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United States Department of Agriculture

Agricultural Research Service

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Colored Mulch Research
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How Scientists are "Tricking" Plants

image of magician pulling a rabbit out of his hat

Colored Mulch Research

Background

Plants are very competitive organisms. They are always competing for space, nutrients, sunshine, and water. Plants must endure hardships such as disease, insects, and weather. They must have defense mechanisms and sensory structures to battle these environmental factors. Plants also have to battle each other. They have a way to detect each other and a method to compete with their surrounding neighbors. Plants have a substance called phytochrome that acts as a sensor to detect changes in the color of light that is reflected from the surrounding environment. Plants use light as a signal that enables them to compete with their surroundings. They do not know if the signal is a neighboring plant, dead plants on the surface of the soil, or even the color of the soil. The plant recognizes far-red light as the signal. If the plant detects an abundance of far-red reflection, it thinks that there must be other plants growing nearby. The phytochrome will then signal the plant to put more energy (photosynthate) in the top of the plant (shoot) instead of in the bottom of the plant (roots). The plant, in effect, is trying to outgrow its competition.

What Is Colored Mulch?

image of strawberries grown over black mulch

If you have ever picked strawberries, you have seen the black plastic that covers the soil of each of the rows in a field. Farmers have used black plastic for years to reduce the amount of weeds near the crop, to warm the soil in early spring, to keep soil from drying out, and to prevent the soil from splashing on the fruit. Because this plastic is black in color, it will absorb the sun's energy and keep more heat underneath the plastic. Since researchers knew that different colors reflected different wavelengths of light, they began to ask questions like, "Would other colors increase plant growth, but provide the same favorable conditions as the black plastic?" The researchers decided to devise some other colors of plastic. This plastic with various pigment combinations is called colored mulch.

What Does Colored Mulch Do?

Colored mulch mimics the reflective patterns of the green leaves of neighboring plants. The plant will sense the increased ratio of far-red to red light as though it is reflected from the nearby plants, when in fact it is just the colored mulch. The colored mulch "tricks" the plant into putting more energy into shoots to outgrow other plants. Some colored mulch even "tricks" the plant into producing more and better tasting fruit.

How Did the Scientists Actually "TRICK" the Plants?

image of green tomatoes growing over red mulch

Plastic mulch only came in black, white, or clear. In order to make the other colors, the scientists first used paint to convert the black plastic to other colors. The scientists then measured the reflection from colored plastic with an instrument called a spectrophotometer, which records the amount of light at different wavelengths reflected off the plastic. The scientists grew tomatoes in soil covered with different colors of mulch to see what would happen next.

What Were the Results?

Tomatoes that were grown over red plastic had larger shoots and smaller roots than plants grown over other colored plastic such as white or black. Since the plastic keeps the soil moist and protected, a slightly smaller root would not harm the plant. For tomatoes, using the red colored mulch gave a 20% increase in the first harvest of tomatoes. This is important to farmers because the first fruit of the season can bring in the most money. For all crops, the key is the amount of far-red light that is reflected. In plastic mulch plots, the plant senses an increase of far-red light and will put more energy into the shoot and less into the root. Therefore, if the fruit is produced in the shoot, it will usually be larger.

Have Scientists Used Any Other Plants in Their Research?

Scientists have also done research with strawberries, turnips, peppers, peas, beans, and cotton. They have used colored mulch to determine if other plants will try to outgrow each other, or put more energy in their shoot. They found certain colors of reflected light can change the flavor of some fruits and edible roots (such as turnips and carrots).

Why Is this Research Important?

When colored mulch is used in agriculture, crops are expected to produce larger fruit and possibly even better tasting fruit. This could mean larger and better tasting fruits and vegetables in grocery stores or in home gardens.

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Last Modified: 8/12/2016
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