration. ⁴ 

Black mulch is used most widely because it blocks solar radiation from reaching the soil. The lack of light prevents most weeds from growing under the mulch. The exception is nutsedge (nutgrass), which germinates under the mulch and can penetrate the mulch layer.^2,5 Clear mulches allow light to reach the soil surface, which allows weeds to grow under the mulch. Herbicides or soil fumigants are usually required to manage weeds when using clear mulches. IRT mulches allow infra-red and near-infra-red light to reach the soil surface, which allows for increased soil warming compared to black plastic mulch. However, light in the PAR region is blocked, preventing most weed growth. Because of the specific wavelengths transmitted, nutsedge growth is usually also inhibited under IRT mulches.^1,5 

Increased root growth is typically seen under mulches resulting from soil temperature and moisture factors, but mulches can also affect the development of the above-ground plant parts. White mulches, which reflect most of the blue light, can stimulate earlier branching, resulting in shorter, more highly branched plants. Other colored mulches can alter the far-red to red light (FR:R) ratio, which can affect plant height and biomass. Red mulches have been shown to increase yields in some crops, including tomato and pepper.^4,6 

Reflective (silver or aluminum-faced) mulches reflect light, which can interfere with the movement of aphids and help repel aphids from the crop. They are most effective early in the season before the canopy covers the mulch. These mulches can help reduce the incidence of aphid-borne viruses.^1,2 

New types of mulches are continually being developed and evaluated to help improve performance and address the drawbacks of existing mulches. Paper mulches have been assessed for helping elevate soil temperatures to address the plastic disposal problem. Paper mulches with darkly colored surfaces were able to raise soil temperatures, increase cucumber yields, and manage weeds in one study. ⁷ 

Much work is going into developing biodegradable mulches, usually based on biological polymers such as starch, cellulose, polylactic acid, and polyhydroxyalkanoates. The advantage of these bio-polymers is that microorganisms can break them down over time. This would eliminate the need to remove the mulch from the field and find a means of disposal. One problem with some bio-polymer mulches developed so far is that they do not entirely break down in a reasonable amount of time. Mulch fragments can remain in the soil, affecting water infiltration and absorption rates. On the positive side, the breakdown products of some of these mulches can serve as carbon sources in the soil resulting in increased microbial activity and biomass.⁸