March marks a significant shift in the gardening calendar, bridging the planning phase with active planting. This month involves intense activity for both indoor seed starting and direct outdoor sowing, preparing the ground for the main growing season. Taking timely action in March is essential because many popular garden plants require a substantial head start to reach maturity before summer heat or fall frost arrives. This window represents the best chance to maximize the potential harvest.
Seeds Requiring an Early Indoor Start
Many heat-loving plants have long maturity cycles, requiring an early indoor start in March to guarantee a harvest before the end of the season. The primary candidates for this early indoor sowing are warm-weather crops like tomatoes, peppers, and eggplant. Starting these seeds now allows them to develop into robust transplants, ready to thrive once the danger of the last spring frost has passed.
Peppers, both sweet and hot varieties, need the longest period indoors, often requiring eight to twelve weeks to develop strong stems and root systems. Eggplant seeds also benefit from a similar head start, sometimes needing up to ten weeks indoors. These seeds germinate best with consistent warmth, ideally between 75°F and 85°F, often achieved by placing seed trays on specialized electric heating mats.
Tomatoes are typically started indoors approximately six to eight weeks before the expected transplant date. Starting them in mid-to-late March ensures the seedlings are not overly large or root-bound when moved outside. Other vegetables, such as long-season leeks and slow-growing brassicas like cauliflower and broccoli, also benefit from a March indoor start, particularly in cooler regions.
Slow-growing herbs like rosemary, thyme, and oregano can be sown indoors in March, as they take a long time to germinate and establish. Certain annual flowers, including petunias, violas, and pansies, also require early indoor attention to produce blooms early in the season. Once seedlings emerge, they require supplemental lighting from grow lights for up to sixteen hours a day to prevent them from becoming weak and leggy.
Cool-Season Crops for Direct Sowing
In contrast to indoor-started warm-weather plants, March is the ideal time to sow frost-tolerant, cool-season crops directly into the garden soil. These seeds germinate in cooler soil temperatures and produce a harvest before the arrival of summer heat, which often causes them to “bolt,” or prematurely flower. The soil must be “workable,” meaning it should not be frozen solid or excessively saturated with water.
Peas are a classic March-sown crop and should be planted as soon as the soil can be prepared, often four to six weeks before the last frost date. Both shelling peas and snap peas should be planted in trenches or rows, requiring support like trellises or netting as they climb. Spinach and lettuce, two highly cold-tolerant leafy greens, can also be sown directly into the ground, with some varieties germinating in soil temperatures as low as 40°F.
Root vegetables like radishes, carrots, and beets are also well-suited for direct sowing in March. Radishes are favored due to their rapid maturity, often being ready for harvest in as little as three weeks. Carrots require a fine, stone-free soil bed for straight root development and should be thinned after germination to ensure adequate spacing.
Other hardy options for direct sowing include kale, Swiss chard, and spring onions, all of which thrive in the lengthening, cooler days of early spring. Preparing the soil involves raking the surface to a fine texture and incorporating well-rotted organic matter to improve drainage and nutrient content. Sowing these cool-season varieties in succession, planting small batches every few weeks, allows for a continuous harvest.
Accounting for Regional Climate Differences
The precise timing for both indoor starting and direct sowing is highly dependent on a gardener’s geographic location and local climate patterns. The most critical factor for determining planting schedules is the average “Last Frost Date” (LFD) for a specific area. The LFD is the average date in spring when the probability of a freeze drops to a statistically low point, serving as the anchor point for all spring planting decisions.
Gardeners determine their region’s LFD by consulting historical weather data, often correlated with their USDA Hardiness Zone. For example, a gardener in a colder Zone 5 might expect their last frost in mid-to-late April, while someone in a warmer Zone 8 may have an LFD in late March. Indoor seed starting calculations are made by counting backward from the LFD based on the number of weeks a specific plant needs to grow before transplanting.
Cool-season crops can be sown outdoors several weeks before the LFD, as their seeds and young plants tolerate light frost. Warm-season crops, such as the tomatoes and peppers started indoors, must only be transplanted outdoors after the LFD has passed and the soil temperature is warm enough, usually above 60°F. Using the LFD allows gardeners to modify general advice for March planting, ensuring seeds are started at the optimal time for their unique microclimate.