Starting seeds in a controlled environment, such as a greenhouse, allows gardeners to significantly extend the growing season. This method provides seedlings with the necessary head start before outdoor conditions are suitable for transplanting. Creating a successful planting schedule requires understanding the local climate, particularly the constraints imposed by winter cold. The USDA Plant Hardiness Zone Map provides a framework for this planning, designating areas based on average coldest temperatures.
Defining Zone 6 and Your Local Frost Date
Zone 6 is defined by the USDA Plant Hardiness Zone Map as having average annual minimum winter temperatures between 0°F and -10°F (-18°C to -23°C). While this designation determines which perennial plants survive winter, the Average Last Frost Date (ALFD) is the more relevant metric for timing annual vegetable crops.
In Zone 6, the ALFD typically falls between April 1st and April 21st. This date represents the historical average after which the probability of temperatures dipping below 32°F greatly decreases. Gardeners must confirm their specific local ALFD, as microclimates caused by elevation, bodies of water, or urban density can shift this date by a week or more.
The Formula for Seed Starting Timing
Determining the optimal seed starting date uses a reverse-calendar method anchored to the local ALFD. The formula involves subtracting the total required indoor time from the expected outdoor transplant date. This indoor time consists of two main components: the weeks needed to grow to a suitable transplant size (‘X’ weeks) and the time required for the acclimatization process known as “hardening off” (‘Y’ weeks).
The ‘X’ period varies by crop, typically ranging from four to twelve weeks, depending on germination speed and desired size. Hardening off is a non-negotiable process that slowly transitions seedlings from the stable greenhouse environment to fluctuating outdoor conditions. This involves gradually exposing young plants to cooler temperatures, direct sunlight, and wind over seven to ten days. Starting seeds too early risks the seedlings becoming root-bound and stressed before the ALFD has passed.
Specific Seed Starting Calendar for Zone 6 Crops
The seed starting schedule groups crops by the length of time they require indoors before transplanting.
Long-Lead Time Crops (10–12 Weeks)
These crops are typically started in late January or early February. This category includes slow-growing items like onions, leeks, and celery, which need a substantial head start to form mature bulbs or stalks. These cool-season crops often benefit from cooler germination temperatures, around 55°F to 65°F, once the initial sprouting phase is complete.
Medium-Lead Time Crops (6–8 Weeks)
These should be sown in late February or early March. This group contains warm-season crops, such as tomatoes, peppers, and eggplants. These heat-loving plants require soil temperatures consistently above 70°F for optimal germination. They should not be transplanted outdoors until two weeks after the ALFD when the soil has warmed sufficiently.
Short-Lead Time Crops (4–6 Weeks)
These are generally started in mid to late March. This includes brassicas like broccoli and cabbage, which can tolerate light frost and are set out a few weeks before the ALFD. Cucumbers and squash are also included, though many gardeners prefer to direct sow them or start them only three to four weeks before transplanting due to their rapid growth and dislike of root disturbance.
Using Greenhouse Environment to Modify Timing
A greenhouse environment mitigates the risks of early planting by offering environmental control. A heated greenhouse can decouple the seed starting date from the outdoor climate, unlike a standard windowsill setup. By maintaining consistent air temperatures and providing high-intensity supplemental grow lighting, a heated greenhouse mimics late spring or early summer conditions.
This controlled environment allows the gardener to safely start medium and long-lead crops two to four weeks earlier than the standard calendar dictates. The greenhouse provides the required warmth for warm-season crop germination and the light intensity necessary to prevent seedlings from becoming “leggy” and weak. Conversely, an unheated greenhouse, such as a cold frame or simple hoop house, offers minimal temperature control during late winter nights. The seed starting schedule must still closely adhere to the local ALFD, as internal temperatures can fluctuate and drop below freezing, potentially killing tender seedlings.