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Irrigation preparedness

Watering tobacco at the right time and in the necessary amounts can lead to higher-quality tobacco in the field and in the barn.

Rocky Womack

While 2005 was a wet growing season for places such as Georgia, one thing is for sure—like death and taxes—another drought will eventually leave tobacco growers scrambling to move irrigation systems. Will you be prepared? When should you irrigate?

“Because of our soils and climate conditions, we will benefit from one irrigation per season,” says J. Michael Moore, Extension tobacco specialist with the University of Georgia. “For prolonged droughts, make multiple passes.”

Some growers tend to overapply water, which can reduce yield and quality. They will wet the soil as much as possible, because they don’t want to move their traveling gun irrigation systems, Moore says. This practice causes nutrients in fertilizer to leach from the soil or damages the roots of plants, essentially suffocating the roots because of the saturated soil.

To keep the plant healthy without overwatering, the University of Georgia recommends keeping the top two feet of soil moist. This will help prevent a shallow root system.

Right amount
During high temperatures in South Georgia, a large proportion of irrigated water can evaporate quickly before hitting the soil. Because of this, “I think growers overcompensate for that loss,” Moore says. “Tobacco is a tough weed. It generally will do pretty well without irrigation provided some water falls.”

Alma, Ga., tobacco grower Daniel Johnson says tobacco doesn’t require a lot of water, especially early in the growth stage. If it is extremely dry—either from lack of rainfall or continual windy conditions—at transplanting, he may sprinkle the soil with just enough irrigated water to wet the soil. Johnson irrigates using pivots, underground irrigation and pipes with water pumped from a pond. He prefers to hold off on the majority of water until layby. From layby to the budding stage, Johnson will water on a schedule, using his common sense to determine how much to apply.

Ware County, Ga., tobacco grower Donald Mixon figures he loses about one-fifth of irrigation water to evaporation, and like Johnson, he uses common sense and experience to decide when his crop needs irrigated water. “I know my soil,” he says. “I know when it needs water.”

In the early stage of growth, Mixon prefers allowing his transplants to take hold and build a healthy root system instead of irrigating too soon. “I do not like to water tobacco when it’s young. I like for it to root down. I like for the plant to get a little dry. Prior to layby it will wilt down a little bit.”

Mixon, who waters using a pivot system, prefers to irrigate at what he considers the most critical time—after layby and on until the plant flowers. He advises other growers to beware of the pivot system. “If a man is not careful with a pivot, he will overwater tobacco, because it’s so easy to push a button and put water on it. Tobacco is basically a dry-weather crop. Water is crucial at crucial times.”

Every season growers struggle to figure out just how much irrigated water is enough. Often irrigation amounts depend on soil type. Moore says sandier-textured soils need smaller amounts of water and will drain faster than clay-textured soils. Those with higher clay content and organic matter require more water to saturate the same thickness of soil.

Irrigation, when needed, promotes faster growth, which allows the plant to mature more quickly. This can cut down on early diseases such as black shank and nematodes as well as reduce insect infestation. In addition, a healthier root system will decrease the chances of plants being blown over by the wind.

Johnson prefers not to saturate the soil. “I don’t want to make the dirt soaking wet,” he says. “I don’t want that plant to blow over. You’ve got to use common sense.”

The application of proper irrigation at the right time and amounts has been shown to reduce sucker growth per plant and produce flowers more uniformly. To produce more and larger leaves, tobacco plants need water the most between the last cultivation and before the plant flowers. “It’s critical to have an adequate supply of moisture during that period of growth,” Moore says.

With proper amounts of water, the plants become more uniform in height, which in turn allows growers to top more of their crop at the same time. They can also apply sucker controls more efficiently. Moore says a uniform plant growth allows growers to top earlier, reducing the growth in the seedhead and transferring that growth into the leaves.

Once leaves thicken up after topping and get a waxy coating, the plant does not need as much water—just enough to keep it alive and prevent suffering in case of excessive heat and drought.

Irrigate or not?
Diseases, especially tomato spotted wilt virus, will keep growers guessing as to whether they should irrigate. One thing is for sure: diseases are likely to be an issue.

“In fields where irrigation water is applied after drought stress, we notice as the plants take up water, plants that are already affected will show symptoms more readily,” Moore says. “It gives the virus a chance to grow and express itself as the leaf expands.”

Most growers would probably say then they won’t irrigate. That could be a mistake, and Moore advises against this theory. “I would not hold off irrigating,” he says. “I don’t believe delaying irrigation would be a management tool, as far as controlling tomato spotted wilt. If you need water, you need water, and you need to apply an adequate amount.”

Mixon agrees and has his own theory about tomato spotted wilt. “If tobacco needs watering, you need to water it. I think if wilt is going to get you, it’s going to get you. You might as well bite the bullet.”

In 2005, Moore says 35 percent of the tobacco plants in Georgia fields started showing some symptoms of tomato spotted wilt, whether it was one leaf or the entire plant. Last year, the state experienced a 15 to 18 percent yield loss from the disease.

“As rainfall continued to add up, the increasing root damage caused plants already infected with the tomato spotted wilt virus to turn yellow earlier than surrounding healthy plants,” he says.

Timing is everything
A good time to irrigate is in the evenings, when the sun isn’t shining so intensely. Early morning is another good time, because the sun can help reduce the lingering of water on the leaves and prevent the spread of diseases such as blue mold, rhizoctonia and brown spot.

A good time to determine when tobacco needs irrigation is when growers spot leaf wilting before 11 a.m., or they may notice the soil color looks ashy and shows little sign of moisture.

“I try not to let it suffer too much,” Mixon says of his tobacco crop. “After layby to the bud forming and flower time is the most critical time for tobacco. I don’t want it to wilt down during that time period. It will start blistering and injuring the tissue in the leaves.”

Irrigation tips
Keeping some important reminders in the back of your mind when irrigating can increase your profits later in the growing season. J. Michael Moore, Extension tobacco specialist with the University of Georgia, suggests following the tips below:

• Because many growers tend to overirrigate, apply what water will saturate the root zone without causing the fertilizer to leach from the soil. Move the irrigation system to other parts of the field, allowing roots in the first field to soak up the water. Come back later to the field where you first applied water and apply some more.

• Don’t apply more water than the soil can absorb. When water runs down the row too fast, it can cause soil erosion and reduce the benefits of irrigating in the first place. It will also waste water that can be applied in other fields.

• Irrigate overnight, when conditions are cooler, so the water won’t evaporate before it hits the soil. Irrigating beginning in the late afternoon on into the early morning hours applies a greater amount of water on the ground and in the root zone. However, when growers cannot pump adequate amounts of water to reach all fields, they may not have a choice but to irrigate night and day.

• Be aware that plants already infected with tomato spotted wilt virus will show the symptoms of the disease more when the plant is watered and growing than when the growth is idle under drought conditions. Nonetheless, apply water when the plant needs it.