Greenhouse growers can substitute rice hulls for perlite in their media without the need for an increase in growth regulators, according to a Purdue University study.
Growing media for ornamental plants often consists of a soilless mix of peat and perlite, a processed mineral used to increase drainage. Growers also regularly use plant growth regulators to ensure consistent and desired plant characteristics such as height to meet market demands. Organic substitutes for perlite like tree bark have proven difficult because they absorb the plant-growth regulators and keep them from getting to the plants. Using bark requires a 25 percent increase in the volume of growth regulators applied.
"We were not sure whether rice hulls, as an organic component, would hold up the growth regulator," says Roberto Lopez, a Purdue assistant professor of horticulture and co-author of a “HortTechnology” paper that outlined the findings. "Testing showed that there were no differences in plants grown with rice hulls or perlite."
Pansies and calibrachoa were planted in an 80-20 mix of both peat and perlite and peat and rice hulls and then treated with several different growth regulators. The plants treated with and without growth regulators and grown in peat and perlite and peat and rice hulls had similar heights and stem lengths.
Finding a waste product to replace perlite could reduce the price of growing media because perlite must be mined and heat processed.
"It’s a really energy-intensive process and, because it’s a mineral, it’s non-renewable," says Chris Currey, a horticulture graduate student and co-author of the “HortTechnology” paper.
Rice hulls are an attractive option, Lopez says, because they can be easily transported on barges and rice growers in the South could increase profits by selling a traditional waste product.
"Often these rice hulls were being burned because there’s not a lot of other use for them," Lopez said.
Syngenta and Fine Americas funded the research. Lopez and Currey collaborated with Purdue research technician Diane Camberato and graduate student Ariana Torres.