The potato is a satisfying garden staple, offering a generous yield for the effort invested. Achieving a successful harvest relies heavily on proper timing, especially in the cool, moist conditions of the Pacific Northwest. The region’s moderate climate offers a long growing season. Gardeners must pay close attention to soil conditions to prevent seed pieces from rotting before they can sprout.
Determining the Ideal Planting Window in Seattle
The timing for planting potatoes in the Puget Sound area is determined more by soil temperature than by the calendar date. Seed pieces are vulnerable to cold, wet conditions, though foliage can tolerate a light frost. Potatoes will not grow until the soil temperature reaches a minimum of 40°F, with 45°F being the safer threshold for planting.
The Seattle area typically experiences its last hard frost in late winter, allowing for an early spring planting window. Gardeners often aim to plant two to four weeks before the average last frost date, which generally falls in mid-to-late March. Planting early yields small, tender “new” potatoes. A second crop can be planted as late as mid-June for a fall harvest of mature, storage-ready tubers.
Seed Preparation and Soil Requirements
Selecting certified seed potatoes prevents common diseases carried in grocery store or saved potatoes. Seed pieces should be cut into chunks approximately 1.5 inches square, each containing at least one or two “eyes” (growth buds). The pieces must cure for one or two days in a warm, well-ventilated area to form a protective callous. This curing process reduces the risk of rot after planting.
A technique called “chitting,” or pre-sprouting, involves exposing seed potatoes to light and moderate temperatures (60°F to 70°F) for a few weeks. This encourages the eyes to develop short, stout, green sprouts, giving the plant a head start once it is placed in the ground. The ideal soil for potatoes in the Seattle region is loose, well-drained, and slightly acidic (pH 5.4 to 6.6). Incorporating well-aged compost into the dense, often clay-heavy local soil improves drainage and structure. Avoid too much fresh organic matter, as it can encourage potato scab.
Planting Techniques and Hilling
Potatoes are typically planted in shallow trenches about 4 inches deep, with seed pieces spaced 10 to 12 inches apart. After placing the pieces, cover them initially with only 3 to 4 inches of soil. Leaving the trench partially filled allows space for the subsequent process of hilling.
Hilling involves drawing soil up around the plant’s stems as they grow, creating a mound. This practice is performed when the stems reach 6 to 8 inches tall, adding soil to cover half the stem’s height. The primary purpose of hilling is to prevent developing tubers from being exposed to sunlight, which causes them to turn green and produce solanine. Repeated hilling, usually every two to three weeks, also increases the total yield by providing more underground stem surface area for new potatoes to form.
Timing the Harvest
Harvest time depends on whether the gardener desires “new” potatoes or fully mature tubers for storage. New potatoes are small, thin-skinned, and tender. They are harvested early while the plant foliage is still green, often around the time the plant begins to flower. This early harvest sacrifices the potential for a larger yield from the plant, but these potatoes are prized for their fresh flavor and quick cooking time.
For long-term storage, the potatoes must be fully mature. Maturity occurs after the foliage naturally yellows and dies back, signaling the end of the plant’s growth cycle. The tubers should remain in the ground for about two weeks after the tops have died down. This waiting period allows the potato skins to thicken or “set,” which is essential for successful storage. After gently digging the mature potatoes, they should be cured in a dark, well-ventilated space at moderate temperatures for seven to ten days. This process further toughens the skins and heals any minor injuries before they are moved to a cool, dark storage area.