Use of plastic mulch is common throughout New York, particularly for vine crops, peppers, and tomatoes. Several types of plastic mulches are available. All protect ground-level fruit from soil pathogens, conserve soil moisture, reduce leaching of mobile nutrients such as nitrogen, and warm the soil. The disadvantages of mulches include the environmental cost to produce and dispose of the plastic and the cost of materials and labor for application and removal. In addition, although they conserve soil moisture, rain and irrigation water may never reach the roots if the soil is dry when mulches are applied.
Black plastic is probably the best weed control measure available and a good alternative to herbicides. Two main disadvantages of using black compared to clear plastic are that (1) soil temperatures are cooler under black plastic than under clear, so black plastic is less effective at stimulating early crop growth and yield; and (2) if black plastic is used with a row cover, air temperatures can become excessive on warm days and damage the crop.
Clear plastic causes warmer soil temperatures than black plastic, resulting in earlier harvest. Some growers also claim that clear plastic leads to larger fruit size and better quality. The main disadvantage of clear plastic is weed control. Clear plastic creates an ideal situation for weeds, and herbicides must be used to prevent harm to the crop.
Infrared-transmitting (IRT) plastic is relatively new and more expensive than conventional plastics, but it may be worth trying because of its special properties. Basically, IRT plastic is a hybrid between clear and black plastic in that it prevents weed growth (as does black plastic) by screening out light energy the weed seedlings need to grow but allows infrared light to pass through, thereby warming the soil more effectively than black plastic. In trials at Cornell University, soil temperatures under IRT mulches have been halfway between clear and black plastic; IRT usually results in greater early yields than black plastic but lower yields than with clear plastic.
Reflective, aluminum-faced, plastic mulch interferes with the movement of aphids, which are insect vectors of diseases such as cucumber mosaic virus. Use of reflective mulches in regions with significant insect pressure reduces the spread of these diseases.
Red, white, and yellow plastic mulches have been tested for their effect on early yield of some crops. Although results have been inconclusive, the theory behind the use of colored mulches is sound. Plant development (e.g., stem elongation and flowering) is sensitive to the ratio of far-red to red wavelengths that strike the leaves and shoots. Different mulch colors affect this ratio and therefore can potentially affect plant development and possibly increase early yield. Initial studies conducted by the USDA and other researchers suggest that certain crops had higher yields with specific colors of mulch, independent of the effect on soil temperature. Research with tomatoes at Cornell showed no significant yield advantage using colored mulches. More conclusive information and guidance for growers may be available at a later date.
Use of photodegradable plastics has increased because of environmental concerns and regulations regarding the disposal of nondegradable types. The products now on the market usually degrade thoroughly once the process begins, but inaccuracy in timing of breakdown has discouraged some growers. It is usually necessary to experiment with a few different formulations to find what will work best for a particular farm management system. Buried edges must be brought to the surface at the end of the season and exposed to light before they will degrade, but these remnants have not been a major problem for most growers. The primary byproducts of degradation are small amounts of carbon dioxide and water, which are relatively harmless. Trace amounts of nickel or other elements (depending on type) may also be left behind. Biodegradable plastics exist, but none are currently being used on a large scale for mulch film in the United States. Another option, recycling of agricultural plastics, requires a considerable infrastructure for collecting, cleaning, and reusing the plastic, that does not yet exist in the United States.
7.1.2 Application and Disposal
Before laying plastic mulch, the soil should be prepared using special precautions. Good soil moisture is essential at the outset because supplemental water applied later through the holes where the transplants are placed usually will not be adequate for maximum growth. Many growers use drip irrigation under the plastic, which is an excellent, although costly, technique for ensuring optimal soil moisture and best response to the mulch.
A tight-fitting mulch, which requires a flat soil surface, will help control weeds by burning seedlings as they touch the plastic. It also prevents a whipping action that can damage transplants on windy days.
Initial fertilizer and herbicide applications must also precede laying of plastic. Late-season supplemental fertilizer applications at the outer edge of plastic can be effective when plants are large enough to have roots in this region. "Fertigation," feeding liquid fertilizer through a drip irrigation system, is another option. See Chapter 8, Soil Management, Section 8.10.6, Fertigation.
Most growers use a commercially-available plastic layer for installation. Disks are used to open small trenches on each side of the plant row, and then the edges of the plastic are buried, so that the plastic can be stretched tightly over the row. Mechanical transplanters work well at putting transplants in through the plastic.
Plastic is usually removed by hand at the end of the season. Thin (1 mil) plastic may tear more easily and be more difficult to remove than thicker or reinforced plastic. Machines can be used if there is considerable acreage to pull. Some growers use two-row potato diggers; others slit the mulch down the middle and then hand pull or spool the plastic.
7.2 Row Covers
The ideal row cover would prevent air temperature from falling below freezing at night, increase growing degree-days, and prevent excessively high temperatures that can have a severe negative impact on growth and reproductive development. Unfortunately, no such material exists. Row covers only offer a few degrees of frost protection and even those few degrees cannot be guaranteed. Some spunbonded fabrics appear to be one or two degrees better than plastics in this regard, but results are inconsistent, and you should not rely on row covers for frost protection at temperatures below 30°F. Their major benefit is associated with more rapid growth by increasing daytime temperatures. Heavyweight, freeze-blanket covers are available for frost protection, but they block much of the sunlight and therefore slow plant growth.
White plastic and fabric tunnels are cooler than clear plastic. These materials minimize the risk of damaging high temperatures, but are less effective in increasing early growth and yield. Some growers hedge their bets by using a cooler tunnel (or hot caps) for part of their acreage and a warmer, riskier system for the earliest yields on the remainder
The decision to use clear, black, or no mulch under a tunnel is critical. Maximum daytime temperatures with a black mulch and row cover can be 10°F higher than with clear mulch or no mulch on a warm, sunny day. This is because the black plastic reradiates much of the incoming solar energy as heat into the air above it rather than allowing the energy to pass through and warm the soil below. In general, to minimize the risk of early yield loss caused by high temperatures, cooler row cover materials should be used with black mulch for weed control.
7.2.2 Installation and Management
Most plastic (polyethylene) row covers are supported by wire hoops. After the crop is planted, 10-gauge, hardened, galvanized wire pieces (36 inches long for four foot plastic, 48 inches for five foot plastic) are bent in the shape of an arch and placed in the soil at three to four foot intervals. The tunnel plastic should be 12 inches wider than the plastic mulch. Hoops should not be placed directly over the plants, so that slits for ventilation or irrigation can be made later if necessary. The height of the tunnel (usually eight to 14 inches) is determined by the clearance required for field equipment and the crop being grown. Commercial row cover equipment is available. With some modifications, any mulch applicator can be converted for use with row covers.
Floating row covers made of lightweight spunbonded or woven fabrics can be placed loosely over plants without wire supports. They are available in widths of approximately five to 40 feet. The edges are held down with soil, large rocks, or irrigation pipe. Floating covers are easier to apply than plastic row covers over hoops, so labor costs are lower. The materials are usually two to three times more expensive than polyethylene film, but the fabric may be reused if it has not been severely damaged by deer or ripped during removal from the field. A unique advantage of fabrics is the exclusion of insects. In addition, excellent yield response has been observed for vine crops. Under windy conditions, abrasion sometimes occurs with upright crops such as peppers and tomatoes.
Proper ventilation is essential. Growers using unvented clear or white plastic must drill holes (1/2 to 3/4 inch in diameter and six to eight inches out from the center line) in the roll before application. Small, four to six inch slits are made after the tunnels are laid if temperatures have become excessive or if overhead irrigation is required.
Allowing excessive heat buildup is the most costly and most common error made by first-time row cover users. In general, if outside air temperatures approach 80°F, additional ventilation should be considered. In unseasonably warm years, covers may do more harm than good and must be removed soon after they are applied.
Normally, covers are removed three to four weeks after transplanting. The plants will be too large for the cover by that time, bees will need access to certain crops for pollination, and daytime temperatures will be too warm. Growers should allow plants to harden by gradually increasing the size of slits three to seven days before complete removal.
7.2.3 Specific Crop Responses
Selection of row cover material and management techniques depends primarily on the sensitivity of the crop to high temperatures. Normally, a cooler tunnel should be used with crops that are more sensitive to high temperatures.
Muskmelon. Use of tunnels, especially those made from clear plastic with 1/2 to 3/4 inch holes for ventilation, has been most successful with this crop. Production can be seven to ten days earlier and yields can be increased. Melons are not particularly sensitive to the high temperatures that may develop with the clear plastic tunnel. Temperatures above 95° to 100°F can be detrimental, however. Tunnels are normally removed when the plant begins to flower.
Summer squash. This is one of the easiest and most responsive crops to grow under tunnels.
Cucumber. This crop is more sensitive to high temperatures than melons but can show a positive response to tunnels. When temperatures are cool, a clear tunnel with holes may be best, but fabric tunnels or white plastic with drilled holes are safest. Some growers use black mulch in combination with a cooler row cover.
Pepper. High temperatures, above 90°F for some varieties, can cause flower abortion and loss of early yields. Adequate ventilation is critical, and ventilated white plastic tunnels have been used (usually with clear mulch) to obtain faster growth and earlier yields.
Tomato. In general, row covers are not recommended for this crop because the high temperature retards yields. Vegetative growth and total yield may be enhanced by tunnels, but early fruit are frequently small or nonexistent. Nevertheless, some growers claim success. If you wish to experiment, start with a cool (e.g., hot-caps, clear-slitted, or fabric) tunnel-mulch system. A high tunnel, 12 to 24 inches, is required.
Cole crops, lettuce, spinach, and celery. Increased earliness of these crops has been reported with tunnels, but the prices paid may or may not warrant the use of row covers. Most of these crops are well suited to the use of floating covers.
Sweet corn. A unique system using clear plastic has been developed for this crop, which allows earlier planting, improved germination percentage, and maturity five to twelve days earlier. The corn is planted in two rows 16 inches apart. The seed is planted in a shallow trench with soil slightly mounded between rows. Clear plastic is then laid over the rows. The plastic is removed when the plants are six to 12 inches high or sooner if temperatures under the plastic approach 100°F or more, which occurs when the ambient temperature is 80° to 85°F. Floating fabric row covers, which have also been used with success, can be left on longer. Row cover systems are particularly effective on the cold-sensitive "supersweet" or "shrunken-2" corn varieties.
7.3 Rye Strip Windbreaks
Rye strips are a fairly inexpensive and easy way to protect young seedlings from cold winds and blowing soil. Vine crops are particularly sensitive to wind. A rye cover crop planted in the fall can supply strips the following spring. Strips 15 to 20 inches wide and spaced ten to 20 feet apart afford the most protection. Strips spaced farther apart, such as in field roadways, are less effective but still may reduce wind damage and promote early growth.
7.4 Raised Bed Systems
For many years farmers have grown vegetables on raised beds composed of soil that has been scooped up from between the beds. Raised beds have not been used extensively in New York or other parts of the Northeast, but the system offers several advantages even though furrow irrigation is not a consideration
- Excess water drains readily from the root zone because the surface of the bed is six to eight inches higher than the bottom of the furrow which improves soil aeration. Plant roots need oxygen to grow and function properly, and good soil aeration also lowers the potential for development of root rot.
- Soil in the upper part of the bed is drier and thus warms more quickly in the spring than does soil on level ground. This promotes germination and rapid seedling growth, a distinct advantage in New York State, where the spring season is often relatively cool.
- The mounded soil gives plant roots deeper soil in which to grow. Established plants withstand drought better on raised beds than on level beds. The added soil depth is also useful when growing crops such as carrots or parsnips, where root length is important.
- Wheel traffic is restricted to a narrow zone between the beds. Even during bed formation and shaping, the equipment is restricted to the furrow area. Some soil scientists urge that beds be maintained in the exact location year after year so as to direct all wheel traffic to specific areas. This idea makes good sense but is difficult to implement.
- Because water drains from the surface of raised beds, fruit and other plant parts do not sit in water after a rain. The soil surface dries more quickly, and air drainage is improved over that of level cultivation; this helps reduce incidence of disease.
No tillage system is perfect. Forming raised beds requires special equipment, and it takes time to throw up ridges and shape them into beds. The soil in the bed must be firmed to prevent planter units from sinking into the soft surface and placing the seed too deep or leaving it at the bottom of a deep depression that will fill with soil in a heavy rain. Unfirmed soil will dry quickly, adversely affecting germination and early plant growth. A roller built to conform to the bed dimensions works well, as does equipment from the West Coast, which presses down the soil as the bed is shaped.
When beds are established on sloping land, rainwater tends to run down the furrows and make ponds in the low areas. Equipment has been developed to make cuts across the beds, that allow the water to move to the lowest part of the field where it can be channeled away. However, this type of equipment has not been used commercially in New York.
If moisture is adequate and beds have been firmed, germination should not be a problem on raised beds. In dry or windy conditions, however, irrigation is essential. Do not plant small-seeded crops, such as carrots, on raised beds if irrigation is not possible. A solid-set irrigation system in place and ready to go is good insurance. Once the plants are growing well and have adequate root systems, they can withstand dry weather because the root systems have been able to spread throughout the large volume of soil contained in the beds.
Short-term deficiencies in rainfall during critical crop-growing periods occur in some parts of New York almost every year, and occasional years of severe drought can be devastating. Complete reliance on erratic rainfall patterns for water needs is becoming less acceptable to New York growers, particularly those who are attempting to expand into markets that demand year-to-year consistency in quality, yield, and time of harvest. Many vegetable crops (e.g., tomatoes, peppers, and cucumbers) can develop physiological disorders and poor quality as a result of relatively minor fluctuations in water supply.
See the Network for Environment and Weather Applications (NEWA) for evapotranspiration information for selected sites.
Scheduling should not rely on plant cues such as change in foliage color or wilting to schedule irrigation. Well before such visible symptoms appear, mild water stress will have slowed the rate of leaf expansion and, depending on the crop and growth stage, may have affected reproductive development, so that fruit maturation becomes unpredictable, and marketable yield potential is irreversibly reduced.
Monitoring soil water status directly is often the best way to determine when to irrigate. This does not have to be complicated or expensive, but it does involve checking soil moisture at the roots because looking at the top inch or two of the surface soil is not informative. In addition, parts of the field with different soil type, slope, and drainage characteristics must be monitored separately.
Several sensors are available that can be placed in the field at various depths and locations to monitor soil water status. Tensiometers, which usually cost between $30 and $50, measure soil dryness using a vacuum-based system. The unit of measurement is a centibar (cb), and the drier the soil, the greater the number of centibars. The texture of the soil influences the soil tension range measured by tensiometers. The placement of tensiometers in the field is extremely important. They should be placed where plant roots are actively growing, usually at a depth of six to 12 inches and within six to 12 inches from the plant's base. When using trickle irrigation, place the tensiometer close to the tape. It may be useful to place tensiometers at various depths to determine how deeply the irrigation or rainfall has penetrated. One caution when using tensiometers. They go off scale easily if the soil dries (above 80 cb), and they must be refilled with water and vacuum pumped by hand to become functional again.
Gypsum blocks and ceramic moisture sensors operate on an electrical principle. They are not expensive, but a voltmeter (usually $150 to $250) is required to use them. They are placed in the root zone as are tensiometers. Ceramic sensors have a narrower operating range than gypsum blocks and therefore tend to be more accurate. A disadvantage of both electrical types is that their calibration may lose accuracy with time, particularly if used for more than one season.
Another approach to scheduling irrigation, usually referred to as the "water budget" method, is much like balancing a checkbook and involves keeping track of "deposits" (rain and irrigation) and "withdrawals" (crop water use). Weather records or evaporation pan data can be used to derive useful approximations of potential crop water use. This is known as potential evapotranspiration or ET. During the months of July and August, expect ET rates of 1 to 1 1/2 inches of water per week with lower values during other months. A newly transplanted crop might use only 20 to 30 percent of these values, but as the crop matures, it will typically reach a maximum between 90 and 110 percent of potential ET. Maximum water use can be much lower (e.g., 50 to 60 percent of potential ET) if plants are widely spaced, plastic mulch and drip irrigation are used, or temperatures are cool, and humidity is high.
Knowing how much water to apply at any one irrigation requires an estimate of the crop's rooting depth and the water-holding capacity of the soil. Approximate rooting depths of various vegetable crops are presented in Table 7.5.1. Light irrigation is needed more frequently at early seedling stages because the plant has only a small soil water reservoir. Later in the season, less frequent but deeper irrigations are used to replenish a larger rooted volume. Information on water-holding capacity is important so as to avoid adding more water at any one time than the soil can hold. Light-textured soils hold less water than do heavy clay (and most muck) soils; thus a grower with a sandy soil will irrigate more frequently and apply less at each irrigation.
Most vegetable growers in New York currently irrigate with hand-portable or traveling gun sprinkler systems. A few processing vegetable and potato growers use center pivot systems. In designing any of these systems, it is important to work with a good engineer who can help you select the correct pump, pipe size, lateral spacing (or timing system), nozzle size for your water supply, crops and acreage to be irrigated, and soil type. The goal is to complete an irrigation cycle in a reasonable time period, apply water uniformly, and achieve a water delivery rate that does not exceed the infiltration rate of the soil.
Table 7.5.1 Approximate rooting depth1 of various vegetable crops
(Down to 1 1/2')
(Down to 2')
(Down to 4')
(Down to 6')
|Muskmelon||Pepper, seeded||Tomato, seeded|
|1These values represent depths to which the roots of mature crops will exhaust the available water supply when grown in deep, permeable, well-drained soil. Soils of many regions of New York do not fit these criteria, and rooting depth may be restricted to the upper 1 to 2 feet regardless of the crop being grown.|
7.5.3 Frost Protection with Sprinklers
Sprinkler irrigation can protect crops against late-spring or early- fall frosts (as low as 20° to 25°F, depending on wind speed and other factors). Heat released as water freezes protects the crop. It is best to start the irrigation before the air temperature reaches freezing and to continue irrigating as long as any ice remains on the plants. Only about 1/10 inch of water per hour is needed, but the sprinkling must be continuous and the sprinklers should make a complete rotation at least once each minute.
This type of irrigation system involves placing low-flow rate drip tubing along the plant row to allow frequent application of small amounts of water directly to the root zone. It is highly efficient, often cutting water use in half compared to sprinkler systems. There is less potential for leaching of nutrients and pesticides into groundwater, and when soluble fertilizer is applied through the system (fertigation), nitrogen fertilizer needs may be reduced by 20 to 40 percent. Foliar and fruit disease pressure is often lower than when using sprinklers because of lower humidity, and the leaves and fruit are not wetted during irrigation. It is particularly effective when used in combination with black plastic mulch. High-value vegetable crops that respond particularly well to the mulch and drip irrigation/fertigation system include tomatoes, peppers, and vine crops.
Many options are available when designing a drip irrigation system, so you should find a dealer with a good engineer who will work with you on the details. The five major components of any system are (1) a pump (seven to 10 horse powers electric or 12 to 20 horse powers gas for a two to five acre system); (2) filters (disk, screen, or sand type); (3) pressure regulators (spring or valve); (4) valves (hand-operated, hydraulic, or electrical); and (5) delivery tubing (PVC or lay-flat hose for main lines; polyethylene line source drip tubing or emitters for plant rows). Two major optional components are also available: an automatic control system (electric clock or computer) and a fertilizer injector (electric or hydraulic pump or venturi suction type).
Delivery rates in a drip system are low, from 1/20 to 1/10 inch per hour, necessitating more frequent irrigation. The system is designed to allow better control of soil moisture near optimal levels. You should not wait one or two weeks and then run the system for ten or more hours in an attempt to apply an inch of irrigation water.
The primary disadvantages of trickle irrigation are the costs of hardware and drip tubing and a manager to run the system. The initial investment in permanent equipment is probably less than for a sprinkler system, but drip tubing only lasts one to two years and may cost $150 to $200 per acre to replace. Total labor hours are usually less than those required for a hand-portable sprinkler system, but more management time will be necessary, at least in the first few years while developing an effective approach to irrigation scheduling and equipment maintenance.
Another problem with trickle irrigation is the clogging of emitters by algae, bacteria, and iron. This can be controlled by chlorination of the trickle system. Periodic treatment, before clogging occurs, will keep the system running efficiently. The frequency of treatment will depend on the quality of the irrigation water. Two to three treatments during the season should be sufficient. More treatments are needed for poorer quality water. Remember, chlorine treatment should occur upstream of filters so as to remove the contaminants from the system.
Trickle irrigation is an excellent way to deliver fertilizers in a timely fashion. See Chapter 8, Soil Management, Section 8.10.6, Fertigation.
Maintained by Abby Seaman, New York State IPM Program. Last modified 2017.