New improvements

Additional heat buildup management in Enza Zaden’s tunnels comes from separate end wall vents.

Field-grown winter vegetables have long thrived in Florida’s high sun and temperatures. Consumers demand quality, and retailers want consistency. Growers want these and high yields, which are all improved with crop protection.

While some of this demand is filled with increasing production in greenhouses, there is also a trend toward growing under passive tunnels. But as any producer with tunnels soon learns, the heat is a double-edged sword, and summer growing becomes impossible.

Veteran Florida field grower William “Skeeter” Bethea, crop specialist- tomato and pepper for the East Coast for seed breeder Enza Zaden, saw the overheating problem and started working on solutions that could extend the effective growing season both by finishing later into the spring and starting earlier for fall production.

Bethea was determined to develop a profitable and productive system for growing short cycle indeterminate tomatoes, peppers and cucumbers.

To do that, he needed to build an attic in his tunnels.


Climate challenged
Enza Zaden specializes in hybrid vegetable seeds that are developed through intense breeding and research programs located in 26 different countries.

Its Florida location is situated in Myakka City, nearly at sea level about 25 miles east of the Sarasota coast on the Gulf of Mexico. While it offers a near-ideal climate, it also presents a full complement of interesting and intersecting challenges.

Winters are mild, with average low temperatures of 48°F. (The lowest recorded temperature is 18°F.) Only five days per year bring freezing temperatures.

At the other end of the spectrum, average high temperatures sit at 92°F. Approximately 134 days per year reach about 90°F. The highest recorded temperature: a sweltering 105°F.

Added to that, Myakka City boasts one of the highest light environments of the entire U.S. East Coast.
While warm temperatures and abundant sunlight are undoubted assets, heavy rains and frost are threats to production.
Thus, protecting crops has been proven to improve quality, consistency and yields.

Bethea estimates that using tunnels in this climate to produce beefsteak tomatoes yields seven times the output of open field production. Tunnel production also provides a greater degree of consistency, a key factor in maintaining markets.


Heating up greenhouses
Understanding the climate and the challenges handed to him, Bethea produces his crops in five gutter-connected, 28-foot-wide and 150-foot-long tunnel spans covered with a single layer of polyethylene. Blocks too large do not ventilate well.

The only ventilation — and that was passive — came from roll-up end walls. That meant heat buildup was an extreme challenge in Florida’s high solar radiation conditions.

Svensson OLS shading helps reduce excess heat for poly tunnels in Enza Zaden’s Myakka City, Fla., location.

The tunnel effectively performs its task of trapping heat in the winter. Excess heat during more intense conditions is only removed via conduction through the cover and primarily ventilation, which is mostly wind-driven. Crop transpiration is also an important mode of cooling the crop, and excess moisture is carried out during the air exchange. Elevation in temperature without air exchange can be dramatic and yield limiting.

Natural ventilation is variable and limited depending on a variety of factors including geometry, temperature, wind speed and direction. Still, it is a very cost-effective way to limit temperature rise.

As he continued the perennial battle against Florida’s legendary heat and humidity and its effect on quality and yield, Bethea strategized. What if he could use reflective shading to create an attic space above head height inside the tunnel to effectively separate the sun and hot cover from the crop?


The right stuff
Knowing Svensson had been researching and experimenting with exactly this strategy, he approached President Kurt Parbst and detailed the situation and factors involved.

Svensson, for decades, has been engineering textiles for commercial horticulture to make crops more productive and profitable. While saving heating energy is probably what Svensson is best known for, its attention has lately been focused on crop improvement via the reduction of overheating.

While it is known that interrupting the path of solar radiation from the sun to the plant is a cost-effective means of reducing heat stress, it is often attacked with inefficient weapons. Svensson has been developing and testing materials that reduce solar radiation while maintaining lower surface temperature of the shading materials. This means that shading efficiency is greatly improved by reducing radiation from the shade that does not contribute to growth.

The resulting effects are reductions in heat stress and drastic improvements in water-use efficiency of the crop.

Discussions of the crop, climate and equipment revealed that the best tool would be OLS 30, an outdoor quality, reflective shading screen with an open structure. While the shade reflects incoming solar radiation, it still allows 74 percent light transmission (26 percent shade). Though the goal was to knock back excess radiation, high PAR (photosynthetically active radiation) light, the energy plants use for growth, is beneficial to vegetable production.

Bethea also soon learned that the porous “ceiling” OLS 30 created in the tunnel also acts as a continuous inlet to allow warm, humid air to rise and concentrate in the attic. This more buoyant air displaces the air occupying the attic and forces it out of the ends of the tunnel, which were modified with custom gable vents that rotate down from the peak, hinged at the shade level.

By efficiently reducing the radiation level and allowing overheated air to separate from the crop into the attic to the other side of a radiant barrier, Bethea experienced a dramatic temperature drop and enhancement to the growing zone.

This small application of technology to a low tech growing practice yields big returns through season extension in spring and fall.


For more: Enza Zaden, www.enzazaden.com or Svensson, www.svenssonglobal.com/us.


Joli A. Hohenstein is marketing and PR specialist for Pen & Petal Inc., a marketing, advertising and public relations agency for the green industry.