A fall potato crop is a second planting, typically undertaken in mid-summer, aiming to harvest fresh tubers as the growing season concludes. Gardeners pursue this late planting to maximize yield and produce potatoes that often store better than those harvested during summer heat. These late-season potatoes grow during the cooler, shorter days of autumn, a period more favorable to tuber development. Successfully timing this crop requires precise calculations based on local climate data.
Calculating the Optimal Planting Window
Determining the precise planting date requires working backward from the average first frost date in your specific location. This date signals the end of the growing season for the potato foliage. A crucial step is identifying the “days to maturity” (DTM) for the chosen potato variety, which can range from 70 to 120 days.
The calculation involves taking the variety’s DTM and adding a buffer of approximately ten to fourteen days to account for the slower growth rate experienced in cooling temperatures and shorter daylight hours. This total number of days is then subtracted from the average first frost date to establish the latest possible planting day. For example, a 100-day variety in a region with an average October 20th frost date should be planted no later than early to mid-July.
Planting must be delayed until mid-summer because potato tuber formation slows or completely stops when the soil temperature consistently exceeds 80 degrees Fahrenheit. The ideal temperature range for tuber development is between 60 and 70 degrees Fahrenheit. For many regions, the optimal planting window falls between mid-June and late July, when the intense heat of high summer is past or can be mitigated with deep planting and mulching.
Selecting Seed Potatoes for a Successful Fall Crop
Selecting the right seed potatoes is important for a successful late harvest, as they must mature quickly before the season ends. Gardeners should prioritize varieties with a shorter DTM, often mid-season types requiring 70 to 90 days to reach maturity. Cultivars known for excellent storage qualities, such as Russets or Kennebec, are also preferred for a fall crop intended for winter keeping.
A challenge with mid-summer planting is that seed potatoes may still be dormant from prior cool storage. To ensure vigorous emergence, the seed potatoes often need to have their dormancy broken, a process called chitting or greensprouting. This is achieved by moving the tubers from cold storage into a warm, bright area, around 50 to 55 degrees Fahrenheit, for two to four weeks to encourage the development of short, sturdy green sprouts.
It is recommended to use certified seed potatoes for planting, as they are guaranteed to be disease-free and will provide the most reliable yield. If a whole tuber is larger than a hen’s egg, it should be cut into pieces, each containing at least one or two “eyes” or growth nodes. These cut pieces must be allowed to “heal” or callus over for at least two days in a warm, humid environment before planting to prevent rot, which is particularly important in the warm, moist soil conditions of summer.
Ground Preparation and Planting Technique
Proper soil preparation focuses on ensuring adequate moisture retention and drainage following the drying effects of summer heat. The planting area should be cultivated to a loose depth of at least ten inches and enriched with aged organic matter, such as well-composted manure, which provides slow-release nutrients. Soil with a slightly acidic pH, ideally between 5.8 and 6.5, also helps to suppress the prevalence of scab.
When planting, the seed pieces should be placed in a trench about four to six inches deep, which is slightly deeper than typical spring planting, to keep them insulated from the extreme heat near the soil surface. The pieces are typically spaced about twelve inches apart within the trench, with the sprouts facing upward. Immediately after planting, the trench should be partially filled with three to four inches of soil and then watered thoroughly to settle the soil around the seed pieces.
Applying a thick layer of organic mulch, such as straw or shredded leaves, helps to regulate soil temperature and conserve moisture during the initial, hot growth phase. This mulch also serves as the material for “hilling,” which involves mounding soil around the growing stems. Hilling protects the developing tubers from light, preventing them from turning green, and encourages the formation of more potatoes along the stem.
Late-Season Care and Harvesting Timing
Late-Season Care
As the fall crop grows, consistent moisture is a requirement, especially once the plants begin to flower and the tubers start to form. Inconsistent watering can lead to developmental problems, such as hollow centers, so the soil must be kept uniformly moist but not waterlogged. As temperatures drop, the risk of late blight increases, so gardeners should ensure good air circulation and monitor foliage for signs of fungal infection.
Harvesting Timing
The final harvest timing is determined by observing the plant’s natural life cycle and the calendar date of the first expected hard freeze. For storage potatoes, the goal is to allow the vines, or foliage, to die back naturally. This signals that the tubers beneath the soil have fully matured and developed thick, protective skins. Growers should stop watering the plants one to two weeks before the anticipated harvest to allow the skins to toughen, which improves storage life.
Once the vines have withered completely, the potatoes are ready to be dug up, ideally on a dry day to minimize the risk of rot. After harvest, the potatoes must be “cured” in a dark, well-ventilated space with moderate temperatures, ideally around 50 to 60 degrees Fahrenheit, for seven to fourteen days. Curing allows minor surface abrasions to heal and further thickens the skin, preparing the fall crop for long-term storage throughout the winter.