How Many Staked Tomatoes Should I Put in My High Tunnel?

Over the last several years, the number of questions I have had about tomato production in high tunnels has increased dramatically. Since I am a plant pathologist, most of the questions I have been asked are about diseases of tomatoes in high tunnels. However, I also have been asked production questions. One particular question about tomato product that may impact disease severity is this: how many staked tomatoes can be grown in a high tunnel effectively?

To be honest, the above question is one that I often ask myself when I observe high tunnels in Indiana. It isn’t necessarily one that is asked by growers. But maybe it should be. It has been my observation that growers often try to place too many staked tomatoes in a high tunnel. The result may include diseased tomato plants due to insufficient air circulation, poor quality fruit and even reduced yields. I was not able to find any information about plant spacing of staked tomatoes in high tunnels. So, I decided to research the subject myself.

Let me take a moment to insert some definitions. A high tunnel is a greenhouse with passive heating. The only heat is that supplied is from sunshine trapped in the structure during the day. The structure is cooled by raising the sides and/or venting the top. Staked tomatoes are determinant tomatoes that grow only to a specific height. This is in contrast to indeterminate tomatoes that are trellised to the ceiling and back down again indefinitely. The study that I will describe here is for tomatoes that are grown in the ground as opposed to in pots are bags.

The 2014 staked tomato study

The 2014 study included two varieties at 5 different plant populations. Both varieties, Red Deuce and Mountain Spring are determinant. Each variety was grown at one of 5 different plant in-row spacing’s (plant population): 18 inches-Florida weave, 20 inches-Florida Weave, 20 inches Spanish trellis, 24 inches-Florida weave, 28 inch-Florida weave. In the Florida weave method, a wooden stake was placed every two plants and twine was woven around the plants and stakes to keep the plants upright. The Spanish trellis technique placed 4 stakes around two tomato plants with an X-pattern of twine woven between the stakes. In each case, twine was tied every 8 to 12 inches of plant growth. It is important to realize that although I varied the spacing of plants within a row, I did not vary the width between rows (center-to-center) which remained at 5 feet for all treatments.

The influence of varieties and plant populations on plant disease

It was my hypothesis that tomato plants grown at narrow spacing’s would have less ventilation, higher humidity and more disease when compared with tomato plants at more distant spacing’s. In the first week of August, the disease leaf mold was observed on the tomatoes in the experiment. I took observations of disease every few days.

While I observed leaf mold on the variety Mountain Spring, I never observed any leaf mold on Red Deuce. This was surprising since Red Deuce was not listed as resistant. It is always a good idea to choose a variety with host resistance, but also note the disease reaction of your plants regardless of how the variety is listed.

Regardless of what I thought would happen, I did not observe more disease on varieties at closer spacing’s. Does this mean that growers should space tomatoes very close together?

I think there are several factors which may have influenced whether the narrow spacing became more diseased. 

  • · If I could have planted the entire high tunnel to a narrow spacing, then ventilation/humidity may have been affected by in-row spacing differences.
  • · Center-to-center spacing may be more important in disease management than in-row spacing. Remember that I did not vary center-to-center spacing.
  • · We use a high tunnel that has a ridge vent. It is possible that such a high tunnel design reduces relative humidity over a design without ridge vents.
  • · These results were for the disease leaf mold. Results with other diseases may vary.

Further, placing tomatoes very close together may have other repercussions—read on.

The influence of varieties and plant populations on weight and number of tomatoes

Growers might be interested in knowing how these treatments affected the yield in weight and number of fruit.

Variety-There was no significant difference in the yield in weight between Red Deuce and Mountain Spring when calculated over the entire season. However, there were significantly more Mountain Spring tomatoes than Red Deuce. (I noted earlier here that the Mountain Spring tomatoes had physiological leaf roll. Given the results of this study after one year, it appears that the leaf roll on the Mountain Spring did not hurt the yield.) Red Deuce yielded significantly larger fruit than Mountain Spring.

Plant populations-There was no difference in the weight of fruit harvested per linear foot when analyzed by plant population when calculated over the entire season. However, the number of fruit per linear foot was less on plants spaced at 24 and 28 inches compared to plants spaced at 20 inches on a Spanish trellis.

The influence of varieties and plant populations on the mean weight of tomatoes

Many growers are able to obtain a premium for larger tomatoes. So, I also looked at whether the variety and plant population influenced the size of tomatoes. When calculated over the entire season, Red Deuce had larger (heavier) fruit than Mountain Spring. Perhaps more important, plants at narrow spacing’s tended to have smaller fruit than plants at more distant spacing’s. For example, plants at 16 inches with Florida Weave and 20 inches with Spanish Trellis had significantly larger fruit than plants spaced at either 24 or 28 inches with Florida weave.

Take Home message

Plant disease-leaf mold disease, while severe on Mountain Spring tomatoes, did not affect Red Deuce tomatoes in this experiment. Plant populations did not influence the amount of leaf mold on tomato plants.

Yield-I was not able to observe any difference in yield in pounds for tomato fruit when I measured over the entire season. However, I did see more tomato fruit on plants spaced closer together. Mountain Spring had more tomatoes than Red Deuce.

Mean fruit weight-Growers who are interested in larger tomatoes will want to avoid the closest spacing’s tried here. Plus, Red Deuce had larger tomatoes than Mountain Spring.

Recommendations

My plan is to repeat this experiment in the summer of 2015. More detailed recommendations will be made when the results of both years of experiments have been analyzed. What follows, however, are my thoughts so far. The most effective between row spacing (center-to-center) should be about 5 feet. In-row spacing should be from 20 to 24 inches.

Watch this space for more information or updates.

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