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Home HortScience Brown Seaweed Mitigate Effects of Drought in Nursery Trees
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LAKE ALFRED, FL—Seaweed and seaweed extracts have been used as soil amendments and fertilizers in agriculture for centuries. Seaweed extracts have been reported to increase fruit yield and quality of citrus and grapes, and to increase drought tolerance of grasses, vegetables, and ornamental crops. A study investigated whether a commercially available seaweed extract could improve drought stress tolerance in nursery-grown citrus trees.

Timothy Spann from the University of Florida's Institute of Food and Agricultural Sciences Citrus Research and Education Center and Holly Little from Acadian Seaplants in Nova Scotia, Canada, published the results of their study in HortScience. Spann said the experiments were designed to help find ways to improve drought tolerance and resulting production uniformity for Florida citrus operations. "Citrus trees grown in nurseries are primarily produced in containers and irrigated with overhead irrigation systems," he explained. "As trees grow, the expanding leaf canopy reduces irrigation uniformity by blocking and deflecting water away from the roots. Uneven irrigation results in a portion of young trees receiving less water than optimal, resulting in uneven tree growth."

Spann and Little tested a commercially available extract of the brown seaweed Ascophyllum nodosum to determine if the product could improve drought stress tolerance and maintain shoot growth under drought conditions of sweet orange nursery trees. They treated 'Hamlin' sweet orange trees on 'Carrizo' citrange and 'Swingle' citrumelo rootstocks with Ascophyllum nodosum applied weekly as either a soil drench or foliar spray. Half of the trees in each treatment were subjected to drought stress (irrigated at 50% of evapotranspiration) while the other half were fully irrigated (100% evapotranspiration).

All trees exhibited similar patterns of shoot growth during the course of the study. Drought-stressed trees did not begin to exhibit slower growth rates until 2 weeks after the initiation of drought stress. By the end of the experiment, seaweed extract-treated drought-stressed trees on both rootstocks grew significantly more than untreated control trees. However, compared with fully irrigated control trees, drought-stressed trees grew less regardless of seaweed extract treatment.

"Our study showed that 50% deficit irrigation reduced the shoot growth of container-grown citrus nursery trees by approximately 30% compared with fully irrigated control trees, but seaweed extract treatment mitigated much of this growth reduction," Spann and Little said. "We found a significant drought-induced reduction in total plant dry weight and a shift in resource allocation from shoot to root growth."

They said that seaweed extracts may be a viable approach for maintaining the growth of citrus nursery trees grown in greenhouse conditions under non-uniform irrigation systems, but cautioned that the study results may not be transferable to other seaweed extracts or species.


The complete study and abstract are available on the ASHS HortScience electronic journal web site:

Founded in 1903, the American Society for Horticultural Science (ASHS) is the largest organization dedicated to advancing all facets of horticultural research, education, and application. More information at


Original Article:

Applications of a Commercial Extract of the Brown Seaweed Ascophyllum nodosum Increases Drought Tolerance in Container-grown 'Hamlin' Sweet Orange Nursery Trees
Timothy M. Spann and Holly A. Little
HortScience 46:577–582. [Abstract][Full Text][PDF]

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