Using Plastic Mulches and Drip Irrigation
for Vegetable Production

Revised 1/01 -- Author Reviewed 1/01 HIL-33

Douglas C. Sanders
Extension Horticultural Specialist
Department of Horticultural Science
College of Agriculture & Life Sciences
North Carolina State University

 

Muskmelons, tomatoes, peppers, cucumbers, squash, eggplant, watermelons and okra are vegetable crops that have shown significant increases in earliness, yield, and fruit quality when grown on plastic mulch. Some less valuable crops such as sweet corn, snap beans, southern peas and pumpkins have shown similar responses. Some of the advantages and disadvantages of using plastic mulches are outlined below. (For additional information, see Bulletin AG-489, Plasticulture for Commercial Vegetables.)

Advantages

  1. Increased soil temperature: At a 2 inch depth; 4 to 5 0F under black mulch or 8 to 10 0 F under clear mulch.
  2. Reduced soil compaction: Soil under plastic mulch remains loose, friable and well-aerated. Roots have access to adequate oxygen and microbial activity is excellent.
  3. Reduced fertilizer leaching: Water runs off the impervious mulch resulting in maximum utilization of the fertilizer.
  4. Reduced drowning of crops: Water is shed from the row area and excess water runs off the field thus reducing drowning and other excess soil water stresses.
  5. Reduced evaporation: Soil water does not escape from under plastic mulch. Plant growth on mulch is often at least twice that on bare soil. The resulting larger plants will require more water, so mulching is NOT a substitute for irrigation.
  6. Cleaner product: A mulched crop is cleaner and less subject to rots due to elimination of soil splashing on the plants or fruits. Note: Beds should be firm and tapered away from the row center. Plastic should be tight to promote run-off. There should be no puddles on the mulched beds!
  7. Root pruning eliminated: Cultivation is not necessary except for the area between the mulched strips. Therefore, roots are not pruned.
  8. Reduced weed problems: Black plastic mulch provides good weed control in the row. Clear plastic will require use of a herbicide or fumigation. Often, weeds between mulch strips can be controlled by a herbicide.
  9. Earlier crops: Black plastic mulch can result in 2 to 14 days earlier harvest while clear plastic can result in a 21-day earlier harvest.
  10. Increased growth: Plastic mulch is practically impervious to carbon dioxide (CO2), a gas that is of prime importance in photosynthesis. Very high levels of CO2 build up under the plastic, because the film does not allow it to escape. It has to come through the holes made in the plastic for the plants and a "chimney effect" is created, resulting in localized concentrations of abundant CO2 for the actively growing leaves.

Disadvantages

  1. Costly to remove: Plastic mulch and drip irrigation tube must be removed from the field annually. Black plastic does not break down and should never be disked into the soil. Clear plastic does break down with time but leaves a messy field. Photo and bio-degradable plastics hold promise.
  2. Greater initial costs: Plastic mulch and drip irrigation will increase cost of production. These costs should be offset by increased income due to earlier harvests, better quality fruit and higher yields.
  3. Increased management: Plastic mulch and drip irrigation must be carefully monitored (daily) to be successful.
  4. Increased soil erosion: Soil erosion in-creases in middles between plastic strips.
  5. Increased crop/weed competition: Weeds can grow out of the holes in close proximity with crops.

Preparation of the Soil - The first step is to take two soil samples in early fall. Have one sample assayed for mineral content and one for nematodes. If the soil test suggests applying lime, apply enough in the fall to reach pH 6.0 to 6.5 using dolomitic lime if magnesium is low. If there is a nematode problem, fumigate the soil at the time the plastic mulch is being laid. By using a multipurpose fumigant (e.g., Methyl Bromide, Terr- O-Gas 67, Vapam), you can get good control of weeds, nematodes and soil-borne diseases. The soil should be free of debris and in good physical condition prior to preparing the beds for fumigation.

Fertilization - Using the soil test report as a guide, apply fertilizer during bed preparation. Consult Horticultural Information Leaflets for specific crop recommendations. Amounts to be sidedressed need to be included in the total fertilizer requirements. Caution: Using fertilizers with ammoniacal N in fumigated soils can result in ammonium toxicity to the crop. Normally, at least 50% of the nitrogen (N) should be in the nitrate (NO3) form.

When using drip irrigation with plastic mulch, one half of the N and K and all of the P should be incorporated at bedding. The remaining N and K should be applied through the drip tube using soluble fertilizers (e.g. calcium nitrate, sodium nitrate, 20-20-20, 15-0-14, or potassium nitrate). Overhead irrigation and fertigation can be used by perforating the plastic. The entire amount of fertilizer may be incorporated in the bed but utilization by plants might be less efficient than with fertigation.

Bedding the Soil - Bedding machines available to growers include Kennco Mfg. Inc. and Riddick Equip. Co. single and multiple row models. With some bedding machines, the soil is lifted and then bedded in one operation (Superbedders). With others, the soil is first lifted in one operation with hilling disks or double disk hillers on a tool bar and then compressed to a uniform height and density using a bed press pan. Note: Be sure that enough soil is pulled up so that the bed has good sharp corners. Bedded rows should be spaced on 5- or 6-ft centers depending on the equipment. A bed with a 30 inch top should slope from the center to the edge with a drop of 1.25 inches, allowing excess rainfall to run off the mulch.

Fall vs. Spring - Laying the plastic mulch and drip tube plus fumigating in the fall has several advantages:

  1. Wet weather conditions are prevalent in the spring and can result in delays in laying the plastic and in transplanting.
  2. Better soil fumigation is possible in the fall because the soil is not apt to be too wet or too cold.
  3. Nematode populations should be the highest in the fall and maximum eradication is possible at that time.
  4. Fall applied mulch often makes it possible to transplant 10 to 14 days earlier in the spring.

* Consult theNCCVR (North Carolina Commercial Vegetable Recommendations, AG-586) or your county extension agent for the most recent recommendations on pesticides.

Fumigating and Laying the Plastic Mulch and Drip Tube - The amount of fumigant actually applied per acre will depend on row width and will be a percentage of the broadcast rate (Consult theNCCVR, North Carolina Commercial Vegetable Recommendations, AG-586). Air temperature should be at least 50F and the soil should be well worked, free of undecomposed plant debris and have adequate moisture for seed germination. If both the air and soil are warm most fumigants should escape through the plastic mulch within 12 to 14 days.

The plastic mulch is generally 4 or 5 ft wide, 1.25 to 1.50 mil thick, embossed (diamond shaped design on film which helps hold mulch tight against the soil) or slick, and comes in 2,400 ft rolls.

For single row crops such as tomatoes, cucumbers, muskmelons, honeydews, watermelons and pumpkins the drip tube should be placed 4 to 5 inches from the center of the bed and 1 to 2 inches deep with the emitters facing upward. There are 1,000 to 7,500 ft of drip tube on a roll depending on the brand. For double row crops like summer squash, okra, eggplant, peppers, beans, peas, lettuce and sweet corn, the drip tube should be placed directly on the center of the bed and buried 2 to 3 inches deep. For 5-ft row centers there are 8,712 linear ft of row per acre, so a grower would require about 3.5 rolls of plastic mulch per acre. For 6-ft centers, 3 rolls of plastic mulch will be required per acre. Note: Take time to adjust the machine so that the press wheels hold the plastic firmly against the bed and the disks place soil halfway up the side of the bed but not on top of the bed. Also, anchor the plastic and drip tube when starting applications, covering it with soil and standing on the drip tube.

PEST MANAGEMENT

Weed Control - For information on weed control under clear plastic mulch and in the row middles between black plastic mulch consult Horticultural Information Leaflet No. 33-D, the NCCVR (North Carolina Commercial Vegetable Recommendations, AG-586) , or your county Extension center. Only approved herbicides can be used between rows of plastic, because this is not a fallow area.

Insect and Disease Control - Good insect and disease control is essential. Consult the current NCCVR (North Carolina Commercial Vegetable Recommendations, AG-586) or your county Extension center for recommendations.

Transplanting - For extra earliness in peppers and tomatoes, large containers (cell sizes 3 to 4 inches) should be used. For the other vegetable crops use 1 to 2 inch cell sizes. Consult Bulletin AG-337, N.C. Commercial Transplants for details on transplant production. Transplants can be set by hand or machine (e.g. Kennco plant-setter, waterwheel or pot transplanters). When transplanting by hand, several tools can be used to make holes in the plastic such as a long handled bulb setter or a sturdy can or cylinder welded onto the end of a handle. The hole should be 2 to 4 inches wide and deep enough for the plants to be transplanted. A hand tobacco plant setter works well once the holes are made in the plastic mulch. With both hand setting and machine setting, the use of a "starter solution", a soluble fertilizer high in phosphorous (P) will often get the plants off to a good start. Examples are 12-52-12, 10-20-10, or 12-48-8.

Irrigation - Drip irrigation is recommended for use with plastic mulches although other types can be used successfully. The frequency of irrigation will depend on soil type and stage of crop growth. Irrometers at the 6 inch and 12 inch depth in the mulched bed are recommended as an aid in determining irrigation needs. Frequent probing with a soil tube near the plant row will also help to keep a check on soil moisture. Normally the area around the drip tube is very soft to the touch and the side of the row away from the tube should be only slightly soft. For more detailed information on trickle/drip irrigation consult a Horticultural Information Leaflet Nos. 33-A and 33-B, The Irrigation Handbook, Dept. of Biological and Agricultural Engineering, Trickle Irrigation in the Eastern United States prepared by Northeast Regional Agricultural Engineering Service, Cornell University, Ithaca, NY 14853, an irrigation specialist, or your county extension center office. DO NOT USE PLASTIC MULCH WITHOUT IRRIGATION.

Double Cropping the Plastic Mulch - Once the first crop has been harvested it is recommended that a second crop be grown on the mulch. This "intensive cropping" results in two acres of production from each acre of actual land. The second crop can be fertilized (1) through the drip line using soluble fertilizers and a fertilizer injector, (2) through overhead fertigation, or (3) by placing fertilizer in holes in the plastic between plants. Consult Horticultural Information Leaflet No. 33-C for additional information on injecting fertilizers through the drip line.

Never plant a field to the same crop twice in one year!

Suggested Spring-Fall Planting Sequences.

Spring

Fall

Peppers

Summer squash, cucumbers or cole crops

Tomatoes

Cucumbers, summer squash or cole crops

Summer squash

Tomatoes or cole crops

Eggplant

Summer squash

Cucumbers

Tomatoes

Muskmelons

Tomatoes

Watermelons

Tomatoes

Honeydews

Tomatoes

Cole crops

Summer squash, pumpkins, muskmelons, tomatoes

Cauliflower

Summer squash, pumpkins, muskmelons, tomatoes

Snap beans

Summer squash, pumpkins, muskmelons, tomatoes

Southern peas

Summer squash, pumpkins, muskmelons, tomatoes

Lettuce

Summer squash, pumpkins, muskmelons, tomatoes

Sweet Corn

Summer squash, tomatoes, or cucumbers

Strawberries

Tomatoes, summer squash, cucumbers, pumpkins

Glyphosate (Roundup and various other trade names) can be used to terminate the first crop. See Horticulture Information Leaflet No. 33-D. Note: Take care to avoid damaging the trickle/drip tube when planting the second crop.

Windbreaks - Strips of rye should be established to protect vegetable seedlings from prevailing winds. Each rye strip should be the width of a typical grain drill (8 to 10 ft) and far enough apart to plant 5 or 6 rows of vegetable seedlings. Well grown rye strips planted in the fall will promote earliness and provide protection for the young transplants. Spring topdressing in February will help assure a good thick rye stand.

When laying plastic in the spring, plant the entire field with rye but be sure to work up the crop area early enough in the spring to minimize crop debris interference with fumigating and plastic laying. Once wind protection is no longer required, mow the rye and use this area as a drive row for spraying and harvesting. Use this drive row for a boom sprayer that covers 2.5 or 3 rows on either side of the drive. An airblast sprayer can also be used.

Reflective Plastic Mulches - The reflective properties of aluminum faced plastic have been shown to interfere with the movement of aphids which spread the watermelon mosaic virus II. This virus causes the green streaking in the fall yellow squash. By using this mulch, a grower would be able to harvest marketable squash for a longer period of time in the fall. Painting the plastic with aluminum paint or white paint increases its reflectivity and cools late planted crops resulting in better fruit quality.

IRT Mulch - InfraRed Transmitting (IRT) mulch is a recent development. These plastics transmit the warming wavelengths of the sun, but not those that allow weeds to grow. These materials result in warmer soils than black plastic, but cooler soils than clear plastics. The IT mulches retard the growth of weeds including nutsedge. Crops grown on IT mulch will develop 7 to 10 days earlier than crops grown on black plastic.

Some Yield Increases - Plastic mulch systems can produce significant yield increases, if managed properly.

Examples of Yield Increases.

Crop

Average yield per acre with plastic and drip

Increase over state average

Eastern cantaloupe

6000 fruits

4X

Western cantaloupe

15000 fruits

5X

Cucumbers

1200 bu

5X

Pepper

1200 bu

4X

Squash

800 bu

4X

Tomato

2500 boxes

3X

Watermelon

3000 fruits

4X

Final Comments - In addition to the machines you can purchase for laying plastic mulch and drip tube and fumigating the soil, custom applicators are also available. With proper planning, good management, attention to details and dedication to all aspects of the cropping sequence, earlier and higher yields are possible using the "intensive" cultural methods described in this publication.


Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact an agent of the North Carolina Cooperative Extension Service in your county.

Published by the North Carolina Cooperative Extension Service


Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.