Turnips, a member of the Brassica rapa species, are a versatile cool-season crop prized for both their root vegetable and their leafy greens. Achieving a successful fall harvest hinges entirely on precise planting time, as the crop requires a specific window to mature before deep winter sets in. The fall turnip crop must race against the calendar to reach maturity and develop optimal flavor. Defining the absolute latest time to sow seeds ensures the plants have the necessary growing conditions for a final, sweet harvest.
Understanding Turnip Maturity Time
Turnips exhibit a relatively quick growth cycle, but the exact Days to Maturity (DTM) varies widely depending on the desired harvest. Varieties grown primarily for their roots, such as ‘Purple Top White Globe,’ typically require 40 to 60 days to reach a harvestable size. Quicker-maturing varieties intended mainly for greens may be ready in as few as 35 days. The greens themselves can often be harvested continuously within a month of planting.
Turnips taste best when they mature in cooling weather. Hot temperatures cause the roots to become woody and the flavor to turn pungent or bitter. As temperatures decline in the autumn, the plant converts starches into sugars as a natural defense against freezing. This process significantly sweetens the roots and greens, requiring the crop to be fully mature before the first hard freeze.
Calculating the Absolute Latest Planting Date
Determining the final planting deadline requires a precise calculation based on local weather patterns and the plant’s growth cycle. The absolute latest date is anchored to the average date of the region’s first hard frost, defined as temperatures dropping below 28°F (-2°C). Turnips can withstand a light frost, but a sustained hard freeze can damage or kill the roots, making them unsuitable for storage.
The formula for the latest safe planting date is: Subtract the Days to Maturity (DTM) of your chosen variety plus an additional 10 to 14 days from the average first hard frost date. The extra 10 to 14 days allow the mature roots or greens to experience cold-induced sweetening. For example, if the average hard frost is November 10th and the DTM is 50 days, the planting deadline would be around the last week of August.
Gardeners focused solely on harvesting greens can use the shorter DTM, which extends the planting window compared to growing for roots. For a dual harvest of both root and greens, the longer DTM for the root must be used to ensure both parts are fully developed. This calculation provides a deadline that accounts for both physiological maturity and peak flavor development.
Techniques for Successful Late Planting
When planting close to the calculated deadline, specific horticultural practices maximize the chances of a successful fall harvest. Selecting quick-maturing varieties, such as those with a DTM on the lower end of the 40-day range, provides a buffer against an unexpectedly early cold snap. Varieties bred for faster development allow the necessary time for the sweetening process.
Soil preparation must focus on creating an ideal environment for rapid root growth, as time is limited. Turnips thrive in loose, deeply tilled soil that is free of rocks and debris, which prevents the roots from forking or becoming stunted. Direct sowing is mandatory, as turnips do not transplant well and any interruption to root growth is detrimental when rushing the maturity timeline.
Consistent moisture is necessary, because dry soil conditions cause the roots to become woody and bitter, negating the benefit of the cool weather. Utilizing season extension tools, such as row covers or cold frames, can offer a two-to-four-week extension to the growing season. These protective measures shield the plants from early light frosts, allowing more weeks for the roots to gain size and sweetness before the ground freezes.