Considerations and challenges


The greenhouse tomato industry has become a major player in the North American fresh tomato industry. The USDA reports that in 2005 well over one-third of all U.S. retail tomato sales came from greenhouse tomatoes. Even though the majority of U.S. greenhouse tomatoes are grown in the warmer regions of the country, greenhouse tomatoes are establishing themselves and finding niche markets all over the country, including New England.

The tomato, an herbaceous vine and its fruit, is a tropical perennial and a member of the nightshade family, with its origins in the Americas. However, it is usually grown as an annual since it will succumb quickly to freezing temperatures. Today, tomatoes are one of the world’s most popular fruits, with many different cultivars being grown. No home garden would be complete without a few tomato plants.

At the same time, though, tomatoes can be a challenge to grow. They are both light and heat-loving; soil conditions must be correct and they need plenty of water. However, too much heat will stress the plant, resulting in poor flowering. Poor pollination results in small fruit, and plants that are allowed to remain damp for extended periods of time are susceptible to molds. Improper nitrogen and mineral levels will also affect the growth of the plant and fruit development.

Dr. Richard McAvoy, professor and extension specialist for greenhouse crops at the University of Connecticut, is an expert on greenhouse tomatoes. According to McAvoy, the growing of tomatoes in general and commercial greenhouse tomatoes in particular is not all that easy. Many things must work in concert to maximize the productivity of a plant. He stresses that with greenhouse tomatoes, the grower has a significant investment in the facility and equipment, and it is essential to maximize plant productivity. The more marketable fruit that can be produced in the greenhouse, the more one can justify the investment and labor costs of a greenhouse. While greenhouse tomatoes can reach top yields of 300 tons per acre per year in states like Arizona, a top yield in New England would be about 100 tons per acre. Most growers in this region average closer to 50 tons per acre.

Flowering and pollination are critical for good fruit development, says McAvoy. However, the additional challenge with tomatoes is that while the plant is flowering and setting fruit, it is also continuing to grow. So a balance needs to be maintained with how many fruit clusters develop and how large the fruit gets before it has a negative impact on the vigor of the plant. Tomato plants begin flowering when still quite young, and fruit clusters establish themselves from the bottom of the plant upward. At any point in the growth process, the plant may be stressed due to inadequate fertilizing, which will affect the vigor of the plant and the flowering and subsequent fruit development.

On an unusually warm day in May, McAvoy visited with Winston Scott, owner of Scott’s Connecticut Valley Orchards in Deep River, Conn. Scott says that greenhouse tomatoes are the most difficult plant he’s ever grown. He’s growing the Trust variety and has about 700 plants in his greenhouse that, to the untrained eye, all appear healthy, already having fruit setting in the third cluster. However, McAvoy pointed out that the leaves were light in color and not expanding fast enough. He explained that the leaf surface gives the plant the capacity to set a lot of fruit and suggested that low nitrogen was most likely the primary issue, but that iron and calcium could also need boosting.

Scott, who grows his plants in coconut husk, commented that after the seedlings were transplanted, they began flowering in just a few days. McAvoy stated that the early flowering was putting extra stress on the plant. The seedlings needed to get their root systems better established before flowering. He also noted that the stem diameter at the top of the plants indicated that they were lacking nutrition. He advises monitoring the stems at about 6 inches below the shoot tip to gauge plant vigor. The stem should be about .5 inch in diameter. Stems that begin to thin are an indication of stress and loss of vigor. Stems that are thicker indicate excessive vegetative growth associated with poor fruit set. McAvoy says many growers measure the stem diameter and leaf length on a regular basis.

Pollination on Scott’s plants was looking good, indicated by the clusters and the induction sequence. McAvoy explained that for a tomato plant, proper pollination occurs in a sequence on each cluster of fruit. The fruit closest to the stem is the first to set and should always be largest on the cluster, with the others progressively smaller. If the sequence is out of order, it indicates there was a problem with pollination. Out-of-sequence fruit should be pulled and discarded, as they usually don’t size properly and rob energy from the rest of the cluster.

McAvoy noted that many of the growers he works with get good fruit on clusters one through five, but after that, the plant and the clusters begin to suffer. He says it takes a high level of sophistication to manage good fruit beyond the fifth cluster. Thinning is important on a single plant, since excessive fruit load on the plant can stress the future blossoming. Too many or too large of a fruit will affect the productivity of the plant, pulling too much energy from the rest of the plant. Once the first cluster gets picked, the plant reinvigorates and takes off.

Temperature is also critical for proper pollination and good fruit development. In a greenhouse environment, a temperature over 90 degrees will stress the plant, and pollination is dramatically and adversely affected above 9 degrees. Each pollen grain makes one seed in the tomato, and McAvoy noted that there’s a high correlation between the number of seeds in the fruit and the size of the fruit.

A tomato with “ghost spot,” which are the first indication of botrytis or gray mold. A lot of ghost spotting in a greenhouse is a sign of a serious gray mold problem.

When the plants are still young, they are literally shaken by hand or mechanically vibrated to achieve a good pollination. This needs to be done every few days, and Scott admits that it’s a tedious job. Bumblebees are also used for pollination and are most effective.

As the tomato plant grows, it’s imperative that it’s trellised and supported so as to keep the fruit off the ground. Water and proper fertilizing and mineral levels will make all the difference as to whether the plant can remain healthy and productive beyond the fifth or sixth cluster. At any point where the plant is stressed, it may affect the shape of the flower’s ovary. Misshapen ovaries will often result in misshapen and unmarketable fruit. This problem occurs when the night temperature is allowed to drop to 55 degrees or lower.

Scott’s greenhouse is on a drip irrigation system, which delivers fertilizer and minerals. McAvoy stressed that nutrient applications through a watering system must be carefully calibrated, and you need to know how much water is going to the plant to be able to gauge and administer the proper mineral levels. Inaccurate water delivery will have a significant effect on the fertilization and mineral application.

McAvoy and Scott with worker named Francis in foreground.

Potassium (K) to nitrogen (N) ratio is important for balancing continued vegetative growth with fruit production. McAvoy recommends that K-to-N be close to 1-to-1 to support plant vigor during seedling development. Then, go to a ratio of 1.25-to-1, K to N, after transplant but before first flowering. Then aim for 1.5-to-1 from first flower to fourth cluster set and then 1.7-to-1 during fruit ripening. The high potassium favors good color development. If vigor begins to slow too much, a producer can drop the ratio to 1.25-to-1 for a few weeks to boost vigor (vegetative development) and then return to the higher ratio for fruit development.

McAvoy suggests adding magnesium sulfate (MgSO4) once the plant gets into the fourth cluster. Magnesium deficiency will show up at the bottom of the plant on the lower leaves and rarely affects yields unless left unchecked for a long time. An iron (Fe) deficiency first shows up on the youngest leaves (top of the plant) and will quickly reduce yields if not corrected at the earliest stages. The phosphorus (P) level should be 50 ppm during the entire life of the tomato crop. The calcium (Ca) level is kept at a minimum of 120 ppm but can be boosted to 180 ppm for more aggressive vegetative growth. Calcium nitrate (Ca(NO3)2) and potassium nitrate (KNO3) are the two primary fertilizers used to adjust K-to-N ratios. Ammonium nitrate (NH4NO3) is also used and is a good option to quickly increase vigor. However, the farmer must avoid providing more than 10 percent of the total nitrogen in the ammonium form.

An example of powdery mildew.
Clusters nearly ready for picking.

If the K-to-N ratio needs to be changed, the calcium nitrate is increased and the potassium nitrate is reduced, which will also change the calcium level as well. McAvoy notes that the farmer also needs to be aware of the competitive ions like magnesium and ammonium. If either of those ions gets too high, they will interfere with the calcium uptake. The calcium and potassium nitrate fertilizers are used in combination with a base formulation such as 5-11-26 (NPK) with micronutrients for complete nutrition. He cautions that with all the different fertilizer products on the market, a person has to pay close attention to the different levels of micronutrients when changes need to be made.

Molds and mildews can be a real challenge for greenhouse tomatoes. Gray mold—Botrytis cinerea—is a common affliction that can damage both plant and fruit. It tends to establish itself when moisture from condensation remains on the plants from thermal cooling in the evening. High humidity and stagnant air will also favor gray mold infections. For Scott’s tomato crop, there were no signs of gray mold. However, he did have a bout with powdery mildew—Oidium neolycopersici—during the middle of June. He explained that one day he walked into the greenhouse and the plants were “all white.” It literally happened overnight. He commented that this had been the first time in years that he’d had to spray for mildew so early in the season and attributed the problem to an unusually humid and rainy June.

Winston Scott’s strategy with growing greenhouse tomatoes is that it allows him to have tomatoes ready for sale at his nearby produce stand by early June before his field tomatoes are ready. He keeps his greenhouse tomatoes producing up through the eighth cluster. By that time, the plants are heading toward 7 feet high and getting too difficult to manage; plus, he’s got other produce on his farm to concentrate on. He says that each year he learns a little more about growing his greenhouse tomatoes, and in spite of the challenges, is committed to doing a better job raising them. They are popular and a good moneymaker.

The author works for Central Connecticut Farmer’s Cooperative in Manchester, Conn., and is an occasional contributor to Growing.