The arrival of March marks a significant turn for gardeners in Zone 6, offering the first real opportunity to begin the season indoors. Starting seeds now is a strategic move, effectively extending the short growing window by cultivating tender plants in a controlled environment. This indoor head start ensures that heat-loving vegetables and slow-maturing flowers reach a robust, transplant-ready size just as the outdoor weather becomes reliably warm. Timing the sowing of seeds precisely allows for earlier harvests and more established ornamental displays than direct-sowing would allow.
Understanding Zone 6 Climate and Timing
The most important metric for a March seed start in Zone 6 is the average last spring frost date. Historically, this date falls within a range, typically from the beginning of April to the third week of April, though local microclimates can push it toward early May. This date is used to calculate the indoor sowing schedule.
Gardeners determine the appropriate indoor starting time by counting backward six to eight weeks from the last frost date. Starting seeds in March aligns perfectly with this timeline, allowing plants to mature sufficiently without becoming root-bound or overgrown before the danger of freezing temperatures has passed. The goal is to produce strong, compact seedlings ready for the garden bed immediately after the soil warms. Planting too early risks seedlings becoming leggy, while planting too late sacrifices valuable weeks of the growing season.
Essential Seeds to Start Indoors in March
March is the optimal time to sow seeds for crops requiring a long period of vegetative growth or those sensitive to cold temperatures. This includes major heat-loving vegetables, which need a 6- to 8-week indoor start for a viable harvest in Zone 6. Tomatoes, peppers, and eggplants fall into this category, requiring warm soil temperatures (70°F to 85°F) for successful germination and early development. Starting these in mid-March ensures they are sturdy, six- to eight-week-old transplants by the time the soil is warm enough in mid-to-late May.
The first half of March is also the right moment for cool-season crops in the Brassica family, such as broccoli, cabbage, and cauliflower. These vegetables thrive in cooler weather and can be transplanted outdoors in late April, several weeks before the last frost date. Giving them a 6- to 7-week head start indoors allows them to size up and mature before the intense heat of summer causes them to bolt.
Slow-growing annual flowers and certain perennial herbs also benefit significantly from a March start. Petunias and snapdragons, for instance, need 8 to 10 weeks indoors to reach blooming size. Perennial herbs like rosemary and lavender, which are notoriously slow to germinate, should be sown in early March to ensure they are well-established when moved to the garden.
Preparing and Starting Your Seeds
Success in indoor seed starting begins with selecting a sterile, lightweight seed-starting mix rather than heavy garden soil. This specialized mix is fine and well-draining, preventing disease and providing a loose structure that delicate roots can easily penetrate. Containers must have adequate drainage holes to prevent the growing medium from becoming waterlogged, a common cause of seedling failure.
Controlling the environment is paramount, especially for warmth and light, which are often insufficient on a simple windowsill. Bottom heat is applied using a warming mat to maintain a consistent soil temperature, speeding up the germination of most seeds, particularly heat-loving varieties. Once sprouted, the heat mat should be removed, and seedlings must be placed immediately under supplemental grow lights for 14 to 16 hours daily. These lights should be positioned just two to three inches above the plant tops and adjusted frequently to prevent the seedlings from stretching and becoming spindly.
Transitioning Seedlings Outdoors
The final stage of the indoor process is preparing young plants for the outdoor environment, a process known as hardening off. Seedlings grown indoors are accustomed to constant temperatures and filtered light, making them susceptible to immediate damage from the elements. This transition begins 7 to 14 days before the planned transplant date, which should be after the last expected frost for tender plants.
Hardening off involves gradually exposing the plants to increasing durations of sun, wind, and cooler temperatures. On the first day, seedlings should be placed in a sheltered, shady spot outside for one to two hours, provided the temperature is above 45°F. Each subsequent day, the plants receive additional outdoor time, and their exposure is slowly shifted to include dappled sunlight, then morning sun, and eventually, full sun. This methodical introduction encourages plant cells to thicken and develop a waxy cuticle, which minimizes water loss and reduces the risk of transplant shock.