March is when Zone 6 wakes up. The average last frost of Apr 1–15 is only weeks away, the soil is warming, and the first hardy crops go outside. It is the busiest seed-starting month of the year — and the month when a little patience pays off more than a lot of enthusiasm.
What March means in Zone 6
Zone 6 sits in the middle of the American garden map — cold enough for a real winter, with average annual minimums of -10 to 0°F, but with a growing season long enough for tomatoes, peppers and winter squash to finish comfortably. The frost-free season lasts about six months, from mid-April into late October. The average last spring frost falls in the Apr 1–15 range and the average first autumn frost in the Oct 15–Nov 1 range — USDA zone averages, not exact dates, so check your local extension service before planting tender crops outside. Our planting calendar by ZIP turns them into a month-by-month plan.
Zone 6's frost-free season starts in mid-April, so March is the bridge month: indoor seedlings grow on, hardy crops go out, and tender crops stay inside a little longer. Peas, spinach, lettuce, carrots and radishes can all be sown outdoors in March, and brassica seedlings can be transplanted out — but tomatoes, peppers and beans must wait until after the last frost, or they will be killed by a single cold night.
March task list
- Direct-sow peas, spinach, lettuce, radishes, carrots and beets. 2. Transplant brassicas, onions and leeks; harden them off first. 3. Start squash, cucumbers and basil indoors in late March. 4. Prepare beds: fork in compost, rake level, and mulch paths. 5. Set up trellises and pea supports before plants need them. 6. Watch for slugs on young seedlings after rain.
What to start indoors now
Start squash, cucumbers and zucchini indoors in late March — they grow fast and should only spend 3–4 weeks inside, or they become root-bound and sulky. Start basil indoors now, and start a second round of lettuce indoors for transplanting into the cold frame. Keep growing on the tomatoes, peppers and brassicas started in February, potting them up into larger pots when roots fill their containers. Use clean pots and fresh seed-starting mix, give seedlings strong light, and keep the soil moist but never waterlogged. Work out exact sowing dates with the seed-starting calculator.
What to direct-sow outdoors
Direct sow peas (60–70 days), spinach (40–50 days), lettuce (45–60 days), radishes (25–40 days), carrots (60–80 days) and beets (50–70 days) outdoors in March, once the soil can be worked. Sow scallions and parsnips (100–130 days) too — parsnips are slow to germinate (14–28 days), so mark the row and be patient. Sow a first round of cilantro and dill in a sheltered spot. Prepare the bed first — fork it over, remove weeds, rake it level — then sow thinly, water gently, and thin seedlings to the spacing in the table below.
What to transplant now
Transplant broccoli, cauliflower and cabbage seedlings outdoors in mid-to-late March, spacing them 18–24 inches apart, and protect them with row cover if a frost is forecast — they are frost-tolerant, not frost-proof. Transplant onion and leek seedlings started in January, setting them out in rows 4–6 inches apart. Harden off all transplants for a week before they go out. Harden seedlings off for a week first, plant on a cloudy day or in the evening, water them in well, and mulch around — but not against — the stems.
What to harvest this month
March harvests are the first fresh food of the year: overwintered leeks, parsnips and Brussels sprouts, plus lettuce and spinach from the cold frame. Cut the first spring herbs — chives and mint are usually up by late March. Indoors, microgreens and windowsill herbs keep producing while the outdoor garden gets going. Harvest in the morning, when plants are cool and crisp, and pick little and often — regular picking keeps most vegetables producing.
Frost protection in Zone 6
March frost is the trap of the Zone 6 year: the average last frost is Apr 1–15, and a warm March week tempts gardeners to plant tender crops weeks early — then a cold night kills them. Keep frost cloth handy for the brassicas and peas, and do not be fooled by one warm weekend. Soil temperature matters as much as air temperature: peas and spinach germinate in cool soil, but beans and corn rot in it, so wait for real warmth before sowing tender seeds. When a cold night is forecast, water the soil in the afternoon, cover tender plants with frost cloth before sunset, and remove the covers in the morning. The Seed-Starting Calculator can help you time plantings around your frost dates.
Planting data for March in Zone 6
All figures below are broad published ranges from standard horticultural references, not exact figures — they vary by variety, climate and soil.
| Crop | Action this month | Days to maturity | Notes |
|---|---|---|---|
| Peas | Direct sow | 60–70 | Sow once soil can be worked |
| Carrots | Direct sow | 60–80 | Slow to germinate (14–21 days) |
| Radishes | Direct sow | 25–40 | Fastest spring crop |
| Broccoli | Transplant outdoors | 60–85 | Space 18–24 in; row cover if frost threatens |
| Zucchini | Start indoors (late Mar) | 45–55 | Only 3–4 weeks indoors |
| Onions | Transplant outdoors | 90–120 | Started indoors in January |
Browse the full planting calendar hub for every Zone 6 month, and see the neighbouring months in this zone:
Frequently asked questions
What can I plant in March in Zone 6?
Peas, spinach, lettuce, radishes, carrots, beets, scallions and parsnips outdoors, plus brassicas, onions and leeks as transplants. Start squash, cucumbers and basil indoors in late March.
Can I plant tomatoes outside in March in Zone 6?
No — tomatoes are tender and will be killed by frost. The average last frost is Apr 1–15, so tomatoes go out in mid-to-late April at the earliest, after hardening off.
When is the last frost in Zone 6?
The average last spring frost falls between Apr 1 and Apr 15. These are USDA zone averages, not exact dates — check your local extension service and the forecast, and keep frost cloth handy through April.
How do I stop slugs eating my seedlings?
Go out at dusk with a torch and pick them off, keep mulch away from young stems, and water in the morning so the soil surface dries by night. Beer traps and copper tape help in small beds.
Why are my carrots slow to germinate?
Carrots take 14–21 days to germinate even in good conditions, and longer in cold soil. Keep the seedbed evenly moist — a board over the row helps — mark the row, and be patient.