Tomato Growing Style: It Depends on Climate

One question that comes up this time of year is tomato pruning, training, and support. Many books and websites tell you to prune indeterminate tomatoes to one stem, pinching out the “suckers.” (These are actually the new branches that sprout in the crotches where leaves meet the stem.) Professional growers use overhead rails, strings, and clips to train their tomato plants as a single vertical stem with clusters of fruit growing from it. Many home growers do the same. For some, it gives great results. For others, it is a disaster. How do you know what will work for you?

The answer is that like most other aspects of gardening, it depends on climate. In places like England, Seattle, or Maine, summers tend to be cool and severe pruning helps the plant to ripen more fruit. Even in some places that get a lot of heat, like the South or Midwest, there is often cloud cover or humidity in the air, which diffuses the sun's rays. Each tiny droplet of water sends the light in a slightly different direction, softening its effect in the same way that a frosted light bulb does. Shadows aren’t as dark, and lighted areas aren’t as bright. This is obvious during overcast weather, but even a bit of haze will diffuse the light to some degree. Plastic hoop houses have the same effect, breaking up the sun's rays so that the light seems to come from all directions instead of one focussed beam.

When the air is dry, nothing bends or breaks up the sun’s rays. If humidity makes the sun more like a florescent light, dry air lets it act more like a laser. Any surface that faces the sun directly heats up, so tomatoes, peppers, apples, and other fruit can literally cook in spots, which we call sunscald. Cherry tomatoes are less susceptible than slicers, because their small round shape has less exposed surface. Large peppers often suffer the worst--their relatively flat sides make a wide target.

Another aspect of the sunscald problem comes when there is water on the leaves in hot weather. A droplet of water acts like a lens, focussing the sun's rays and killing the leaf tissue. I've seen holes in leaves where droplets were, and whole leaves turn brown when they were coated with water. I've learned that I have to be very careful about any water getting on leaves during sunny, windless times. I've also seen that I can safely wet plants when wind keeps them in motion, or when they are inside a hoop house where the plastic diffuses sunlight.

I once had neighbors who learned to farm in the Northwest. They spent hours training tomatoes to single stems with elaborate supports–only to lose them all to sunscald in our unforgiving sun. Growing tomatoes outdoors where I live, we need leaves to shelter the fruit. As a bonus, the leaves make sugars to power growth and sweeten the tomatoes. We leave the plants to branch freely, so they grow into large, heavy masses. This in turn affects how closely we can plant. If I were going to prune off all the side branches, I would need to raise many more seedlings and plant them more closely. Since I let the plants branch, I have fewer, larger plants.

How do you know which kind of sunshine you have? Of course, if sunscald has been a problem for you before, you have your answer. But what if you are new to growing, or have moved recently, or have changed your garden style? Or what if your weather seems to have changed? One way to find out is to search online for "average summer afternoon humidity" in your area. Areas with 50% humidity or more have much less problem with sunscald. Areas that are near the ocean or in the eastern half of the country usually have enough cloud cover or humidity to soften the effects of direct sun.

Areas in the Plains, mountains, Southwest or inland from the Pacific coast  The sunscald zone tends to have clear blue skies much of the summer and get little or no summer rain. Your own eyes and skin can tell you a lot. Is there mist or haze when you look into the distance? Does sweat evaporate or stay on your skin? Does bread get dry and hard if you leave it out in the air? If evaporation is fast and complete, then humidity is low. If in addition there is no cloud cover, you have the conditions for sunscald.  You may want to use leafy cover, shadecloth, or other strategies to cut the sun's intensity.

Peppers are even more prone to sunscald but the solution is different: group them in a block, so that most of the light comes from above, not from the side. It helps to position them where a shadow falls for a couple of afternoon hours. And I usually grow basil, bush beans, flowers, or some other low, bushy companion between them and the sun. Peppers are native to riverbeds and arroyos in the Southwest, where the canyon walls give shelter and sunlight comes from directly above. A bed of peppers surrounded by basil may not look like a Southwestern arroyo, but once you know what conditions a plant needs, you can figure out how to provide them in the garden. 

There is one type of tomato pruning you should do no matter where you live.  Once the plant is growing well, you should remove the leaves at the base of the plant. At transplant, I remove the leaves that touch the ground. As the plant grows, I remove the leaves below the first truss of flowers.

Tomato vines are constantly putting energy upward and out to the growing tips, so the lower leaves naturally atrophy and start to die. Mites, thrips, and other pests climb up from the ground to infest those lower leaves. Removing them keeps the plant healthier. Another way to keep disease and pests away is to mulch any bare earth around the plants. You can safely remove all the leaves up to the first truss of fruit, which will be shaded by the leaves above. One final bit of pruning I like to do: a month before the first frost I cut off the growth tips to let the plant concentrate on ripening existing fruit.