Dry-farmed tomatoes - how to grow them

Q: I've been reading about dry-farmed tomatoes, how water is withheld from the growing plants so that the roots have to go down deep to find their own water. It makes sense that the flavor ends up being more concentrated this way, but I'm wondering if the same effect could be achieved just by watering much less when the plants start fruiting? Do the roots really need to go all the way down to the groundwater? I guess that would make them less likely to dry out completely (versus being dependent on someone watering them), but does it affect the flavor all that much?

A: Because dry-farmed tomatoes are said to have superior flavor, I can understand why you'd want to try the technique at home. You may be able to improve flavor somewhat by watering less after fruit set, but may not be able to replicate what some farmers can do.

Dry-farming tomatoes is recommended only for areas with at least 20 inches of rain, which allows the soil to be fully charged with water by spring, and where significant marine influence somewhat cools summer weather, slowing evaporation. Soils need to be highly water-retentive, so soils with moderate to high clay content are best. Sandy soils dry out too quickly. Plants are set 4 to 6 feet apart and weeds are kept out. With such wide spacing, tomato plants don't compete with one another, and their roots range wide and deep to find enough moisture. Under ideal conditions, one could harvest a crop with perhaps only a watering at planting time. Production is lessened and fruit smaller, but it becomes sweeter as summer progresses.

I asked Mike Cahn, Cooperative Extension irrigation adviser for Santa Cruz, Monterey and San Benito counties, if the roots have to reach groundwater. He said it's unlikely that they would, because roots need oxygen and soil has little air deeper than 4 to 5 feet, which is above many water tables. "Most soils have some sort of impeding layer below 4 or 5 feet (clay layers or hardpans) that would further prevent such deep root growth," he added.

For an example of how a farmer might dry-farm tomatoes, I spoke to Liz Milazzo, field production manager at the UC Santa Cruz Farm and Garden Program. She says they begin their process in November by planting a cover crop consisting of 45 percent bell beans, 10 percent oats and 45 percent vetch to increase fertility and moisture retention. They dig this into the soil between March 1 and early May, as soon as rains let up long enough for the soil to be dry enough to turn. Ideally, they'd plant the tomatoes at least three weeks after turning the cover crop under, and no later than mid-May. If late rains soak the soil again, they have to let it dry a bit before digging to plant. If rains fail to fall after the cover crop has been turned in, they water deeply, then let the soil dry to the correct moisture before planting.

They grow large, purposely leggy seedlings, remove leaves except near the top and "bury the plants up to their necks." (Tomato plants will grow roots from buried leaf nodes.) They use the variety 'Early Girl', a common choice for dry-farming, but any early-fruiting variety with big plants might succeed. This year, they are also growing 'Cherokee Purple' and 'Chianti Rose'

Two weeks after planting, they cultivate the top 6 inches of soil to create a "dust mulch" with the intention of reducing water loss from the surface of the soil. They water only if the plants threaten to wilt, and when the first fruit ripens, they cut out water altogether. The first picking or two after that might not taste much different, but later pickings become sweeter.

It seems the formula for dry-farming depends on the conditions. Where they are ideal, you might not have to water at all. Where rainfall, soil or spacing is less than ideal, you'd have to water deeply occasionally as the plants grow, and might not be able to stop watering altogether when the harvest starts. It's possible for backyard gardeners to practice a form of the technique despite less-than-ideal conditions.

One note: Based on a review of numerous studies that attempted to evaluate whether a "dust mulch" actually preserves soil moisture, Linda Chalker-Scott, an extension urban horticulturist at Washington State University, says no, it doesn't. The theory, steeped in tradition, is that hoeing up the surface of dry soil breaks the capillaries through which water would evaporate. Hoeing might reduce weed competition, but Chalker-Scott says it does not reduce water loss; it exposes the soil to erosion, and, in the majority of cases, it does not increase crop yield. Read her article online at links.sfgate.com/ZKKT.

Pam Peirce is the author of "Golden Gate Gardening" and "Wildly Successful Plants: Northern California." Read her blog at www.goldengategarden.typepad.com or e-mail her at home@sfchronicle.com.

This article appeared on page L - 3 of the San Francisco Chronicle