Q: Is it true that putting red plastic under tomato plants will increase yields by twenty percent?
A: A great deal of research has been done on different color plastic mulches in the last decade. Scientists have tried red, blue, and mirrored mulch to see if they affect plant yields and insect damage. Red mulch seems to have the most effect on tomato and pepper harvests but the color of the mulch is very important. Experimenters found that different red mulches reflect light differently; some even fade in color so quickly that no effect is found. Other research found that blue and shiny aluminum foil mulch repel aphids and thrips.
Beyond its effect on yields, plastic mulch also conserves moisture, blocks weeds and warms the soil to encourage growth. That said, I encourage you to try different plastic mulches and see how they affect your tomatoes. Just make sure to collect and dispose of the mulch properly each season.