While tomatoes can be kept as a perennial, most of the time gardeners opt to treat it as an annual. It can be a PITA to keep it alive and happy when it hits the hot season. You could cut it back and coddle it for regrowth- but you might find struggle with it during the heat. Some gardeners take cuttings to clone out if it's a particular fave, but that too takes some coddling during the heat. So many gardeners opt to just take them down if they are mostly spent from moths of harvesting and it's just hitting too hot for good produciton.
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