Sweet potatoes are generally ready to be harvested after about 90 to 130 days in the ground, depending on the variety and growing conditions. Here are the key details to determine the right time to harvest:

• Timing: Most sweet potatoes mature between 90 and 130 days after planting. You can use the days to maturity for your specific variety as a guide (e.g., Beauregard variety takes about 90 days).
• Signs of readiness:
  • Yellowing leaves, especially the lower leaves, indicate that the tubers are ready. The foliage may also start to droop.
  • The soil around the plants may mound up as the sweet potatoes push up the soil.
• Avoiding issues: Harvesting too early results in smaller, starchy, less sweet potatoes. Harvesting too late may cause the potatoes to become woody, reduce storage life, and risk frost damage.
• Harvesting method: It's advised to cut back or mow the vines a few days before harvesting (but not too early to avoid toughening the skins) and then gently dig the tubers up with care to avoid bruising.
• Climate considerations: In cooler climates, harvest before soil temperatures drop below 50°F and definitely before the first frost, which can damage the sweet potatoes.
• Curing: After harvesting, curing sweet potatoes for 7 to 10 days at a warm temperature improves their sweetness by converting starches to sugars.

So, the best time to harvest sweet potatoes is typically in early fall when the leaves start yellowing and about 90 to 130 days after planting, before frost hits, using these indicators for optimal flavor and storage.