Fluctuating growth rates cause tomatoes to suddenly expand. This might happen after a period of drought followed by generous watering, or low temperatures followed by a sudden sunny spell. The split might then become infected, causing the tomatoes to rot.
Solution
Organic
Water plants regularly and consistently, so it's not feast then famine for the plants. Ventilate greenhouse tomatoes during very hot spells and provide shading. Promptly pick off any split tomatoes before they start to rot.
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