Archived content - Plastic Mulches for Commercial Vegetable Production

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Introduction

The use of plastic mulch to achieve earlier and larger yields of commercial vegetables is increasing. This is especially true for warm season species such as peppers, corn, tomatoes and the vine crops. Warm season crops in Saskatchewan grow rather slowly under the soil temperature conditions which prevail early in the growing season.

Dr. Doug Waterer of the University of Saskatchewan, with the aid of PAWBED funding, investigated the benefits of plastic mulches for vegetables grown under Saskatchewan conditions.

Plastic Mulch

Various types of plastic mulch are available. Clear, black, and infrared transmissable (IRT) mulches were evaluated in this study.

Clear and IRT plastics have the greatest warming potential. They are transparent to incoming radiation and trap the longer wavelengths re-radiating from the soil. Black mulches are limited to warming soils by conduction only and are less effective.

In addition to soil warming, weed control is a major consideration in mulch technology. Clear mulches allow profuse weed growth which may negate the benefits of soil warming. Black mulches provide effective weed control.

Wavelength selective mulches, such as the infrared transmissible mulches (IRT), have become available. IRT mulches blend the soil warming characteristics of clear mulch with the weed control ability of black mulch. IRT mulches are more expensive. The benefits of IRT mulches, as with other mulches, must be balanced against the cost of material and labour for installation, removal and disposal.

Additional benefits of plastic mulches are their ability to conserve soil moisture, and their potential to direct rain water to the crop plants.

Installation

The objective of using plastic mulch is to enhance soil temperature. The mulch should be installed in the fall or as early as possible in the spring to allow soil warming to occur before seeding or transplanting.

Mulch applicators are available commercially or can be manufactured. They generally consist of a plastic roll holder, two furrow openers, and two land wheels followed by two furrow closers mounted on a tool bar. The furrow openers create a trench on either side of the mulch. The two land wheels follow the furrow openers and hold the plastic in the furrow. The two furrow closers refill the trench, firmly anchoring the plastic. A 15 to 23cm (6-9") border of plastic is usually buried leaving 76 to 91 cm (30-36") of mulch exposed. Rototilling prior to installation creates a smooth lump free surface which helps to ensure a tight union between the plastic and the soil for more efficient heat transfer.

Planting

Planting can be done either in a single or double row on top of the mulch. Plant arrangement should be designed to make maximum use of the space available.

A plug mix planter or a water wheel planter are the most efficient means of planting or transplanting. Hand planters are available for hand seeding through plastic or transplants can be set by hand. The planting hole should be kept as small as possible to reduce weed competition in the plant row.

Research Results

Soil temperature under clear mulch was greater than 10o C (50o F) by mid April and remained above 10o C for the rest of the season. Under other types of mulches the soil temperature dropped below 10o C on several dates. Clear mulch produced the earliest harvest and highest yields of several crops when weed pressures were low to moderate and provided the most favorable cost/benefit ratio.

Chemical weed control with recommended herbicides was achieved under clear mulch. There was, however, a lasting inhibitory effect on the growth of peppers and corn even though there were no symptoms of herbicide injury. Effective chemical control measures must consider the shift toward heat tolerant weed species such as pigweed and portulaca which tend to occur under mulch. Wavelength selective mulches present a viable alternative where weed pressures are excessive and chemical control is not an option. The cost of this mulch, however, is greater.

Vegetable yield increases achieved under black mulch as compared to bare ground are not always sufficient to justify the additional cost of material, installation, and removal of the mulch. Black plastic mulch is used primarily to conserve moisture and to provide weed control.

Minitunnels

A combination of row covers or tunnels and plastic mulch often results in higher yields. A less expensive alternative of covering a trench with clear perforated plastic was evaluated on corn, tomatoes, cucumbers and muskmelons. The minitunnels were not as effective as the standard mulch/cover combinations but further study is warranted. Weed control continued to be a problem under the clear plastic minitunnels.

Rain Harvest

Areas of limited irrigation options which must rely on rainfall may benefit from the rain harvest potential of plastic mulch. Rainfall is funnelled to the plant positioned in the bottom of a shallow trench lined with plastic mulch. Excessive spring rains may be harmful to the developing seedling. No benefit is gained without rainfall.

Response of Individual Crops to Mulches

Cucumbers

Variable results were reported but clear and IRT mulches were superior. Benefits from row cover application were observed.

Peppers

Early development was advanced with clear and IRT mulch. Weeds flourished under the clear mulch. Benefits from a floating row cover or from tunnels in addition to mulch was observed. Floating row covers must be supported above the plant canopy to avoid abrasion damage to the crop.

Corn

Clear, followed by IRT, and then black mulch provided the best early and total yields, although gains were insufficient to cover the additional cost of the mulch. Row covers were as effective as clear mulch in enhancing earliness, and they may be retained on the crop for a longer period. Row covers were also effective in preventing excessive heating of the crop.

Melons

Early harvest was best under clear mulch if adequate weed control is achieved. IRT mulch was superior if weeds were a problem. Total yield at the first frost was similar for all mulches. Benefits from row cover application were observed.

Pumpkin

Higher yield, more even maturity, and better orange color were observed with pumpkin grown under IRT plastic mulch. Pumpkin, with its high moisture requirement, responds to the moisture retention provided by mulches. Initial observation indicated row cover application enhanced the weight per pumpkin, but further testing is required.

Zucchini

Clear mulch and IRT produced the best results.

Tomato

Mulch combined with a row cover appeared to enhance vegetative growth but not fruit yields. Non-covered plants were smaller but produced greater fruit yield.

Squash

The IRT mulch produced the highest yield.

Asparagus

Most herbicides for asparagus are not recommended for seedlings. Black mulch was a cost effective alternative to hand weeding, and enhanced the growth of asparagus.


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