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Some tomatoes develop green shoulders (or yellow shoulders), even when the rest of the fruit ripens. It’s annoying because you want the whole tomato, obviously, to be red and tasty.
The phenomenon is not caused by a pest or a disease, but is directly related to the weather. The good news is that once you understand why tomato tops go rogue, you can take steps to prevent it from happening.
There are a couple of reasons that tops of tomatoes stay green or yellow while the rest of the fruit ripens. The first has to do with lycopene production. Lycopene is a plant pigment which gives tomatoes their red color.
The ideal temperature for lycopene development is 65-75º F. When temperatures rise above 75ºF and stay sustained, lycopene production is inhibited.
The irony is that tomatoes like heat. Plants tolerate higher temperatures than 75ºF consistently throughout the summer. But it’s the fruit’s exposure to the direct sun that dictates what happens on its shoulders. The upper portions of tomatoes generally receive the most exposure to heat or sunlight. As sun strikes tops of tomatoes, temperatures in the fruit rise, inhibiting lycopene. Without precautions, those portions cannot produce lycopene. They stay green.
Another reason tops of tomatoes may stay green has to do with chlorophyll, the pigment that gives plants green color. Excessive heat prevents chlorophyll from breaking down. So when ripening green tomatoes are in the direct, hot sun for hours on end, chlorophyll hangs on. Put together chlorophyll’s stubbornness with lycopene’s inhibition issues, and green reigns where red is meant to break through on tomato tops.
Carotene, another pigment in tomatoes, produces yellow and orange. It is less affected by heat. When higher temps and hot sun strike tomato tops, carotene (yellow) shines through while lycopene (red) is squelched. The lower part of the tomato is often protected from direct exposure by the top of the fruit. Thus your tomatoes' shoulders can remain yellow while the rest of the tomato ripens to red.
When temperatures are high and stay that way, exposed tomato shoulders may not ripen evenly along with the rest of the fruit. Pigments are the culprit. If the variety’s carotene makeup is on the low side, then the fruit will likely exhibit green shoulders. Higher carotene content means the tomato shoulders will be yellow. Chlorophyll throws a monkey wrench in the whole mix.
But the problem is not insolvable! No, you cannot control the weather. But when you protect tomato plants from excessive direct sun, you can allow them to ripen evenly.
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