Acorn squash generally reaches maturity during late summer or early fall, approximately 80 to 100 days after planting. A ripe squash feels hard and is difficult to pierce when pressing lightly on the rind with a fingernail.
Once the stem of the acorn squash begins to harden and dry and the vegetable has turned a deep green color, it can be harvested by cutting through the stem with a sharp knife. To avoid damage to the vegetable and plant, the squash should not be pulled from the stem. Acorn squashes can be stored for up to three months in a dark, cool place.