Tomatoes 101, part II: a production guide

This is the second part in a two-part series about greenhouse tomato production. You can read part one here.

Pollination: While tomato flowers can be self-pollinated and do not require a different cultivar to act as a cross-pollinator, tomato flowers must be actively pollinated to maximize the number and quality of fruits. For large greenhouses, bumble bees are most commonly used as a pollinator. For smaller plantings, vibrating wands placed on flower clusters or a gentle shake of the plant by hand are sufficient for pollination. If pollinating by hand, save time and effort by pollinating only those flowers that are ready. Evidence of a flower ready for pollination includes petals that are reflexed or curving up, as well as anthers with pollen that is a vibrant yellow (not a pale green) color and may also have a rough appearance, indicating the pollen is mature and ready for dispersal.

Pruning and training: The first type of pruning or training you may do is to bifurcate your tomato plants. Instead of planting seedlings and training a single main leader, a second leader is allowed to develop; the stems are then run in opposite directions in double rows. This reduces the number of transplants required, but can delay the time until the first tomatoes are harvested. Whether a single or bifurcated transplant, stems are trained to twine using the “lean and lower” system. Stems are trained to vine on twine suspended from a trellis above the crop, usually at the same height as the gutters, at an angle; as the crop reaches the wire, they are lowered down and allowed to continue to grow vertically (Fig. 1). Greenhouse tomato cultivars are indeterminate, meaning the plants should be grown with a single leader and axillary branches or “suckers” should be removed to maximize productivity (Fig. 2). If allowed to grow, they divert energy and growth away from desirable vegetative growth and fruits. It is generally best to remove suckers as soon as they are clearly identifiable; they can often be rolled out with your fingers. Larger suckers should be removed using a pruning shear or another tool so the main stem is not damaged. Foliage may also be removed from plants during this process. Old leaves are removed starting when the third or fourth cluster of fruits are starting to form, eventually up until the oldest cluster starts to ripen (Fig. 1). In addition to removing the lower foliage, you may also remove one or two leaves between fruit clusters during the summer when light levels are high; this will help balance the vegetative and generative growth of the plant. Depending on the type of tomato grown, you may need to prune fruit clusters. While this is not done for cherry, grape or cocktail tomatoes, some beefsteak or cluster varieties may need to have fruit thinned in order to promote better development of fewer fruits; the least developed or underdeveloped fruits are the ones that should be removed. In order to maximize the number of ripening fruits that can be harvested before terminating the crop, tomato plants are topped four to six weeks prior to the end of the crop. This will focus the remaining energy on the developing fruits instead of new growth that won’t produce marketable fruits before the crop is terminated.

With any of this pruning, be sure to sterilize any tools you are using between plants to avoid spreading diseases among your crop.

Pests: The two primary insect pests are thrips and whiteflies. Thrips are particularly problematic because, in addition to damaging plants and fruits, they serve as vectors for tomato spotted wilt virus. Thrips screening around vents and entryways are used to try and minimize this pest, but chemical and/or biological control may also be used. Parasitic wasps (Encarsia formosa) are a commonly used biocontrol for whiteflies.

Diseases: One of the most common diseases you will find on tomato plants in the greenhouse is powdery mildew. Dehumidifying the greenhouse is one of the best ways to prevent this disease. Viruses including tobacco mosaic virus and tomato spotted wilt can be devastating. These viruses are best managed by prevention: controlling potential insect vectors such as thrips; employing strict sanitation and hygiene policies for employees; and sterilizing tools used for pruning in between plants.

Physiological disorders: Physiological disorders can be observed on tomato foliage, flowers and fruits. One of the most common disorders observed on tomato foliage is intumescence — protrusions on foliage that look like little bumps. The cause of this disorder is unclear; it has been attributed to a lack of ultraviolet light, as well as high humidity when temperatures are warm and light levels are high. Flower abortion is the most common flowering disorder and is usually related to either excessively warm air temperatures (>86° F) or a unit heater or CO₂ burner that is not functioning properly and releasing ethylene from incomplete combustion. Blossom end rot is one of the most common fruit disorders and is caused by a calcium deficiency in fruits. While this can be caused by nutrient solutions low in calcium, it can also occur when there is adequate calcium available, but uptake is inhibited. Catfacing, where the fruit looks puckered from not fully expanding, is the result of incomplete pollination.

Harvesting: The appropriate time to harvest a tomato depends on which type of tomato you are producing. The earliest greenhouse beefsteak tomatoes are harvested is around the “breaker stage,” where the fruit is starting to turn from greenish-yellow to pinkish-red; harvesting at this stage is more for producers who will be shipping their crops farther distances and need firmer fruit to reduce susceptibility to mechanical damage during transport. For local markets or cluster varieties, fruits are to be harvested when they have over 90 percent color; this enhances the flavor of the tomatoes and creates a higher-value product. Regardless of variety, all tomatoes will be harvested by hand. Beefsteak tomatoes may be able to be twisted off by hand (taking care to leave the green calyx on), while cluster varieties should be cut from stems to avoid tearing stem tissue.

Postharvest care: Once fruits are harvested, they should be cooled down. The temperature depends on the degree of maturity; fruits that are fully ripened should be held between 46 and 50° F, while those with less color should be kept a few degrees warmer (52 to 54° F).

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu