March in USDA Hardiness Zone 5 marks the transition to the active growing season. The average last frost date typically falls between mid-April and mid-May, meaning planting decisions must balance early starts with the persistent risk of freezing temperatures. Successful gardening requires a bifurcated approach: beginning tender, long-season crops indoors while utilizing the cool, damp soil for cold-tolerant, direct-sown vegetables. This strategy provides a head start on the short Zone 5 growing season.
Starting Seeds Indoors
March is the optimal window for initiating warm-season vegetables that require a prolonged period to mature before the summer heat. Heat-loving crops like tomatoes, peppers, and eggplants need eight to ten weeks of indoor growth to develop into robust transplants ready for the garden after the final frost has passed. Starting these seeds too late results in smaller plants that produce fruit later in the season, while starting them too early can lead to leggy, root-bound seedlings.
These young plants demand bright light. A dedicated grow light system, often using full-spectrum LED bulbs, must be positioned inches above the seedlings for 14 to 16 hours daily to prevent stretching. They should be planted in a sterile, fine-textured seed-starting mix to ensure proper drainage and minimize the risk of fungal diseases like damping-off. Maintaining a consistent temperature, often with the aid of a heat mat, helps ensure swift and uniform germination.
Many cool-season crops also benefit from an indoor start in March. Members of the Brassica family, such as broccoli, cabbage, and cauliflower, are best started five to seven weeks before transplanting. This allows them to establish a strong root system for setting out during the cool spring weather. Starting these plants inside ensures they reach maturity before the heat of summer causes them to bolt, or prematurely flower.
Direct Sowing Cool Season Vegetables
Hardy annual vegetables tolerate cold soil and can be sown directly into the garden as soon as the ground is workable. These crops thrive in the cool, moist conditions of early spring and falter once the weather turns hot. Garden peas can germinate in soil temperatures as low as 40°F, and should be sown in mid-March for the earliest harvest.
Leafy greens such as spinach and lettuce tolerate a light frost once established and are suitable for March direct sowing. Spinach is cold-hardy, with a minimum germination temperature of 35°F, making it one of the first seeds sown. Planting small batches of these fast-growing crops every two weeks (succession planting) ensures a continuous, manageable harvest rather than a single flush of produce.
Root vegetables like radishes and carrots, which dislike being transplanted, can also be sown directly into the soil late in the month. Radishes germinate quickly, often within a week, and their rapid growth makes them a good marker for rows of slower-germinating seeds like carrots. While carrots prefer a more friable soil, both can be planted once the soil has dried out enough to be easily worked without compacting.
Planting Bare Roots and Dormant Perennials
March is the time to plant dormant, non-seed material to prevent transplant shock. Root crops like onion sets (small, immature bulbs) can be planted six weeks before the last expected frost date, typically toward the end of March. Planting these sets early allows the onions to size up during the cool spring days.
Perennial vegetables, which return year after year, should be planted now before they break dormancy. Asparagus crowns and rhubarb roots are commonly planted in early spring while the soil is cool but not frozen. This timing allows them to settle in and establish the deep root systems necessary for years of production. Seed potatoes are also planted when the soil becomes workable, well before the last frost, allowing them to sprout underground.
Dormant ornamental plants, including bare-root roses, shrubs, and perennials like peonies or clematis, should be planted before their buds swell. Planting while dormant minimizes the stress of root disturbance. This ensures the plant dedicates its first energy to establishing new feeder roots in the surrounding soil before the demands of summer heat arrive.
Understanding Soil Temperature and Frost Dates
Outdoor March planting in Zone 5 must be governed by environmental factors, not just the calendar date. The average last spring frost date, typically falling in late April or early May, dictates outdoor planting timing. Any outdoor planting, even of hardy crops, carries a risk that an unexpected frost could damage or destroy the seedlings.
Soil temperature directly influences seed germination and root activity. While air temperatures fluctuate in March, soil temperature remains stable and can be measured with a soil thermometer placed a few inches deep. Peas struggle to germinate if the soil is below 40°F, and cool-season greens prefer a temperature in the 40°F to 50°F range for sprouting.
A cold, wet March can delay the soil’s warming process, which slows germination. Planting seeds into excessively cold or saturated soil can lead to seed rot rather than successful sprouting. Waiting until the soil has drained sufficiently and the temperature has reached the minimum threshold for the target crop is necessary for successful early spring gardening.