Researchers have found that with greenhouse plants, applying pesticides to the bottom, not the top, works better and saves labor costs, too.

A University of Georgia study compared using subirrigation to apply pesticides in commercial greenhouses to putting pesticides on the potting soil surface. Subirrigation, also known as ebb-and-flow, is widely used in Europe.

"It's a closed system where the growing area is flooded with water or fertilizer from a holding tank," said Marc van Iersel, a horticulturist with the UGA College of Agricultural and Environmental Sciences.

"After the potting medium absorbs the water," he said, "the flow reverses and the water flows back into the holding tank."

This system is known to work well for applying fertilizers. But there had been no research into whether it would work for applying pesticides.

The Pesticide Becomes Part of the Plant

"We used systemic pesticides for the study," Van Iersel said, "as they are much more effective for much longer than spray pesticides. Plants take up the systemic pesticides, and when insects feed on the plants, they feed on the pesticide."

For the study, Van Iersel and CAES entomologist Ron Oetting used poinsettia plants that were infested with whiteflies. Whiteflies are major pests of poinsettias, pansies, petunias and many other greenhouse plants.

The pesticide used for the study was imidacloprid, the active ingredient in Marathon.

"This pesticide is traditionally applied to the surface of the potting medium, either as a drench or as granules," he said. "The water carries the pesticide down into the pot. Using subirrigation, the pesticide is delivered from the bottom ... by capillary action to the surface of the potting medium."

The scientists found subirrigation much better at maintaining the pesticide levels in the plants. Nine weeks after the application, the hand-watered plants showed far lower pesticide concentrations than the subirrigated plants.

It Kills Adult and Immature Insects

"This suggests the pesticide is leached from the potting medium with repeated hand-watering," Van Iersel said. "With subirrigation, the pesticide isn't leached from the pots and remains available to the plants for much longer."

Subirrigation also proved better at killing both adult and immature insects. Killing immature insects is an essential part of a control program, Van Iersel said.

"Preventing reproduction is much more important than just controlling the adult insects," he said. "After all, a few adults won't cause much damage. But if they reproduce rapidly, populations can quickly build up where they can be a serious problem."

Van Iersel says the bottom line is to apply the pesticides the same way you water the plant.

Besides being an effective way to apply pesticides, subirrigation is also a huge labor-saver.

Kills Pests and Saves Time

"Normally, a greenhouse worker would have to apply 2 ounces of pesticide to each plant," Van Iersel said. "Using subirrigation, you just mix up the pesticides and apply them all at one time."

With subirrigation, the pesticides are always contained, which cuts down on the amount of pesticide runoff from greenhouses.

"When applying pesticides from the top of the plant, anytime you overwater, the pesticides leak out onto the ground," he said.

Subirrigation is becoming more prevalent in greenhouses in the northern United States and Canada. But so far, few greenhouse growers in Georgia have installed the system.

"In Georgia, everyone is waiting for their neighbor to do it first," Van Iersel said. Subirrigation costs about $7 per square foot to install.

"It's expensive initially," he said. "But growers can earn back the cost in labor savings in just a few years."

As a result of this research, Olympic Horticultural Products, manufacturers of Marathon, now lists subirrigation as a legal method of applying their product to greenhouse plants.