Sweet potatoes are a warm-season crop, meaning they require extended periods of heat to produce a harvest. Unlike standard potatoes, which grow from tubers, sweet potatoes are grown from vine cuttings called slips and are highly sensitive to cold weather. Timing the planting correctly is the single most important factor for success in temperate climates, such as Zone 6. This guide focuses specifically on the planting schedule and necessary preparations for gardeners in Zone 6, ensuring the slips are put into the ground at the optimal moment.
Understanding Zone 6 Climate Constraints
The United States Department of Agriculture (USDA) Plant Hardiness Zone 6 is defined by its average annual minimum winter temperature, but for warm-season crops like sweet potatoes, the spring and summer temperatures are more relevant. The primary challenge in this zone is the relatively short growing season and the risk of a late spring frost. Sweet potato slips are extremely tender and will suffer severe damage or death if exposed to freezing temperatures or prolonged cool weather.
The average last spring frost date for Zone 6 typically falls between early to late April. Following the last frost, air and soil temperatures must warm considerably for the plants to thrive. Sweet potatoes require air temperatures to be consistently above 65°F, and soil temperatures must be well above the minimal survival threshold.
If the soil remains too cold, the planted slips will experience thermal shock, which severely stunts their growth and may encourage rot. This need for warmth dictates that planting must be delayed well past the point when other, more cold-tolerant vegetables are set out.
Pinpointing the Ideal Planting Time
The calendar date for planting sweet potatoes in Zone 6 is secondary to the actual temperature of the soil. Planting should occur only after the soil at a depth of four inches has reached a consistent temperature of at least 60°F. The optimal temperature for rapid growth and establishment is actually closer to 65°F, with some sources recommending up to 70°F for peak results.
For many locations in Zone 6, this critical temperature threshold is not met until two to four weeks after the average last frost date. This usually places the ideal planting window between the end of May and the first two weeks of June. Planting too early, even just a week or two, into cold soil results in slow, weak growth and poor root formation, ultimately leading to a reduced harvest.
Gardeners can measure the soil temperature accurately using a simple soil thermometer, checking the reading in the morning before the sun has had a chance to significantly warm the surface. This physical measurement is far more dependable than relying solely on a calendar date, which can vary significantly depending on the year’s weather patterns. Delaying planting until the soil is sufficiently warm ensures the slips can immediately begin forming the storage roots that will become the final harvest. Planting too late, however, may not leave enough time for the tubers to fully mature before the first autumn frost arrives, typically between mid-to-late October in Zone 6.
Essential Steps Before Planting
Before placing the sweet potato slips into the garden, several preparatory steps are necessary to ensure their survival and vigorous growth. Sweet potatoes are started from slips, which are rooted sprouts taken from a mature tuber, rather than from seeds or whole potatoes. If purchasing slips by mail, they may arrive looking wilted or without fully formed roots, and they require immediate attention.
The slips must be “hardened off” by gradually introducing them to outdoor conditions over several days to a week before transplanting. This process involves exposing them to increasing amounts of direct sunlight, wind, and cooler outdoor temperatures, which toughens the tender plant tissue. This prevents severe shock when they are permanently moved to the garden bed.
Soil preparation should focus on creating a loose, well-draining environment, often achieved by planting in raised beds or earthen mounds. Mounding the soil helps it warm up faster and improves drainage. It is important to avoid over-fertilizing with nitrogen, as this encourages excessive vine growth (leaves) at the expense of developing the edible storage roots.