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Q. Only the bottoms of my tomatoes are turning red. The tops turn a yellowish orange. Can you tell me what's happening?
A. Your tomatoes are facing a condition called yellow shoulders (sometimes called green shoulders.)
Pigments are the culprit, specifically lycopene, carotene, and chlorophyll.
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. So the tops of tomatoes, most exposed to sun and heat, ripen more slowly.
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. 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. This pigment 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. That's why you may get some green streaks in the top of your tomatoes along with yellow and orange.
You can help your tomatoes ripe evenly when you provide shade during hot spells. You can also limit pruning, allowing tomato foliage to thicken and provide extra shade relief for tomatoes during the heat of the summer.
Learn more about yellow and green shoulders here.
Good luck and happy gardening!
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