Are your tomatoes getting sunburn?
Even if you have a green thumb you might still end up with green shoulders. In the right context that makes more sense. Tomatoes, whether on the vine or windowsill, which are exposed to intense prolonged sunlight do not mature properly. No matter how long they are left in the sun to ripen they will always have “green shoulders,” areas near the top that fail to turn red. Experienced gardeners know this and do what they can to keep developing tomatoes partially covered. But this year, some common foliar diseases were much worse than usual and most tomato plants lost a good deal of their foliage. This left fruit at the mercy of UV radiation.
Plants such as lettuce and chard can get by even if they’re 50 percent shaded, but to be successful, a tomato plant requires full sun—eight hours a day at minimum—to be happy. This is for the benefit of the plant, though, not the actual tomatoes. Leaves are solar panels that make sugar, the life-blood of plants, which use it to build everything from stems to flowers and fruit. Tomato plants need loads of light to crank out those luscious “love apples,” as they were called by sixteenth-century Europeans who assumed they were poison and grew them as an ornamental for a century or so.
John Denver sang that sunshine on his shoulders made him happy, which has not exactly been my experience. Sunshine in general does make me happy, but if it lands on my shoulders for longer than about three seconds it makes me sunburned, even with lotion on. (In that regard, being of German and Irish extraction may render me one step removed from vampirism.) I have to wonder if the observation that light-skinned people turn red when exposed to the sun is what makes a lot of folks assume light-skinned unripe tomatoes also need a good sunburn to redden up. Whatever the reason, it is a common belief that tomatoes ripen better in the kitchen window than in a dark place.
The red color is due to several compounds, most notably lycopene. Many people believe it has significant health benefits and it is widely available as a supplement. Some of these claims are unproven, but the Mayo Clinic states: “A number of studies suggest that lycopene may have antioxidant benefits. These effects are believed to help prevent asthma caused by exercise, although the method is still unclear. Early studies also report that tomato-rich diets may benefit people with asthma.”
Here’s why tomato sunburns are green: As the tomato matures, chemical signals induce chlorophyll to start breaking down. This in turn triggers Lycopene production, which is localized, not systemic through the fruit. However, intense sunlight can override the mechanism that leads to chlorophyll breakdown, and little or no lycopene is made in those areas with a “sunburn.”
There are other reasons to keep tomatoes off the windowsill. Light reduces the nutritional value of foods. Many vitamins, including A, B12, D, K, E, Folic Acid, Pyridoxine, and Riboflavin, deteriorate when exposed to light. Amino acids are also light-sensitive. The amount of damage varies depending on light intensity and length of exposure.
Unripe tomatoes of mature size can be brought indoors, wrapped individually in newspaper and spread out on a tray to ripen. Temperatures between 50 and 70F are best. Peek at them every 5-7 days to get the good ones and weed out any that develop bad spots. That way you’ll have tomatoes well into the fall and none of them will give you the green shoulder.
Paul Hetzler is a horticulture and natural resources educator with Cornell Cooperative Extension of St. Lawrence County.