Introduction

The impending loss of methyl-bromide as a soil fumigant for vegetable crops after 2005 has crop producers and researchers searching for alternative means to protect their crops from soil-borne pests. One possible alternative involves avoiding the soil altogether by growing crops in perlite-filled, lay-flat plastic sleeves using soilless cultural techniques.

Growers around the world, as well as in the United States, are using hydroponic techniques to produce many horticultural crops. Horticultural crops produced worldwide in 1998 using soilless culture valued at approximately $4 billion (Carruthers, 1998). Benefits of using soilless culture include the ability to optimize water and nutrient supplies to the crops throughout the various growth stages and the ability to grow crops in areas where the soil is not naturally conducive to crop growth or is simply not available. Recent work evaluating hydroponic techniques for producing food for astronauts on space missions highlights these capabilities (Porterfield et al., 2000).

The strawberry industry in Florida is, by necessity, seeking alternatives to the use of methyl-bromide for eliminating soil-borne pests. The primary production area for strawberries in Florida is in Hillsborough County, located in the southwest region of the state, where approximately 6,500 acres of strawberries are produced annually with a market value of nearly $180 million. Although many alternatives to methyl-bromide are being investigated, no consensus has been reached concerning which alternative(s) will be predominantly used long-term in this industry. Alternatives being investigated include flooding, solarization, and numerous chemical compounds. So far, no single alternative is as consistent and effective as methyl bromide for controling weeds, nematodes, and fungal pathogens. In addition, the cost and label restrictions of some alternatives may prohibit their widespread use.

Recent work by Hochmuth et al. (1999) has shown that strawberries can be grown outdoors hydroponically on a commercial scale on small farms in North Florida. Also, several growers in Central Florida have experimented with various hydroponic systems with varying degrees of success (personal conversations).

The objectives of this study were to evaluate the use of perlite-filled, lay-flat plastic sleeves for commercial, outdoor, hydroponic strawberry production in Central Florida and to demonstrate for growers the practical application of the perlite-based system.

Methods and Materials

A 2X3 factorial experiment was instituted at the Gulf Coast Research and Education Center (GCREC) in Dover, Florida. Experimental factors were strawberry variety and growing system. Two varieties of strawberry used were ‘Sweet Charlie’ and ‘Camarosa’. Plug transplants of these varieties were established in white or black plastic sleeves filled with coarse perlite or conventionally, in the soil. Filled perlite sleeves were approximately 10 ft long, 6 in. deep, and 7 in wide. Two sleeves were placed side by side atop raised beds (2 ft wide and 9 in high) covered with black polyethylene mulch. Small slits (0.5 in) were made 1 in from the bottom of the sleeve every 12 in to allow for excess water to leave the bag and for the leaching of excessive nutrient salts. Trickle irrigation tubing with 4 in emitter spacing and a flow rate of 0.18 gal/ft/hr was inserted along the top of the sleeve between the perlite and plastic sleeve. Trickle irrigation was connected to a proportioner (Dosatron, Clearwater, FL) which supplied water and nutrients three times a day for 7 minutes. The proportioner was set to supply 85 ppm NO₃-N prior to flowering and 125 ppm NO₃-N afterwards from a complete nutrient solution (Table 1). Irrigation length was determined to allow for 20% leaching to remove any build up of salts inside of the sleeve. Eight plants spaced 12 in apart were planted into each sleeve through X shaped incisions along the top of each sleeve. Plants grown in the soil were planted in a double row atop beds (as described above) fumigated with a 67% methyl bromide, 33% chloropicrin mixture applied at a rate of 310 lbs A^-1. Spacing between soil grown plants was 12 in. in row spacing and 12 in. between row spacing. Water and nutrients were applied to these plants in accordance with current University of Florida recommendations (Maynard and Olson, 2002).  Harvest was begun on 22 Nov, 1999 and 1 Dec, 2000 for the two seasons, respectively.

Data were collected for mortality, number of stolons produced by plants, marketable yield, disease incidence, unmarketable fruit, and root mass at the end of the season (2000-2001 season). Data were analyzed using SAS statistical software, proc GLM procedures. Analysis revealed significance for year, year by variety interaction, year by growing system interaction and year by variety by growing system interaction effects; therefore, data were separated by year and analyzed.

Results and Discussion

Dramatic differences between hydroponically and traditionally grown strawberries were noted for runner (stolon) growth. No runners were observed to grow from the strawberry plants grown in the perlite bags either year while traditionally grown plants produced an average of 7.2 and 5.3 runners per plant during the 1999-2000 and 2000-2001 seasons respectively. This has economic implications as runners must be removed from the strawberry plants grown in soil in this area at a cost of $44 - $90 per acre.

The 1999-2000 season was close to ideal, with favorable temperatures and low amounts of rainfall. Early (Nov - Jan) marketable yields and number of berries per acre of plants grown in either white or black bags surpassed the yields of traditionally grown plants (Table 2). Total yield and number of berries per plant were not affected by growing system (Table 2). Early yields were not different for either variety grown (Table 3). Significant differences were observed between varieties for total yield, ‘Camarosa’ produced a 48% greater yield and 36% more berries than ‘Sweet Charlie’ (Table 3). There were no significant differences in the number of diseased or cull fruit due to growing practice (Data not shown).

The 2000-2001 growing season was less than ideal with low temperatures reducing yields industry wide by approximately 50%. A severe iron deficiency, confirmed by tissue analysis, occurred in plants grown in perlite filled sleeves. This was probably due to low root zone temperatures. In other crops, such as cauliflower, it has been seen that micro-nutrient deficiencies can occur when night time temperatures are low and day time temperatures favor rapid growth. Iron deficiency is not uncommon in certain strawberry varieties in Florida during cold weather. 'Camarosa' was more affected than 'Sweet Charlie'. Supplemental iron was applied both to the media and foliage; however, the deficiency persisted until temperatures increased at the end of February. If means could be found to increase the root zone temperatures, iron deficiencies could be eliminated. Plants grown in soil produced a significantly higher early yield of fruit than those grown in perlite filled sleeves with no difference in number of berries produced per plant (Table 2). Total yield and number of berries per plant was significantly higher for plants grown in soil than those grown in perlite filled sleeves. Early and total yield of ‘Camarosa’ was significantly greater than ‘Sweet Charlie’ with no significant difference in number of berries produced (Table 3).

Upon examination of root systems of plants grown in perlite filled sleeves it was determined that ‘Sweet Charlie’ had a significantly greater dry mass accumulation in the total, upper (3 in), and lower (3 in) half of the sleeve than ‘Camarosa’ (Table 4). In addition, when the length that roots grew along the bottom of the bag was measured, ‘Sweet Charlie’ roots grew 2 in longer than those of ‘Camarosa’ (Table 3). This disparity between the two varieties was unexpected, as ‘Camarosa’ produces a much larger and vigorous bush than does ‘Sweet Charlie’. Differences in root mass may explain why ‘Camarosa’ showed more severe Fe deficiency symptoms than ‘Sweet Charlie’.

Under ideal or warmer weather conditions open field hydroponic production of strawberry may provide improved yields and reduce runner production compared to traditional culture. However, under low night/high day temperatures, yields for hydroponic open field production may encounter significant losses compared to traditional methods. Therefore, it is strongly recommended that hydroponic production of strawberry take place in protected culture in subtropical regions of Florida.

Literature Cited

Carruthers, S. 1998. Hydroponics- A global perspective- The future outlook for hydroponics in the 21^st century. Practical Hydroponics and Greenhouses. 42:53.

Hochmuth, R., L. C. Leon, and D. A. Dinkins. 1999. The development and demonstration of an outdoor

hydroponic strawberry production system for North Florida. Proc. Nat. Ag. Plastics Conf. 28:165-167.

Maynard, D.N. and S.M. Olson (eds.). 2002. Vegetable production guide for Florida (SP170). Univ. of Florida and Citrus and Veg. Mag.

Porterfield, D.M., T.W. Dreschel, and M.E. Musgrave. 2000. A ground-based comparison of nutrient delivery technologies originally developed for growing plants in the space-flight environment. HortTech. Jan-Mar. 10(1):179-185.