Vegetable gardening is more popular than ever. As many as 42 million householders are growing vegetables at home or in community gardens, according to the 2013 annual report from The National Gardening Association. That’s up 17% from 2008 to 2013, with no end in sight. Ornamental greenhouse growers are taking notice and adding or looking to add vegetables to their production. Making that shift requires some adjustments, starting with the structure that houses the crop.
Peter Konjoian, President of Konjoian’s Horticulture Education Services, is a rare blend of a grower and a scientist. He shares his vegetable research regularly with Greenhouse Grower magazine readers in his “From Flowers to Food articles,” and works with growers around the world through his research and consulting company. We caught up with him to ask his advice on what small- and medium-sized growers need to consider when retrofitting an existing structure or purchasing a freestanding, Quonset-type structure for growing vegetables.
Grade-A Greenhouses Yield Grade-A Vegetables
For those wanting to retrofit a structure, Konjoian’s first bit of advice is simple – pick the very best structure on your range. He says that oftentimes operations add greenhouses over a period of several years and each one varies depending on its age, location, ventilation technologies, and growing environment. When making the shift to growing vegetables, growers should settle for nothing less than their top-performing greenhouse because vegetables are particular about their growing conditions. Also, vegetable production is likely to utilize the structure year round compared to seasonally for ornamental crops.
“Growers sometimes think vegetables have such a quick turn-around that they can throw them anywhere in the greenhouse with some ebb-and-flood or nutrient film channels, and the crops will turn out just fine,” Konjoian says. “Some vegetables do have a quick turnover, and they don’t require the work that many ornamentals do, but that doesn’t mean they should be treated like second-class citizens in terms of the environment.”
Humidity Control Is Priority No. 1
Part of choosing the best structure available is picking out the driest one, and concrete floors are a plus for good drainage and absorption of excess moisture. Considering that powdery mildew, a condition favored by high humidity, is one of vegetables’ worst enemies and can ravage crops like lettuce seemingly overnight, it pays to ratchet up the humidity control when growing vegetables. It is especially important for hydroponic growers because of the added moisture the growing systems add; even though those systems are covered, humidity is still a concern.
Air circulation within the structure is an important system to incorporate. Horizontal air-flow fans installed correctly will eliminate cool or warm pockets and assist in preventive disease and insect control.
Ventilation and humidity control are closely associated. Whether a grower is actively ventilating a greenhouse with exhaust fans or passively ventilating it with ridge vents and roll-up sidewalls, good airflow is instrumental to success. Konjoian says there is no advantage to one type of ventilation system over the other; however, growers should keep a couple of factors in mind when determining what type of structure and ventilation system they will use.
First, with passive ventilation there is more surface area open for insects or disease spores to enter the greenhouse, compared to actively vented houses where inlets have much less square footage. Second, the temperature gradient in actively ventilated greenhouses that are longer (around 150 feet) can be as much as 10 degrees from the inlet end to the exhaust end.
Second, spending money on dependable environmental control that includes dehumidification cycles will pay dividends in disease management especially during winter production of crops susceptible to powdery mildew, Botrytis, and other pathogens.
Another consideration is growing vegetables in close proximity to ornamentals. It is better to separate the two crops to avoid pesticide drift or accidental applications and to prevent disease spread between the two. Some type of barrier is required in gutter-connected houses with multiple bays, and growers must factor this in when retrofitting them. Growers with freestanding structures have the advantage in this area because the entire house can be shifted over completely, which is preferable and much less complicated.
Lastly, supplemental lighting will be required in northern latitude locations for fall, winter, and early spring production of vegetables. High pressure sodium lamps are currently the most common fixtures used; however, exciting LED technology is creating opportunities to deliver tailored light spectra to individual crop species.
Strength Important With Freestanding, High Tunnels
For growers who plan to purchase a freestanding, Quonset-type structure, Konjoian says the same general rule applies as for those with existing structures. Don’t skimp on price or quality. If a grower is planning to grow a trellised crop like tomatoes or cucumbers, he says they need a greenhouse with the structural strength to support the weight of the crop, which means thicker diameter hoops that aren’t always found on cheaper structures. The greenhouse should also be as tall as the grower can get with generous sidewall height for more usable space.
Heating a freestanding structure is another factor. Two options include heat distributed by hot water circulating through pipes or tubes or warm air blown through a hot-air furnace. Typically, the hot-air system is the cheaper option in terms of installation and upfront costs, Konjoian says, but the hot-water circulating system is usually better long term. It costs a little more to install, but uniformity of temperature is a cut above hot-air systems.
Finally, there’s the greenhouse covering. Growers who are planning on three-season or year-round production want to make sure the polyethylene covering they choose has the infrared (IR) characteristics for added insulation. Quality films last four or five years, Konjoian says, and there is no reason for a grower not to use them.
“Growers can find cheaper plastics, but with a five-year IR film the heat savings in just one year pays for the extra cost of the film itself,” he says.