University of Maryland Extension

Don't cook your crops! High tunnel heat management

Neith Little
High tunnel with sides rolled up for passive ventilation

It’s that time of year again. You’re out in the fields, busy, excited, maybe a little overwhelmed, getting your annual crops into the ground. The day started off cold and misty, but it cleared up just before noon and you got in a long, hot day of work planting transplants. Triumphant, you come back to your high tunnel to find all the rest of your transplants and your early crop of greens and herbs wilted and drooping.

Don’t let this happen to you. On a sunny day, a closed high tunnel can reach temperatures that my old friends in Massachusetts would describe as “wicked hot.” Even in the winter, when outdoor temperatures hover around freezing, an unvented high tunnel can reach near 90 degrees F, which is plenty to cause heat stress in cool-season crops. On a sunny spring day like we’ve been having lately, when outdoor temperatures near 80, I would not be at all surprised to see the inside of a high tunnel reach 120 degrees F.

So what can you do to manage the temperature in your high tunnel?

First off, consider investing in a thermometer. It can be difficult to believe the high temperatures your tunnel is achieving until you see the numbers yourself. Even an analog $5 thermometer is a good check, and if you are willing to spend a little more, digital thermometers are available for about $30 that will record minimum and maximum temperatures. Higher end digital thermometers on the market, costing about $200, will even text you an alert when the temperature passes a set threshold. Whatever thermometer you choose, remember to install it somewhere in your high tunnel that is out of direct sunlight, which will throw off the measurement.

Passive ventilation is the standard, go-to method to cool high tunnels. Most high tunnels are designed with roll-up sides, creating a cross-draft of air. Some have removable end walls or ceiling vents. The key is to invest the management time necessary to make use of these design elements. Think of a high tunnel almost more like livestock than like outdoor plant crops: when you’re growing in a high tunnel, it’s going to require daily attention. Keep an eye on the weather, check the temperature, and open or close the ventilation as necessary to maintain the temperature within an acceptable range. Cool season crops like spinach tend to prefer 60 to 65 degrees F, while warm season crops like tomatoes are more comfortable between 70 and 75 (Jett 2010).

In addition to passive ventilation, some farmers use fans, whitewash, or shade cloth. Fans can be used to either vent air out the end-walls, or circulate air within the high tunnel. Whitewash can be sprayed onto the plastic, increasing the amount of light reflected off the plastic and thus reducing the amount of light (and heat) transmitted through the plastic into the tunnel. Whitewash is relatively inexpensive, but can be washed off by rain over the course of the season, and can be labor-intensive to wash off yourself when you finally want the high tunnel to heat up again in the fall. Similarly, shade cloth can be stretched over the tunnel to reduce light infiltration. Shade cloth will be more expensive up front than whitewash, but when taken care of a shade cloth high tunnel cover can be reused for multiple years. Choosing the correct grade of shade cloth, and deciding when to put it on can have big impacts on warm season crop productivity (Trinklein 2012).

To learn more about high tunnel management, check out the following resources:

Neith Little
Urban Agriculture Extension Educator, Baltimore City

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