H.Y. Hanna

Growing high-cash-value vegetable crops under cover can exceed expectations. Europeans and Canadians are producing most of their fresh-market vegetables in greenhouses. Other countries are expected to do the same in future years.

The estimated gross farm value of Louisiana greenhouse tomatoes was nearly $1.5 million for 2010. Many convincing reasons to grow fresh-market tomatoes under cover include: 

• Higher yield per unit area.
• Favorable hygienic conditions that lead to vine-ripened fruit without spoiling. 
• Production of a healthy fruit with less exposure to pesticides. 
• Significant reduction in fresh water use. 
• Great efficiency in using fertilizer without groundwater pollution. 
• Using soilless culture in places that lack the existence of flat farmland and/or topsoil to produce a crop.

 Perlite, pine bark, coco fiber and rock wool are popular soilless media for growing greenhouse tomatoes. Perlite is an excellent medium to grow the crop because of good physical structure that balances nutrient and air holding for healthy roots. Also, if recycled properly, perlite can be reused time after time to grow successive crops for cost-effective tomato production, saving natural resources.

Two methods have been developed at the LSU AgCenter Red River Research Station to recycle perlite for repeat use. Each method encompasses two steps to recondition perlite – first with some mechanical means and then by disinfecting the medium with hot water. In 2010 a third method was developed to recondition and disinfect the perlite in one step for cost-effective tomato production.

To recycle perlite with the new method, all tomato plant material, including the crown roots, is removed from the greenhouse after the last harvest, and perlite is simultaneously reconditioned and treated with hot water. This method combines the penetrating power of hot water from a water breaker (or nozzle) mounted on the wand of a pressure washer with the operator’s action to do the job. The worker moves the water breaker front and back, up and down, and sideways in the grow bag for at least one minute, using at least 4 gallons of near-boiling water in each 5-gallon grow bag. This action allows the water power from breaker to loosen the plant material from the perlite, leach excess salt and disinfect the medium at the same time.

The new method is as effective as the previously developed methods in reducing residual salts in the medium at significantly reduced labor and recycling costs. Hot water at 190 degrees Fahrenheit or above raises perlite temperature higher than the temperature range that kills fungi and nematodes known to cause economic losses. Tomatoes grown in perlite recycled with this method produce similar yields to tomatoes grown in new perlite or perlite recycled with the previously developed methods.

The simultaneous perlite reconditioning and disinfecting method was the third in a series of recycling methods developed at the LSU AgCenter Red River Research Station. This method should offer the smaller grower another tool to recycle perlite for cost-effective tomato production and save natural resources. It is less time-consuming and more cost-effective than the previously developed methods.

Replacing perlite every growing season to produce a new tomato crop is costly, and recovering the expense by increasing the selling price may not work well in a competitive market. Repeat use of old perlite as-is to grow successive tomato crops without reconditioning to restore the original structure, desalinating to remove excess salts and disinfecting to control root pathogens is risky because of media compaction, salt buildup and pest contamination.

Using the new method, a grower can recondition, desalinate and disinfect the root medium in one step. This method also eliminates labor time and effort to remove old media from the greenhouse, transport it to a land fill or a vacant field for disposal and fill other bags with new perlite. The rental of a hot-water pressure washer and the cost of a heavy-duty water nozzle can be the only out-ofpocket expenses a grower has to pay. The labor can be done by the grower.

The AgCenter has been growing tomatoes in recycled perlite for 16 years with no root disease problems or yield reduction. The hot-water treatment is more desired than treating media with methyl bromide or any other fumigants. And fewer chemicals used in food production can contribute positively to human health and improve environmental conditions for better living. The AgCenter has produced more than 1.27 million pounds of tomatoes grown in 1,200, five-gallon grow bags filled with recycled perlite. In 1996, 800 cubic feet of perlite was purchased, and it has been recycled and reused every year since then.

Efficient management of limited space greenhouse tomato operations by reducing production costs and improving plant yield should help producers do well in a competitive market.

H.Y. Hanna, Professor, Red River Research Station, Bossier City, La.

(This article was published in the summer 2011 issue of Louisiana Agriculture magazine.)