Why Aren’t My High Tunnel Tomato Plants Growing Faster?

Beginning about now and lasting through mid-April, I am often asked by high tunnel tomato growers why their crop is not developing as rapidly as they expect. Troubleshooting covers a wide range of possible explanations. As various ones are considered and ruled out, the possibility they have overlooked the role of soil temperature becomes more important. The high tunnel may be heated, and the crop may have been irrigated and fertilized aggressively, but there is usually no record of the soil temperature, which greenhouse growers know is very important and work to optimize. After all, root growth significantly influences shoot growth and root growth is influenced by soil or root zone temperature.

In my view, we know far too little about soil temperatures in high tunnels — what the optimal ones are at any time and how to achieve them. Still, discussing this with people in Ohio and other states and having done some research on the topic, I was asked to summarize findings at the recent Mid-Atlantic Fruit and Vegetable Convention in Hershey PA (https://www.pvga.org/wp-content/uploads/2022/01/Mid-Atlantic-Convention-Program-22-website.pdf). The subject of the presentation was “root zone heating and root zone temperatures for high tunnel growers” and what follows are a few messages from that presentation.

Root systems are rarely seen but their size, form, and function influence every aspect of the crop, including the size of the canopy and crop marketable yield and profit potential.

Root systems are hard-wired to follow general patterns as they develop. However, conditions surrounding root systems influence their development significantly. Further, those conditions include temperature and are partially set by the grower. So, growers are partially responsible for root system development and function. While a “strong” canopy is good evidence of an equally strong root system, without another canopy to compare it to, it is difficult to be sure it is as strong and productive as it could be. This indicates that a little on-farm experimentation can go a long way in helping optimize total crop management. It also reminds us that since we usually cannot see roots while experimenting or farming, we often need to rely on tracking factors we can measure and that are known to influence root system development and function.

Research findings suggest that tomato growth and production tend to be greatest at root zone temperatures of 65-70 degrees F. This begs two questions.

First, are root zone temperatures in your high tunnel in the optimal range as often as possible? Do you measure soil and irrigation water temperatures? We have recorded soil temperatures every fifteen minutes for various entire seasons in high tunnels and open fields at OSU-Wooster/OARDC and some of the data are shown below (click to enlarge, if needed). Notice the description of the situation in which the readings were taken and when soil temperature readings were in the optimal range. These readings may or may not represent your farm or crops. However, the data may give clues as to the potential temperatures in your fields and high tunnels and encourage you to record those temperatures directly. Reliable, easy to use, inexpensive instruments are available for doing that.

About irrigation water – much of it draws from wells and surface sources and can be very cold (from the crop’s perspective) fall through spring. Although it has not been tested to my knowledge, passing well, surface, or municipal water through drip lines in a high tunnel, heated or not, may be unable to bring its temperature to 65-70 deg F. So, irrigation in the earliest part of the season may amount to bathing roots in water well below the optimal temperature for tomato and other crops and heating the air may overcome that issue only partially.

This brings us to Question 2. Are you convinced that your returns on investments in high tunnel heating, especially of the air for early season tomato production, are as high as possible? If the air temperature is high but soil temperature is low, are you getting as much from the relatively short photoperiods as you could? In early spring, crops may be more limited by a lack of sunlight than below-optimal air temperatures (and excessive heating during extended low-light periods may be counterproductive). We cannot change daylength or cloud cover, but we have some control over air and soil temperatures and may benefit from bringing investments in them into alignment with daylength. For example, should heating increase with daylength? What is the return on investment in aggressive air heating when daylength is very short soon after transplanting?

Addressing those questions opens doors to exploring the relative value of investments in air, soil, or combined heating. That is a subject for other discussions and articles, but it is worth asking if investments in air heating are returning as much as we expect based on the air temperature alone. The 11/6/21 issue of VegNet included an article on root and air heating in fall-time high tunnel leafy vegetable production (https://u.osu.edu/vegnetnews/2021/11/06/soil-heating-effects-on-days-to-harvest-quality-and-regrowth-of-three-high-tunnel-and-fall-grown-vegetable-crops/) and our previous research included spring season experiments, too. Individual crops respond differently to air and soil temperature due to biology and other reasons. For example, the growing tip of lettuce plants is closer to the soil surface than the growing tip of tomato plants and, therefore, may be more strongly impacted by root zone temperature and heating over brief periods.

The point here is that investments in high tunnel heating may be most effective when taking the whole cropping cycle and rotation into account. High tunnel management systems, including temperature, can be designed around one or a set of crops – i.e., around optimizing income from one crop or across the year. Of course, this would occur on a farm by farm, market by market basis. This spring and season, as you are able, consider taking a moment to examine your high tunnel temperature management practices and ask if they maximize your entire annual profit potential.

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