Field tomatoes are a long season crop with high water requirements. An average cultivar requires about 40 cm (15.7 in) of water over the growing season, with the need for moisture increasing until full fruit load is developed. The most critical times for moisture are during flowering, fruit set, and fruit sizing.

When tomatoes experience moisture stress:

  • fewer flowers develop per truss, lower numbers of fruit are set (lower yield)
  • total soluble solids increase (higher soluble solids means higher recovery in the processing plant and generally improved taste of fresh or processed tomatoes)
  • fruit size decreases (lower yield; may result in unmarketable fresh market fruit and greater harvest loss in mechanical harvesting of processing tomatoes)
  • blossom-end rot increases and loss can be significant in susceptible varieties (affected fruits are unmarketable; can lead to black specks and high mould counts in processed product; if severe, loads or fields of processing tomatoes can be rejected; Agricorp offers a salvage benefit for processing tomato fields severely affected by blossom-end rot)
  • plant canopy temperature increases (an additional stress to the plant; associated with reduced transpiration and photosynthesis therefore reduced growth; at fruit temperatures above approximately 30°C, development of red colour is inhibited)

Tomatoes are more tolerant of moisture stress than crops such as pepper and cucumber. They can adjust their physiological processes to conserve water while maintaining some growth. Early exposure to moisture stress makes the plant more tolerant of moisture stress later in the season. While this allows the tomato plant to survive where some crops would suffer irreversible damage, prolonged water stress does reduce yield as the plant uses energy to make these adaptations.

Although the plant can survive dry conditions, optimal yield and quality will not be achieved. Irrigation of tomatoes can result in higher and more consistent yields, better quality, less blossom-end rot, and less cracking.

Fields with good soil structure and higher organic matter will hold more moisture and allow roots to penetrate better. Compacted layers will limit the spread of roots. In tomato, 85 per cent of the root system is typically in the upper 30 cm (12 in) of soil, but tomato roots can extend beyond 1 m (39 in) in depth. Cover crops can be grown and the residue used to mulch the soil, reduce water loss, and moderate soil temperatures. However, this requires advance planning and could have a detrimental effect in a wet spring. In irrigated fresh market tomato production, plastic mulches can be used to conserve water. Minimizing tillage can also reduce moisture loss.

In the early 1980s, Dr. C.S. Tan at Agriculture and Agri-Food Canada found that the marketable yields of processing tomato were directly related to seasonal evapotranspiration (ET; plant water consumption). Below 88 mm of ET, tomato produced no marketable yield on a Fox sandy loam. Above 88 mm, yield increased linearly, to an optimum when 300-400 mm of water was available to the crop.

Optimum seasonal ET would be expected to be higher with current higher yielding varieties and production practices. Recent research on processing tomatoes in Ontario has shown yield increases of up to 81 per cent on a range of soil types with the use of properly scheduled irrigation. However, under dry conditions irrigation is most critical on the lighter textured soils with low water holding capacity. It is estimated that approximately 40 -50 per cent of the processing tomato acreage is being irrigated in 2007.

For more information on dry conditions and low water go to our Adverse Weather page.