Several types of plastic mulch are available which can enhance earliness of vegetable crops. Which, when and how these mulches might be used for maximum benefit can be confusing. In addition, there is some research that suggests that colored mulches may enhance earliness or total yields in vegetables. What follows is a description of some of these plastic mulches, and how you might optimize your use of these on your farm.
Plastic mulches can reflect, absorb or transmit incoming sunlight, the extent of which depends on the type of mulch. In the case of enhancing the earliness of vegetable crops, the mulch should maximize the transmission of the soil-warming portion of sunlight (the near infra-red radiation) and minimize the amount of visible light that is transmitted. This visible light can increase the weed growth under the plastic.
Regardless of the plastic mulch you select, the following practices will enhance the effectiveness of the mulch to warm the soil and provide good weed suppression:
Use raised beds with plastic mulch. These beds should be firm (can walk on it without sinking too far). This will increase early season soil warming and drainage out of the bed. Water is more easily shed off the top of the bed, keeping the mulch cleaner. Raised beds can also reduce disease pressure in the crop, by lifting plants up out of low zones in the field.
Stretch plastic tightly over the top of the bed. A tight fit to the bed surface increases heat transfer between the plastic and the soil. Air space between the plastic and soil can lead to a loss of 2° to 4°F of soil warming. This tight fit also can reduce weed pressure under the mulch. As weeds emerge, the cotyledons hit the hot plastic and may be burned.
Use an embossed plastic. The diamond shape pattern visible on the plastic gives it additional stretch compared to a smooth plastic. An embossed plastic acts like an ‘accordion,’ stretching while you lay it, then shrinking slightly to form a tight seal over the surface of the bed.
Apply the plastic mulch close to estimated planting date. If certain mulches that transmit some visible light (clear, IRTs) are applied too far in advance of planting, weed growth under the mulch may compete negatively with the crop. Generally, crops can out-compete these weeds, if established well in advance of the developing weeds.
Clear mulch is the most transparent to infrared radiation and warms the soil to the greatest extent of all plastic mulches. For warm season crops, such as vine crops, earlier yields and higher yields can be achieved with clear mulch. However, clear mulch is also transparent to visible light, which contributes to weed growth under the mulch. With early sweet corn, herbicides are available to deal with this challenge. For other crops, you may have to rely upon the crop growing and covering the plastic before weeds get established under the mulch. When no herbicides are available for use under the mulch, it is recommended that you apply the clear mulch close to planting time, to minimize any weed competition during crop establishment.
As a result of grower trials and tribulations with managing clear mulch, researchers in New Hampshire developed Infra-Red Transmitting mulches (IRT). IRT mulches warm the soil intermediate to clear and black mulch and are pigmented to reduce the amount of visible light transmitted (reduced weed growth under the mulch). Some of these IRT mulches are either green or brown, depending on the manufacturer. However, not all green or brown mulches are IRTs. IRTs contain very specific pigments that give them the unique ability to transmit a maximum of near infra-red radiation and a minimum (14%-16%) of visible light. Some visible light must be transmitted in order to maximize soil warming.
The benefits of IRTs are greatest for early season plantings. At later planting dates, crop growth and yield usually are not different between IRT and black plastic. During springs that are unusually cloudy and cool, there will be little difference in yields between an IRT and black mulch. Remember – these IRT mulches depend upon incoming sunlight to warm the soil. Because of the higher cost of this specialized mulch, it is recommended that you only consider using IRTs for your earliest planting dates, on tomatoes, peppers, and vine crops. If you are using tunnels with tomatoes and peppers, the air temperature over the IRT will not be as warm as over black mulch, reducing the risk of blossom crop caused by high tunnel air temperatures.
While the term IRT is not trademarked, it is associated with the company which currently owns the license to produce it (APR Industries). Some of IRT mulches available (all embossed) to growers in the Northeast include:
IRT-76. This 1.25 mil plastic mulch was the first of the IRTs released commercially. The name ‘76’ describes the amount of infra-red radiation transmitted by the mulch (76%). Only 14%-16% of the visible light is transmitted, resulting in fairly good weed control under this mulch.
IRT-100. Same as IRT-76 but 1.00 mil thickness. By making it thinner, the company can sell it cheaper! This mulch has the same transmission characteristics of the IRT-76.
IRT-200. This IRT has been pulled from the market. It was similar in thickness to the IRT-100 (1 mil). Manufacturers tried to reduce the price further by reducing the amount of pigment compared to the other two IRTs – so more visible light was transmitted (20%-25%). This led to problems around the country with poor weed control under this mulch. There were also reports of this mulch breaking down early, in the field, making removal difficult.
PlastiTech Wavelength-Selective mulch. This is a new IRT-type mulch that has good pigment characteristics and is being tested around the region. For more information, contact PlastiTech at (800) 667-6279.
There has been much discussion recently about using specific colored mulches on different vegetable crops to achieve earlier or higher yields. When used by a roadside stand, these mulches can certainly draw quite a bit of attention. However, the research results from studies examining red mulch under tomatoes is very conflicting. In some cases, a yield benefit was observed; in others, no benefit. Different tomato varieties also appear to respond differently to the red mulch. The scientific theory behind using red mulch is sound – a specific pigment system in plants (phytochrome) responds to specific red wavelengths of light. If we enhance those wavelengths in the plant canopy, we should be able to change plant growth. The effect of red mulch, like that of IRT mulches, depends on sunlight levels. Part of the variation in results in the Northeast may be related to the differing levels of total sunlight and average air temperatures.
Red mulch can act like an IRT much and will increase soil temperatures compared to black mulch. Some of the benefit to tomatoes is suspected to be due to this temperature effect. If used for the early plantings, some yield benefit may be observed with red mulch due to soil warming. When used with main season plantings, the effects of red mulch may be more variable. Red mulch, however, is more expensive than IRT, requiring careful consideration of crops and markets you are targeting. Start small if you are considering experimenting with these colored mulches. There have been problems with both the product quality and visible light transmission (poor weed control under many of these).
