269 Increasing Winter Strawberry Production in North-Central Florida Using Passive Ventilated Greenhouses and High Plant Densities Ashwin Paranjpe, Daniel J. Cantliffe Elizabeth M. Lamb, Peter J. Stofella Horticultural Sciences Department and Charles Powell University of Florida, 1251 Fifield Hall, Indian River Res. and Education Centre P O Box 110690 University of Florida Gainesville FL 32611-0690 Institute of Food and Agr. Sciences USA 2199 South Rock Road Ft. Pierce, FL, 34945-3138 USA Keywords: Fragaria x ananassa, protected culture, plug transplants, transplant conditioning, soilless substrate, cost threshold. Abstract Winter production of strawberry (Fragaria x ananassa Duch.) under protected structures can give several advantages to producers over field production. The objective of this research was to determine how increasing plant densities (10.8, 11.7, 12.7, 14, 16.9, 18.3, 20, and 22 plants per m 2 ) of 'Sweet Charlie' strawberry grown in a passive ventilated greenhouse might improve yield without adversely affecting fruit quality. Plant densities were derived with four between-row spacings (65, 60, 55, and 50 cm center-to-center) and two within-row spacings (17.5 cm and 35 cm plant-to- plant). Strawberry plants were grown in a ‘Hanging Bed-Pack’ trough system (Polygal Industries, Ramat Hashofet, Israel) that were suspended 1.8 m above the ground level and filled with 6.5 cm 2 sieved pinebark. Marketable yield per m 2 increased linearly with an increase in plant density throughout the season. Early yield per plant was not affected by plant density, however, total yield per plant decreased as plant density increased. Regardless of treatment, the average berry size for early and total yield was 20 g and more than 90 percent of the yield was marketable. A cost threshold was developed by comparing the value of higher early yields when prices are highest, to increased cost of plug transplants at high plant densities. A plant density of 16.9 plants per m 2 led to break-even yields while higher plant densities from 18.3 to 22 plants per m 2 led to increased profits. Thus, high plant populations were required to maximize profits from strawberries grown in passive ventilated greenhouses. INTRODUCTION The loss of methyl bromide in the year 2005 (U.S. EPA. 2002), strict regulations on water use at transplanting and for frost protection, rapid urbanization, increasing labor cost, and potential low productivity during winter (Nov to Feb) due to cold weather are some of the major concerns for the Florida annual strawberry industry. Presently, almost 100 percent of the strawberry production in Florida is done in open fields on raised beds fumigated with methyl bromide and covered with plastic mulch. The majority of production occurs during January and February, when the market price for strawberries is high. Because the Florida strawberry industry is characterized as competitive, maximizing yield is critical for maximizing profits in the higher-priced off-season winter market. Protected cultivation can offer a viable alternative to Florida strawberry growers to meet this challenge. Strawberries are grown under protective structures in the Netherlands, Belgium, Italy, Spain, U.K., (Lieten, 2001), Australia, Israel, and many other countries. Maximum space-utilization by increasing the plant density without compromising yield per plant and fruit quality is critical for the profitability of a protected strawberry operation. Proc. XXVI IHC – Berry Crop Breeding Eds. P. Hicklenton and J. Maas Acta Hort. 626, ISHS 2003 Publication supported by Can. Int. Dev. Agency (CIDA)