Does Plastic Mulch Color Make a Difference on Small Field Plots?   

In 2002 the ASCFG funded a field study undertaken by the University of Maryland Cooperative Extension to quantify whether there was a measurable difference when using different color plastic mulches for producing field grown Eustoma grandiflorum (lisianthus) and Antirrhinum majus (snapdragon).
    
Work on different colored plastics has been conducted on large production fields in California and in vegetable production fields in South Carolina. Robb et al. 2001 published the results of using aluminized and black plastics on reducing insect feeding on large cut flower plots in California. Karen Robb and her coworkers found a positive impact from using aluminum colored plastic in reducing insect feeding damage on cut flowers. (Effect Of Plastic Reflective Mulches And Plant Covers In Controlling Insect Pests And Weeds In Goldenrod, Julie P. Newman, Christine Payne, and Karen L. Robb). The cut flower fields treated with the aluminum plastic were relatively large resulting in significant reflective surfaces that deterred migratory insects from entering the fields.
    
Besides the benefit of insect deterrence, the color of the mulch is reported to increase plant vigor, and in the case of vegetable production, increase yield per acre. A USDA study, (Far-Red Light Affects Photosynthate Allocation And Yield Of Tomato Over Red Mulch. Crop Science, Michael Kasperbauer, USDA Costal Plains Research Center, Florence, S.C.) found different colors of mulch resulted in different responses that led to increased yield in vegetables and decreased insect populations in many cases.
    
In California production of cut flowers is often on large acreage and production of one plant species may be more common. Though the impact of plastic mulch color has been shown on large acreage production of cut flower and vegetable crops little if any research has been conducted on small-scale production fields that are more typical for specialty cut flower producers. Many of the ASCFG’s East Coast growers produce cut flowers on small plots, growing a mix of cut flower species. Our trial evaluated whether different colored plastic really made a difference on small plot production.

Materials and Methods

2001 Preliminary Trial. In 2001 a preliminary field trial was conducted at the Central Maryland Research and Education Center to evaluate several different colored plastic mulches including yellow, red, black, white, silver and green. In this exploratory trial the colored plastic mulches that appeared to have the most positive impact on lisianthus production were black plastic, white plastic with a black underside, silver plastic, and white plastic. Yellow colored plastic, though pleasant in appearance, was a disaster. Yellow plastic was great if your goal was attracting cucumber beetles, thrips, and winged aphids. Weed growth through the planting holes was greater with the yellow plastic compared to other color plastic mulches in the trial. The yellow plastic allowed enough light penetration for weeds to grow, raising the plastic. Cut flowers grown on green plastic did not significantly differ in plant height, number of flower buds or vigor of the plant from black plastic in our preliminary trial.
    
2002 Trial. For the field trial conducted in 2002, we evaluated those plastics that made the first cut in the 2001 trials: black, white with black underside, silver, and red plastic. All of the plastic layers were 1 ml in thickness and were purchased from commercial sources including Trickl-eez, Plastitech, and Clarke Agriculture Plastic Company.
    
The field plots at CMREC were soil tested and the pH and nutrient levels adjusted before the trial so the plots were relatively uniform. The pH was adjusted to 6.2 and phosphorus levels brought up to the optimum level determined by the University of Maryland Soil Testing lab. The area was tilled and a plastic laying machine was used to install the four different colored plastics. Four completely randomized blocks were used for the trial, with each plastic run being 40 ft in length and 4 ft wide (3 ft exposed surface as raised bed). All the beds were trickle irrigated using 2 rows of flat T-tape with 12″ on-center emitter holes. The standard weed control material used by cut flower growers is black plastic and the other plastic mulches were compared to it.
    
Philip Katz of PanAmerican Seed Company supplied plant material used in the trial.
PanAmerican supplied both snapdragon and lisianthus 288 plugs for these trials. The plugs were planted and overhead irrigated for 14 days until they rooted into the soil, then they were switched to trickle irrigation.
    
Plants were established in May of 2002.  Tenax plastic netting was placed over the plants at 16″ height for the lisianthus and 24″ height for the snapdragons. Each of the plants received a slow release fertilizer using the brand Osmocote 20-4-8 with a 3-month release.
    
Over a 3-month period Suzanne Klick (technician) with the aid of a student laborer recorded the number of harvestable stems per plant. In addition the number of open flowers and insect presence and level of damage if insects were present were recorded into an EXCEL spreadsheet.

Results and Discussion

Impact of plastic color on insect populations and  damage to flowers:
         
Based on previous work by Robb we expected dramatic results but actually the impact on insect levels did not differ significantly with the different color plastics. Thrips populations were at a very low threshold level with under 0.02 thrips (average per 40 stems harvested and examined) on all of the treatments. In 2002 the summer was hot and dry summer with one of the worst droughts on record. This dry, hot weather did not fare well for thrips populations in our trial plots. In 2004 we are having a warm, humid summer so far, and thrips populations are much higher in many outdoor growing beds. Regardless, in the summer of 2002 when we conducted the trials the thrips population never reached damaging levels in any of the treatment blocks so it is impossible for us to say that thrips are driven out by the reflective surfaces of white or aluminum coated plastic mulch.
    
Normally we would experience problems with aphids on the snapdragons. With the hot, dry summer weather aphid populations never reach high levels in any of our treatment blocks. There was great activity of hover flies and lacewings in all the treatment blocks and these predators kept the populations of aphids from reaching significant, damaging levels. One observation: if you don’t spray aphids with broad-spectrum pesticides then predators will be very active and are very efficient in collapsing aphid populations in cut flower fields.

Stem Length:
    
Stem length for lisianthus and snapdragon is an important factor for some markets. For wholesale production, the longer the stem length the better. In retail markets stem length is not necessarily as important. In our study, stem length was influenced by plastic mulch color. Lisianthus produced the longest stem length when grown on the silver plastic mulch. On  average there was an increase of over one-inch stem length for silver. Surprisingly, black plastic beat out white plastic providing slightly longer stem length. White showed an average .75-inch growth over red plastic. For the snapdragons, the mulch color that showed the greatest influence was black, with a one-inch increase in length over silver. The other colors, each with an average one-inch difference behind black were silver, white and then red.  
    
Earlier in the season temperature was measured under the plastic. The measurements showed the black to have a significantly higher soil temperature (5-10F degree) compared to the silver and white. The black plastic may have allowed the soil to warm earlier allowing the plants to form denser root systems and grow faster. The white and silver reflected more radiant energy and may have caused denser stemmed plants.

Stem Quantity:
    
Stem quantity and accompanying number of flower buds is are important factors in marketability of cut flower crop. The color of plastic mulch that showed the greatest influence on the lisianthus was silver. Silver mulch produced an average of .25 additional stems per plant over the white plastic mulch. Black plastic was very close behind the white again with an average .25 additional stems per plant over the white plastic. Red showed the lowest number of stems at .9 stems per plant behind the black plastic.  
    
The snapdragons’ number of stems was also influenced by plastic mulch color. White showed the greatest number of stems per plant over the next mulch color of silver. The white showed 2.25 additional stems per plant as compared to the silver mulch. The black was very close to the silver with silver showing only .15 additional stems per plant. Black outperformed red with an additional 1.4 average stems on the black as compared to the red plastic mulch. Although black plastic mulch is considered the standard for most cut flowers, silver and white plastic mulch can provide more saleable stems. The question is whether this increase in the number of stems justifies the higher expense for colored plastic.

Price Considerations

When doing a price comparison several factors need to be considered. Embossed plastic will last much longer in the field. It will stay close to the ground if installed properly and will not stretch when it gets warm. Plastic mulch is available in several different widths and several lengths. The figures below were obtained from catalogs in the 2002 season. When comparing four feet wide and a length of 4000 feet per roll the average pricing per roll is as follows:

Black$74.00
White on Black$107.00
Red$109.00
Silver$116.00


This works out on a per square foot basis as follows:

Black$0.0046
White$0.0066
Red$0.0068
Silver$0.007



Black plastic is obviously the cheapest price plastic mulch. White plastic with a black backing (to reduce weed growth under the plastic) is slightly higher in price than black plastic but should be a less expensive alternative if you are looking for improved quality from the white reflective surface. The silver plastic mulch is the most expensive but if you are considering covering a large area it may be worth the expense if you feel you might see the insect reduction that Robb reported in California. We certainly saw improved plant growth for plants grown on silver colored plastic mulch.

We should comment that when our technicians were installing the plant material on the silver plastic mulch they had to wear dark sunglasses to reduce the sun glare from the highly reflective surface. If your workers are looking for the best tan then have them plant into silver or white plastic covered beds.

Summary    

In summary, a producer has many factors to consider when selecting the color of mulch to use. The best results were shown to occur when large areas were covered with the same color plastic. While most growers in the Mid-Atlantic region do not produce this way, many benefits were still noticed from many of the colors.  While no color of mulch proved to be the best in all cropping situations, silver showed the greatest influence overall for the two species of cut flowers used in this trial. Black plastic mulch was very close in its overall effect on stem quantity and stem length. If a producer needs to select one color over all others based on just those factors, silver mulch would be the one to select. If the producer has the ability to select different colored plastic mulch for different varieties of flowers then other options may be chosen.

References:
2001. Julie P. Newman, Christine Payne, and Karen L. Robb
Effects of Plastic Reflective Mulches and Plant Covers in Controlling Insect Pests and Weeds in Goldenrod. University of California Davis Campus

1998. Kasperbauer, M. J., and P. G. Hunt, Far-Red Light Affects Photosynthate Allocation And Yield Of Tomato Over Red Mulch. Crop Science. 38(4): 970-974.