Structurally sound success

Passive ventilation is great for saving energy, but a ridge vent is needed in larger houses to allow hot air to rise up and out, pulling cooler air in. Side vents alone require some wind movement to be effective and they cannot keep a wide, tall house sufficiently ventilated.

Production of greenhouse and high tunnel crops is booming, for a number of reasons. Among consumers, there’s a growing demand for the wide range of high-quality products that come out of these structures, from flowers and herbs to fruits and vegetables. On the grower side, there’s a variety of technology to utilize, from the simple to the highly sophisticated, depending on your budget and your production needs. While it may be impossible to fool Mother Nature, greenhouses and high tunnels do the next best thing by providing some control over the elements, thus promoting yield and quality while protecting against losses due to unfavorable weather.

It takes good management to make the most out of your greenhouse investment (I’ll call all these structures greenhouses for now, since a high tunnel really is just a bare-bones greenhouse.) I’ve seen enough problems during my farm visits to make up the following list of things to consider as you try to make the most of greenhouse growing.

Layout for the future

If you’re new to this business, think long and hard about where you put that first greenhouse, because it may not be your last. Many growers I know started with one or two, but now have many more than they ever imagined. In hindsight, many would have laid these out in a more organized fashion to better accommodate materials handling, farm traffic, snow removal between houses and multihouse heating systems, to name a few issues affected by layout .

Where will the water go?

Consider the worst-case surface water conditions when you set up a greenhouse, especially if you will be growing crops in the ground. In rainy years, I see many farms struggle with excess water flowing into the house because perimeter drainage is insufficient. That causes all kinds of problems from delayed planting to cold soils and root rots.

If you’re building new greenhouses, be sure they are designed for the maximum snow load and wind in your area!

Some farms don’t have water problems inside the greenhouse, but outside: the driveways and walkways were not properly designed to deal with high traffic in the early spring when the ground is really wet. Plan ahead to prevent mud, ruts and soil erosion.

Err on the side of strength

There’s a time and a place for low-cost, homemade growing structures. Typically, this is early in the season for temporary protection of relatively fast crops like lettuce. So-called “field houses” can speed production and be very profitable, and it may not make sense to spend much money on them. However, for longer-season crops that you’d like to grow in a greenhouse for many years, stronger is better. The house should be designed to withstand the maximum snow load and highest winds in your area. Don’t skimp on the quality of steel pipe, the spacing of hoops or the extra structural features, such as purloins and cross-braces, that provide strength and stability.

Change the plastic covering

Greenhouse films last longer than regular construction types of plastic films since they contain additives that make them resistant to degradation from ultraviolet light. However, most greenhouse covers are only designed to last three to four growing seasons. After that, the amount of photosynthetically active radiation (PAR, the kind of light plants need to grow) may decline significantly; some of this loss is due to changes in the plastic that can reduce transmission of certain PAR wavelengths, but a lot is simply due to the accumulation of soil, particulates, mold, scratches and other features that collect over time on greenhouse covers—both inside and out. Pushing the lifespan of your greenhouse cover may be penny-wise and pound foolish if it reduces crop growth or quality.

Know your growing media

Commercial peat-based growing mixes that contain conventional fertilizers typically provide very consistent performance. I’d still ask my supplier for the nutrient analysis of the mix so I know what I’m working with and can keep track of what works—or doesn’t work—for certain crops. In the case of organic growing mixes, which contain compost and natural fertilizers, it’s a bit trickier to get consistency from batch to batch. I suggest that growers not only ask their supplier for the analysis, but also send a sample in for a saturated media test well in advance of planting. That gives you a record of a mix’s characteristics from year to year, and allows you to make adjustments if needed. Most land grant university testing labs offer this greenhouse soil test.

Know your water

Many university testing labs also offer a greenhouse water test, which includes pH alkalinity (dissolved carbonates and bicarbonates). To maintain proper pH (and thus nutrient availability) in your growing media, you need to avoid pH problems with your water. If the pH is too high, you can add some type of acid, but it takes more acid to decrease the pH of water with high alkalinity. Water can also contain excess soluble salts, which harm roots and can lead to nutrient deficiencies. High sodium is another possible problem. Annual water testing is an inexpensive management tool to help optimize crop nutrition.

Be vigilant for pests

A lot of growers simply keep an eye out for problems as they work in their houses. A more systematic approach is to have a scouting form and a person assigned to fill it out on a regular basis, like every week. That makes sure all greenhouses and crops are examined closely and frequently, which increases the chances of catching insect or disease problems early, when they are easier to control. A sample scouting form is at

Another good tool is a low-cost hand lens to aid in the observation of small problems, before they become big ones. Hang one of these up in each greenhouse. Placing sticky yellow cards throughout the greenhouse is another low-cost way to stay ahead of potential insect pests.

Weed-free and puddle-free floors help prevent pest problems. Oops,the hose was left on the ground!
Yellow sticky cards are a low-cost tool that helps with insect pest monitoring.
All pots and trays should be cleaned and disinfected prior to reuse. Be aware that lots of residue in the wash water will reduce the effectiveness of the disinfecting solution.
This system of a wire and clips makes it easy to manage the hose when watering, avoiding tangles. It also minimizes the potential spread of disease after the nozzle lays in the ground.

Natural ventilation

Natural ventilation uses less energy than mechanical ventilation, but a greenhouse needs the right setup to maintain good ventilation naturally. Smaller, narrow houses can get away with roll-up sides as their only form of ventilation, but even these may suffer when there is little or no wind since that is what drives sidewall ventilation. Larger houses really need both sidewall and ridge vents to assure good air movement. That way, the greenhouse is vented by both wind and thermal gradients.

Mechanical ventilation

Fans can be used to draw cool air into the greenhouse, but remember, they will pull from the point of least resistance. Quite a few houses I visit have fans running with doors open nearby, cracks in the endwall where the fan is located or even the sides rolled up. These situations diminish or even eliminate the fans’ effectiveness. Using high-quality thermostats to control the operation of fans and vent motors can reduce unnecessary operation and lower electricity costs.

Sanitation is a state of mind

I’m not suggesting you have a dirty mind if you don’t make greenhouse cleanliness a priority. I am suggesting that some management time and employee training focused on greenhouse sanitation is a reasonable, better- safe-than-sorry approach to preventing pest problems.

Recommended sanitation activities include regular washing of tools, containers and equipment that comes in contact with plants or growing media using a greenhouse disinfectant. Employees should be aware that they can carry pests from one house to another and know when to wash their hands and clean off tools to minimize that risk.

A common problem I see in greenhouses is hose ends lying on the floor, which is a good way to spread disease, especially in houses with dirt floors. It’s pretty easy to set up hooks to keep these hung up. Benches and other working surfaces should be disinfected in between crops and kept as free of debris as possible.

Under the benches, all weeds should be removed while still small and efforts made to avoid standing water. Any trash containers in the greenhouse should be emptied daily. If any plants are discovered to have insect or disease problems, they should be put in a plastic bag to minimize spread of spores or bugs and then removed from the greenhouse.

Outside the greenhouse, weeds should be removed and turf kept closely mowed to limit the habitat for pests. Compost piles, dead plants, old pots and other breeding sites for insects and disease should be located as far from the greenhouses as practical.

There are many excellent greenhouse management Web sites; one that I use a lot is UMass Extension Greenhouse Crops and Floriculture at

The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office and longtime contributor to Moose River Media. Visit to discuss this article!